Ultra-Low-Level Measurements of Argon, Kryptonand Radioxenon for Treaty Verification Purposes

7/31/2019 Ultra-Low-Level Measurements of Argon, Kryptonand Radioxenon for Treaty Verification Purposes

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ESARDA BULLETIN, No. 36

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Ultra-Low-Level Measurements o Argon, Krypton

and Radioxenon or Treaty Verifcation Purposes

Paul R.J. Saey

Preparatory Commission for the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO), Provisional

Technical Secretariat, Vienna International Centre, P.O. Box 1200, A-1400 Vienna, AustriaE-mail: [email protected]

Abstract

Various noble gases are created in nuclear processeslike burn-up o nuclear uel, target irradiation andnuclear explosion. Being chemically inert, they wontreact with the ambient environment or deposit on

the ground once entered into the atmosphere butkeep on travelling and only disappear due toradioactive decay. They are, thereore, very goodtracers or revealing specic nuclear activities andcan help in veriying non-prolieration treaties.

Krypton-85 and radioxenon isotopes areanthropogenic isotopes produced through ssion ouranium or plutonium. The analysis o krypton in theatmosphere could help, e.g. in veriying compliancewith the Nuclear Non-Prolieration Treaty bymonitoring nuclear uel re-processing activities. The

detection o the radioxenon isotopes could giveindications e.g. on a nuclear explosion, clandestinenuclear reactors or other violations o non-prolieration treaties.

Argon-37 is an anthropogenic isotope producedwhen e.g. ssion neutrons react with calcium inrock. Its identication in the lower troposphere or insoil gas can be an indication or the detonation o anuclear device and can be used to veriy theComprehensive Nuclear Test-Ban-Treaty e.g. duringan on-site inspection.

Other noble gases like argon-4 and various short-lived krypton isotopes may be used or nuclearsaety monitoring or reactor operation surveillance.

This paper will describe how these noble gases arecreated, measured at ultra-low sensitivity level andused to trace back a violence o treaties dealingwith nuclear arms control and non-prolieration andpropose additional applications o these new ultra-low environmental measurement techniques.

Keywords: treaty verication; environmental

monitoring; radioxenon; noble gas; low-levelmeasurements

1. Introduction

In the world o today, more and more sensitivemethods have to be used and are being developedto deal with the threat o nuclear prolieration,nuclear terrorism and nuclear arms. It can be

expected that a possible violator tries to hide theseintentions and the clandestine preparation acilities.Traces o emissions originating rom these processesmay provide key inormation to reveal hiddenactivities. Thereore, monitoring o the atmosphereand environment in general is o vital importance orvarious treaty verication purposes.

Since uranium and plutonium are the key compo-nents o the nuclear reactions related to ssion, theirbehaviour need to be known or nuclear non-proli-eration as well as in monitoring nuclear explosions.

In the burn-up o nuclear uel and in the nuclearexplosions two sources o radioactive material aregenerated:

Fission products: these are direct products romthe nuclear ssion. Even when remotelymeasured, they may give inormation on thematerial used in the core o the nuclear device;

Activation products: neutrons produced in thession interact with the surrounding materialsand also the core itsel. As a result, substantial

part o the material gets radioactive due toneutron capture. Activation products may give agood indication o the environment where thession took place. Further they containinormation on which materials the device wasmade o.

Because in ssion the nucleus to split in anasymmetric manner, the ssion yield curve or theseelements (mass o ssion products versus atomicmass o the ragments) has two asymmetric peaks,one in the area zirconium through to palladium and

another at xenon through to neodymium. The ssionyield is a unction o the ssioned nuclide and theincident neutron energy (except or the case ospontaneous ssion). Figure shows the typical

Scientifc papers


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bimodal curve or ssion-product yield. The curveshows that during ssion, among others, relativelylarge quantities o radioxenon noble gases areproduced (up to almost 7 % o the ssion products).On the other hand, 85Kr has only a ssion yield o 0.3% and less. Table lists the cumulative ssion yieldor our relevant radioxenon isotopes.

2. Noble gases

The noble gases are: Helium (He), Neon (Ne), Argon(Ar), Krypton (Kr), Xenon (Xe), Radon (Rn) and

Ununoctium (Uuo). They have a complete outerelectron shell and are, thereore, chemically inert.Noble gases are inert and are thereore dicult tohide. For nuclear verication purposes, we will ocuson the 37Ar, 85Kr and our radioxenon isotopes andisomers.

2.1. Argon-37The atmosphere o earth contains around 0.93 %stable Ar. The name is derived rom the Greek argosor lazy or inactive. It was discovered by LordRayleigh and Sir William Ramsay in 894 during anexperiment in which they removed all o the oxygenand nitrogen rom a sample o air. The melting pointo argon is -89.35 C and its boiling point is -85.85C.

The radioisotope 37Ar (t/2

= 35.04 days) is producedin the atmosphere: 40Ar(n, 4n)37Ar. When 37Ar decays(electron capture), it emits no photons but onlyelectrons (Auger electrons).

It can also be the product o a reaction with ssionneutrons on calcium (Ca): 40Ca(n, )37Ar [2]. Ca ismostly contained in soils. The neutrons need to

Figure 1: Fission yield in % or several nuclear explosion relevant nuclides: 235U, 238U and 239Pu, or fssion

induced by fssion spectrum neutrons () and high energy neutrons (14.7 MeV) (he) respectively [1].

Fission Product Hal-lie Time unit 235U

235Uhe

238U

238Uhe

239Pu

239Puhe

3mXe .934 d 0.05 0.06 0.05 0.06 0.05 0.0733mXe 2.9 d 0.9 0.29 0.9 0.8 0.24 0.4233

Xe 5.243 d 6.72 5.53 6.76 6.02 6.97 4.8635Xe 9.4 h 6.6 5.67 6.97 5.84 7.54 6.8

Table 1: Cumulative fssion yields in percent or six fssion modes relevant to nuclear explosions, induced by

fssion spectrum neutrons () and high energy neutrons (14.7 MeV) (he) [1].


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have an energy in the range o MeV to let the (n, )reaction happen. Such neutrons are produced in anuclear explosion. I, thereore, 37Ar is ound in soilgas, this is a certain proo o a nuclear explosion. Itsconcentration is strongly dependent on the calciumquantity o the environment o an explosion. Ater

an underground nuclear explosion,

37

Ar can migrateto the surace into the atmosphere along aults andcracks, driven by e.g. the barometric pressure (lowpressure weather systems) over several weeks andmonths.

2.2. Krypton-85

Around 0.4 % o the earths atmosphere containsstable Kr. The name derives rom the Greek kryptosor concealed or hidden and it was discoveredin Great Britain in 898 by Sir Ramsay and Morris

Travers in residue let rom evaporating nearly allcomponents o liquid air. The melting point okrypton is -57.36 C and its boiling point is -53.22C.

The radioisotope 85Kr (t/2

= 0.76 years) is mainlyproduced during ssion, although a small amount isproduced in the atmosphere (rom 84Kr). It is a beta-emitter (99.56 %maximum beta energy: 690 keV)but 0.43 % decays to an excited level, ollowed bya gamma ray o 54 keV.

Due to its relative long hal-lie, it remains or manyyears in used nuclear uel or in soil ater anunderground nuclear explosion. It is released whennuclear uel is dissolved or plutonium extraction orother purpose. There are no other relevant sourceso 85Kr than the atmospheric nuclear weapon testsin the past and the reprocessing now. Kalinowski [3]calculated that during the production o kg oweapons grade 239Pu (which contains less than 7%o 240Pu) about .5 2.0 03 Bq o 85Kr is released.I 85Kr detection system is sensitive enough, it could

be used to monitor or discover reprocessingplants.

Further, due to its good geophysical properties, itslong hal-lie and its chemical inertness, 85Kr is usedor several applications in geosciences, e.g. tracingthe fow o ground- and ocean water.

2.3. Radioxenon

The earths atmosphere contains approximately0.009 % o stable Xenon (Xe). The name derives

rom the Greek xenon or the Stranger. Xe wasalso discovered in 898 by Sir Ramsay and M.Travers in residue let ater evaporating liquid air. Itis a heavy, odourless and colourless noble gas with

element number 54. Melting point o xenon is -.7C and its boiling point is -08.2 C.

Naturally occurring xenon consists o seven stableand two radioactive isotopes (24Xe and 36Xe, bothwith very long hal-lives). Beyond these stable orms,20 other radioactive isotopes have been ound.3m

Xe,33

Xe,33m

Xe, and35

Xe are some o the ssionproducts o 235U, 238U and 239Pu. The major part oradioxenon isotopes is manmade however, thespontaneous ssion o uranium in the natureproduce very low levels o radioxenon.

As can be seen in the Table , 33Xe has highproduction rates in 235U, 238U and 239Pu ssion. Thehal-lie o 33Xe is 5.2 days; this is perect ordetection systems since it is not accumulated in theatmosphere and it lives long enough to be detectableater atmospheric transportation to a monitoring

station. This isotope is thereore typical detected invarious environmental samples.35Xe has a very high thermal neutron capture crosssection, so it can absorb thermal neutrons very well.As thermal neutrons are needed to maintain thenuclear chain reaction in a reactor, 35Xe is a poisonthat can slow down the chain reaction or even stopit.

3. Ultra-low level measurement techniques

Radioactive noble gases can be extracted rom theenvironment with high eciency, as they are inert.On the other hand, they have low solvability and arepresent in the ambient nature in very lowconcentrations.

Many methods are used to measure radioactivenoble gases. Among them are Low-Level DecayCounting (LLC), Mass Spectrometry (MS),Accelerator Mass Spectrometry (AMS), ResonanceIonization Mass Spectrometry (RIMS), Atom TrapTrace Analysis (ATTA) and additionally or the photonemitters, gamma and beta-gamma coincidencespectroscopy [4; 5]. In the ollowing we will ocus onthe most common techniques used or nuclear non-prolieration and nuclear explosion monitoring: LLCor 37Ar and or 85Kr and gamma and beta-gammaspectroscopy or radioxenon analysis.

Some systems have recently been designed ormobile noble gas collection and analysis (e.g. ARIX-III, MARDS and SAUNA-II OSI). The measurementtechniques used to reach the required ultra-low

activity concentration levels needed in mobilemeasurement systems requires that the systemsare optimised or certain radioisotopes o interest.The techniques used with an emphasis on the


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analysis o radioactive noble gases are briefyreviewed in this paper.

The unit o detectability, minimum detectableconcentration (MDC), is dened as the smallestconcentration o radioactivity in a sample that canbe detected with a certain probability. When

detecting the activity, one can dene two dierentways to make wrong conclusions:

Erroneously detecting radioactivity, when in actnone was present (Type I error)

Not detecting radioactivity, when in act it ispresent (Type II error).

Improvement o detection capability and ability tomake right conclusion are also key issues orverication work with ultra-low level measurementtechniques.

3.1. Argon-37 measurement systems

The worldwide background o 37Ar is very low.Thereore, special ultra-low level counting techniquesare used or measuring the activity. The measuremento its low decay energy (2.8 keV) is done with speciallow-level gas proportional counters.

The only laboratory worldwide that can measure37Ar at ultra-low levels is situated at the University oBern, Switzerland. This laboratory has ultra-lowbackground measurement chamber 35 meters

underground, and it is using the LLC method.

For the purpose o on-site inspections (OSI) in theramework o the Comprehensive Nuclear Test-Ban-Treaty (CTBT), the Institute o Nuclear Physics andChemistry, China Academy o Engineering Physics,Mianyang, China, has developed the MovableArgon-37 Rapid Detection System (MARDS). Thissystem extracts the argon rom sampled air andmeasures the 37Ar radionuclide with a gasproportional counter [6].

3.2. Krypton-85 measurement systems

The U.S.A. used the 85Kr detection technique alreadyin 95 to monitor the Soviet production o Pu. Laterthey also monitored other countries (OperationBluenose). Air was collected in gas bottles, whichwere shipped to a laboratory or measurement.

As its gamma emission is too small to be measuredin environmental samples directly, it is a challengeto measure the activity concentration o this isotopewith a good sensitivity, as the samples may be small

and the measurement time can be limited. Acommonly used method is the LLC. In this methodthe krypton is rst reezed out and then measured ina gas proportional counter [7]. This measurement

should ideally be perormed in a low-backgroundenvironment.

The Noble Gas Laboratory o the German FederalOce or Radiation Protection (BS) has analysedamong other things 85Kr in the environment sincethe early 970s. Bieringer and Schlosser describe

in [8] their routine analysis the ollowing steps:. Enrichment, purication and separation o the

noble gas ractions by cryogenic adsorption anddesorption and gas chromatography;

2. Measurement o the integral beta activity by gascounting in proportional counters anddetermination o the stable krypton volume bygas chromatography.

New automated and mobile systems that has adetection capability o 85Kr down to 0. Bq/m3 with

a 6 hour measurement time, like e.g. the FrenchSPAARK system (Systme de prlvement etdAnalyse Automatique du Radio Krypton) [9].

3.3. Radioxenon measurement systems

The rst radioxenon measurements date back tothe second world war, when American airplanesfew low over Germany, sampling air and trying tond traces o a German nuclear reactor and weaponsprogramme [0]. During the nuclear testing era,33Xe was measured both onsite and osite by the

tester. Because o the detection capability o theearly equipment, the traces o 33Xe were detectedonly i the nuclear test was atmospheric or badlycontained underground test.

The International Monitoring System (IMS) is themonitoring system that will veriy the CTBT [].According to this Treaty there will be at its Entry IntoForce (EIF) 40 stations worldwide capable omeasuring the relevant Noble Gases [2]. The othertechnologies used or its verication are radioactiveparticles, seismology, hydroacoustic- and inrasoundmonitoring. In addition to this, the radioactivesubstance detection requires atmospheric transportmodelling (ATM). Since commercial noble gassystems with suitable detection capability were notavailable, the Preparatory Commission (PrepCom)o the CTBT Organisation (CTBTO) tasked theProvisional Technical Secretariat (PTS) o the CTBTOin 999 to perorm an International Noble GasExperiment (INGE) according to the requiredspecications o the noble gas monitoring.

Four countries announced their willingness toprovide radioxenon sensors: the USA made asystem called ARSA, Russia constructed ARIX,Sweden built SAUNA and France oered the


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SPALAX system. The design criterion or all o themis that the MDC o 33Xe should be mBq/m3 or lessor a 24-hour sampling period [3]. Three o thesystems, the ARSA, the ARIX and the SAUNA, areusing very similar measurement techniques andproduce two-dimensional beta-gamma coincidencespectra [4;5;6]. The SPALAX system is based onhigh-resolution gamma spectroscopy [7]. Theoriginal ARIX system was based on beta-gatedgamma spectroscopy [8], until this technique wasmodied to beta-gamma coincidence spectroscopyin early 2007.

For the purpose o on-site inspections, mobileversions o the ARIX and the SAUNA systems havebeen developed. These systems have proven to beoperational during a eld experiment set up inSeibersdor, Austria, in the summer o 2006. TheSwedish mobile SAUNA-II OSI system has alsobeen deployed in the Republic o Korea, immediatelyater the announced nuclear explosion o theDemocratic Peoples Republic o Korea, to clariywhether the explosion was in act a nuclear weapontest or not [9].

The BS e.g. applies the integral counting methoddescribed above with krypton also or radioxenonmeasurements. All xenon isotopes in the sampleare determined simultaneously. 3mXe and 35Xeactivities can be calculated with decay rates o

these isotopes by measuring the sample in time-slices.

3.3.1 Sampling and purication o xenon

The noble gas systems developed to measure theour CTBT relevant isotopes at ultra low level arebased on similar collection principles. Air is sampledat high volume with an air fow that has to be largerthan 0.4 m3/h. This air is cleaned: aerosols, water,Rn, Ar, N

2, O

2, CO

2, etc. are removed, among others

with lters and by heating. The next step is theextraction o xenon gas rom the air. This is perormed

with high eciency by adsorption o xenon ontocharcoal ollowed by thermal desorption o thexenon and depending on which system, also withmolecular sieve columns. The stable xenon volumeo the concentrated gas is quantied by means o agas chromatograph.

3.3.2. Nuclear measurement o radioxenon

isotopes

Ater a 2 hour cycle, between 0.5 and 3 ml o stablexenon has been extracted, which is enough or a

low level nuclear measurement. This is based onone o the ollowing two methods: two-dimensionalbeta-gamma coincidence spectroscopy or high-resolution gamma spectroscopy.

The - detector has a sodium-iodine (NaI) crystalwith a drilled hole, where the gas fows in. The holeaccommodates a plastic scintillator cylinder. Gammapulses are counted through photomultipliers in botho the ends o the crystal and beta pulses arecounted with the scintillator cylinder. The electronicsystem counts the gamma, the beta and thecoincidence pulses typically 2 hours with noblegas sample inside. Beore each measurement, aquality control source (e.g. 52Eu) enters the cell andis measured. As some xenon diuses into the plasticscintillator, a gas background is measured or hours to count this possible memory eect o aprevious sample.

The detector consists o a p-type broad energyHigh Purity Germanium (HPGe) detector. Thedetector type has minimal dead layer despite it is o

p-type. The gas fows in a sample cell, which ismade o low background aluminium. The cell islocated on top o the germanium crystal. Somemeasurement systems have carbon bre window toprovide improved X-ray detection. The gas sampleis typically measured or 24 hours. A quality controlsource is mounted above the cell. This sourcecontains a mixture o radioisotopes that emit gammalines at other energies than radioxenon does. Thepurpose o this source is to check the stability o thedetector.

The xenon sample is archived in separate archivebottles, one or each sample. Ater 5 7 days, thesebottles are emptied, fushed with a clean gas andthey can be re-used.

CTBTO is using remote analysis o the measurementresults. Ater the measurement is done and spectraldata saved, it is transmitted through PTS GlobalCommunication Inrastructure to the data centre oranalysis, review and nal conclusions. [20]

3.3.3. Spectral analysis o radioxenon isotopes

The our major xenon isotopes emit all photons (X-rays and/or gamma rays) in coincidence with betaand conversion electrons. X-rays are at 30 keV (withslight shits caused by the dierent nuclear mass othe various isotopes) and have a total branchingratio o about 50%, except or 35Xe, which has justa 5% X-ray branch. The strongest associatedconversion electrons in coincidence with the X-raysare 29.4, 98.7, 45.0, and 23.8 keV or 3mXe,33mXe, 33Xe and 35Xe, respectively. Other strongcoincident decay modes are up to 346 keV energy

beta decay o33

Xe in association with a 8.0 keVgamma decay, and up to 90 keV energy beta decayin 35Xe, which is ollowed by a 249.8 keV gammaray (see Table 2).


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In gamma and beta-gamma spectroscopy, 33Xeand 35Xe isotopes are identied by their photonpeaks. I the isotopes 3mXe and 33mXe have a lowactivity in a sample, the photon peaks may be belowthe detection capability o the measurement systemsThus, only the 30 keV X-ray peak may be visible.The beta signal can give the missing inormation to

calculate the activity o the metastable isotopes. Ithe beta-signal is not measured (e.g. or the SPALAXsystem), the analysis is still possible through acomplex gamma peak and x-ray peak deconvolutiontogether with a hal-lie analysis o all the preliminaryspectra measured on the sample. This method hasbeen developed at the PTS [2].

4. Environmental monitoring o noble gases

Environmental monitoring is a powerul tool indetecting low levels o radio-isotopes in theenvironment. The isotopes o interest can be oundin soil and vegetation, in the hydrosphere and theatmosphere. We will concentrate on monitoring othe atmosphere. To be able to perorm environmentalmonitoring with the scope o nding anomaloussignals that could reveal important inormation,several actors have to be known and to be takeninto account while evaluating the results o themonitoring. The regional background o the selectedradionuclide is important as well as climatological

and meteorological behaviour at the sampledlocation.

4.1. Environmental background

To monitor the environment or certain radioactivenoble gases, one has to be able to distinguish thereal signal rom a background signal. The noblegases o interest have ollowing global backgroundcharacteristics:

Almost all 37Ar is natural;

Most o the85

Kr is anthropogenic all over theworld; Nearly all radioxenon is anthropogenic and its

concentration can be regionally dierent.

37Ar background is very low and rather stable (in theorder o mBq/m3).

Due to its long hal-lie, 85Kr, has a large backgroundin the whole northern hemisphere. Its worldwidebackground started increasing dramatically by aactor o one million ater the rst nuclear weapontests in the atmosphere took place in the late orties

and it is continuing to increase with the installationo large reprocessing plants.

The worldwide background o 85Kr is currentlybetween .0 and .4 Bq/m3 [24]. This value varieswith seasonal infuences, atmospheric dilution andlarge releases at reprocessing plants. Due to dencenuclear installations, the concentration is higher atthe northern hemisphere.

Although radioxenon isotopes have a shorter hal-lie, they are produced at most nuclear acilities and

can thereore be ound in wide regions around wherenuclear acilities are. To distinguish clandestinenuclear acility or a nuclear explosion rom othersignals, the worldwide background (activityconcentrations and also activity ratios) has to bestudied careully. The background o radioxenonand the ratio o dierent radioxenon isotopes isdepended rom several actors and sources and canvary over a ew orders o magnitude. The dierentsources can be:

Nuclear Reactors: mainly 33Xe Fuel reprocessing plants: mainly 3mXe Hospitals: mainly 33Xe and 3mXe Nuclear Explosions: mainly 33Xe, 35Xe and 33mXe

UNSCEAR (United Nations Scientic Committee onthe Eects o Atomic Radiation) reports regularly onemissions rom nuclear reactors worldwide [25]. Thereport contains global noble gas release inventoryand in several cases it has also the radioxenonisotope 33 is listed separately. Kalinowski et al.

[26] have collected a database o worldwideradioxenon release data rom various sources. Table3 gives an overview o the order o magnitude oradioxenon release rom dierent kind o acilities.

Isotope Energy X-ray [keV] (K

and K2) Intensity* [%] Energy-ray [keV] Intensity [%]

3mXe 29.62 44.4 63.930 .933mXe 29.62 46. 233.22 8.2 [22]33Xe 30.80 40.9 80.997 38.035Xe 30.80 2. 249.77 90.0

Table 2: The our relevant radioxenon isotopes and their most intense -ray and X-ray (rom the Evaluated

Nuclear Structure Data File [23]).

* These values are the weighted averages o the K

and K2

X-rays. The intensities are the sum o these two K

lines


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To understand the radioxenon background, severalstatistical studies have been or are being perormedwith the experimental stations o the INGE network.During 2007, 8 systems will be measuringradioxenon isotopes in dierent parts o the world(see Table 4). Some o these stations have beenoperating already longer than ve years. Based on

current experience the radioxenon background canbe categorised into our groups:

Radioxenon is not expected: no radioxenonisotopes present (e.g. at Tahiti in the SouthernPacic) [27];

Regular but low radioxenon background o oneor two isotopes: regular presence o 33Xe and or3mXe at very low concentrations (less than mBq/m3) (e.g. on the Arctic station oLongyearbyen, Spitsbergen) [28];

Regular radioxenon background o 33Xe (~ 00mB/m3) and occasionally other isotopes at lowlevel (e.g. in the European station on theSchauinsland mountain in the Black Forrest,Germany) [29];

High radioxenon background with many isotopes:all isotopes are regularly present at dierentactivity concentrations (up to ew Bq/m3) (e.g.the station o Ottawa, which is surrounded bynuclear power industry and a large

radiopharmaceutical production acility) [7].It has been shown that the environmentalconcentration o 33Xe in Central Europe is ew mBq,in Scandinavia around 0 % less and in the highArctic (Spitsbergen) another 0 % less [30]. Further,there was a 20 old decrease o the environmentalradioxenon activity concentration in the late 980s,which happened due to improvements in the nuclearuel rod cladding and reactor containment systemsand longer decay times beore the noble gases arereleased into the atmosphere. [3]

As can be seen in Table 4, the global radioxenonbackground has not been characterized in manyareas containing nuclear acilities and thus

radioxenon emissions. Thereore, more radioxenonbackground studies are needed.

4.2. Radiopharmaceutical radioxenon sources

Radioactive pharmaceuticals are used in nuclearmedicine or diagnosis (e.g. imaging) or or treatment.Common isotopes are e.g. 99mTc, 3I and 33Xe. 99mTc

is used a lot in pharmacy due to its hal-lie o 6hours and low gamma energy at 40 keV, 3I is usedwidely in the world or thyroid treatment. 33Xe (hal-lie o 5.2 days and 38 % intensity 8 keV photonenergy) is used or measuring the physiologicalparameters o lung ventilation and to image thelungs. It is also oten used in an isotonic solution toimage blood fow, particularly cerebral blood fow.99mTc is the daughter nuclide o 99Mo (t

/265.94 h).

The most common way to produce the ssionproduct 99Mo is the irradiation o highly enriched

uranium (HEU, up to 95 % o 235U). It can also beproduced by using low enriched uranium (LEU, lessthan 20 % 235U) or via neutron activation (n, reaction)o 98Mo in a high neutron fux reactor. Ater theirradiation, 99Mo is separated chemically and thendistributed to the end customer, e.g. hospitals. Inthe hospitals, 99mTc is extracted just in time beorebeing used on a patient.

When ssion is used or the production oradiopharmaceuticals, also radioxenons areproduced. They will escape into the atmosphere,especially during the chemical process producing ahigh radioxenon background [32]. First measurementsand preliminary estimates using ATM indicate thatthese sources might dominate the radioxenonbackground, as their releases can be up to threeorders o magnitude above the ones rom nuclearpower plants.

4.3. How noble gases enter the atmosphere

4.3.1. Release rom a nuclear acility

Fission product noble gases are created within theuel material o nuclear reactors. There can becracks in the cladding (the outer layer o the uelrods, e.g. Zircaloy), which will allow some noble

Type o acility Order o magnitude o radioxenon release

Hospitals ~ 03 Bq

Research laboratory ~ 06 Bq

Nuclear power plants ~ 09 Bq

Radiopharmaceutical plants ~ 02~ 03 Bq

kton nuclear explosion underground ~ 03~ 05 Bq

kton Nuclear explosion atmospheric ~ 06 Bq

Table 3: Order o magnitude o radioxenon release rom dierent nuclear acilities and events


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gases to leave the uel rods and enter the primarycooling material (e.g. water or gas). More quantitiescan be released during start-up and shut-down othe reactor, due to thermal stress o the claddingmaterial in these phases. Further, some noble gasesare created rom ssion o traces o U or Pu incooling material itsel. Rarely, noble gases canescape the uel due to a process disturbance or anaccident. The isotopic composition o the releasecan be very dierent depending on in what kind ocircumstances and how ast the release is occurring.Also depending on the containment, the emissionsmay stay variation o time in the release pathway,during which they are decaying. Some acilities usespecial retention lines, where high radioactive gasesdecay up to several tens o days, beore they enterthe stack to leave the acility and enter theatmosphere. Reprocessing plants release noble

gases during the processing o nuclear uel, most othem during the dissolving o the uel. Also inradiopharmaceutical plants most o the releasesoccur during the chemical process (see 4.2). Allthese delaying actors have to be taken into accountwhen ratios o environmental radioxenons will bestudied.

Appelhans and Turnbull [33] have calculated in detailthe release raction o noble gases in light waterreactors, while Kalinowski and Fister [34] havesimulated radioxenon ratios o light water reactors

under dierent circumstances.

4.3.2. Source term o radioxenon

in a nuclear explosion

During ssion o uranium or plutonium in a nuclearreactor, thermal (slow) neutrons are used, whereasduring a nuclear explosion the great amount ossion is induced by ast neutrons. Most o thenuclides in the explosion device undergo ssionwithin a microsecond.

There is little time or activation build-up in a nuclear

explosion whereas there is sucient time orproduction o many activation products in a nuclearreactor. These dierences produce dierentradionuclide abundances. Since a nuclear blastproduces dierent radionuclide abundances, nuclideratios may be used or source identication.

The energy produced in a one kiloton (kton) nuclearexplosion is equivalent to an explosion o 000 tonso TNT, which equals 02 calories = 4.2 02Joules.

The average total energy produced in ssion o one235U atom is 200 MeV = 3.2 0 J (with 000 MeV =.602.00 J), the average total energy released inssion o one plutonium-239 atom is 20 MeV = 3.5

0 J. This energy takes the orm o the ssionragments, instantaneous gamma-ray energy, kineticenergy o ssion neutrons, beta particles rom ssionproducts, gamma rays rom ssion products andneutrinos rom ssion products. About 80 MeV isimmediately available as energy rom each ssion

event, which means that there are around .45 023

ssions per kton. With

A = activity [Bq]

= decay constant

N(t) = number o atoms

t1/2

= hal-lie [s]

an upper and lower emission value can be calculated.According to Table , 239Puhe

has the lowestcumulative ssion yield or 33Xe (4.86 %) and 238U

he

has the highest (6.02 %). Thereore, depending onthe ssion material inside the nuclear device,between .08 06 Bq and .33 06 Bq o 33Xe willbe released in a kton nuclear explosion.

However, rom the mass 33 isotopes that areproduced during the explosion, the 33Xeconcentration is initially very low. De Geer [pers.comm.] calculated the dynamics o the isobar chains

or the our considered radioxenon isotopes. Aterthe explosion o a 239Pu device used, 33Xe reachesits maximum concentration (as calculated above)ater 2.8 days due to in-growth.

4.3.3. Release rom underground

nuclear explosions

For an atmospheric or near surace nuclearexplosion, all the debris is in the atmosphere andcan be more easily measured by e.g. a station o theparticulate radionuclide component o the IMS. In

this case, radioxenon stations give only little addedvalue. I, however, the explosion is underground ordeep under water, only noble gases might leak outand the radioxenon stations o the IMS will be theonly ones that will be able to proo that the explosionwas nuclear [35; 36].

Ater the Partitial Test Ban Treaty got into orce in963, most nuclear explosions were perormedunderground in drilled vertical holes or in minedtunnels. The goal o the state perorming the test

was to acquire the experimental inormation o thenuclear device or and at the same time contain theexplosion umes, i.e. prevent that any radioactivematerial would reach the atmosphere.


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The time a gas needs to reach the surace isdependent on its diusivity, the power o theexplosion and on the underground environment(amount o ractures, humidity, geological structureand aults etc.).

Schoengold et al. [37] reported that up to 20 Bq/m3

o33

Xe could enter the atmosphere ater anunderground explosion at the Nevada Test Site,USA. Oten, however, the activity concentration o33Xe was not reported ater a nuclear test as it wasbelow the detection capability o the equipmentused that time.

Dubasov [38] reported on releases o 33Xe romunderground tests in e.g. the Novaya ZemlyaArchipelago (Arctic Russia) in the late eighties,measured in subsoil gas and in the atmosphere.Some atmospheric samples contained up to 620

mBq/m3. A MDC between 50 400 mBq/m3, 35days ater underground 30 50 kton tests wascalculated or the equipment used at that time. Theactivity concentration in subsoil gas samples wasound to be around 2000 times higher than theatmospheric concentrations measured. In Kurchatov,around 00 km rom the test site, also 85Kr with 20times higher than continental background wasmeasured, with a MDC o 3.7 Bq/m3. All releases onoble gases below the MDC level where declared tobe complete contained explosions. In another

report, Dubasov [39] inorms about noble gasreleases in the Semipaltinsk test site (nowKazakhstan) between 96 and 990. There, theMDC or 33Xe was around 400 Bq/m3, whereasactual equipment measures close to 0.2 mBq/m3.

The Soviet Union made 8 tests with a total yield lessthan with 50 kt in a tunnel on the Novaya ZemlyaNorthern Site on the 20 October 990. Theseunderground test were detected in Sweden withradioxenon measurements, approximately 24 mBq/m3 o 33Xe was detected by ater these tests [DeGeer, pers. comm.].

In 996 a Non-Prolieration Experiment (NPE) wasperormed. A conventional explosion o kton, wasdetonated in Rainier Mesa (Nevada Test Site) at 400m depth. It included also sulphur hexafuoride (SF

6)

gas and 3He to simulate argon and radioxenon.Carrigan et al. [40] concluded that 33Xe would bedetectable 50 days ater detonation, whereas 37Arwould need 80 days to reach the surace. The mainmechanism driving the release in this case is

atmospheric pumping. The dierence o the releasetime is due to the dierent physical properties(diusivity) o these moleculesthe most diusiveentered the atmosphere the latest because they had

gone deeper into cracks and were thereore betterhidden than the lower diusive molecules. Accordingto Carrigan et al., ater a kton ssion detonationthe total release o 33Xe could be around 9.7 02Bq in a period o weeks till months ater thedetonation due to seepage.

Releases o radioactive material ollowing anunderground nuclear test are generally categorizedas ollows:

Unintentional release o radioactive material tothe atmosphere due to ailure o the containmentsystem (000 % o the created noble gases(primarily 85Kr and 33Xe) could enter theatmosphere);

Prompt venting due to high pressure o theexplosion and other dynamic eects (pushes gas

through cracks and ssures in the bedrock, ~ 0%);

Venting due to opening o tunnels to measure thedetonation eects or due to removal o themeasurement materials. This can happen daystill weeks ater the event (controlled tunnelpurging)

Drilling o holes etc. (operational releases);

Natural atmospheric removal, low pressure ispumping out the gases stored in the ssures andcracks under ground (late-time seeps due toatmospheric pumping) (~ %).

Immediately ater an underground nuclear explosion,the materials around the device are vaporised dueto the enormous heat. When the heat lowers down,vaporized gases condensate to particles, thisprocess is complex and involves many chemicaland physical processes. Because the chemical andphysical properties o the materials change as aunction o time, the materials escaping rom the

cavity and remaining there may dier. Somesubstances stick aster to the wall o the cavity,whereas others are more volatile and can move acertain distance in cracks beore they condense.This has also an eect on the gases that are createdin the explosion, especially i the decay productsare changing the phase rom solid to gas (e.g. 33I 33Xe).

4.4. From a source to a detector

In general, the probability that a detection systemcan measure a signal rom nuclear activities isdepended rom:

the amount o released noble gases;


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the atmospheric dispersion between the sourceand the monitoring system;

the MDC o the measurement system;

the local environmental background.

Since noble gases are chemically inert, they do not

react with particles or water vapours o the cloudsduring their atmospheric transport. The deposit onthe ground does not occur either, but theirconcentration in the air shall decrease due toradioactive decay and dilution. They are, thereore,very good tracers or nding specic nuclearactivities, like i.e. nuclear explosions.

Once an environmental noble gas measurementsystem has identied certain relevant radionuclides,it is o key importance to nd the possible releasepoint o these measured nuclides. To perorm thesecalculations, dierent atmospheric transport models(ATM) can be used. The PTS o the CTBTO usesroutinely models based on Hysplit or Flexpart [4;42]. These models benet orm possible inormationo event time that can be available or examplethrough seismic, inrasound or hydro-acousticdetection.

There can be two scenarios: the geographic locationo the source is thought to be known or it isunknown.

In the case the source is thought to be known, ATMcould strengthen a hypothesis or could excluderegions where a release probably took place. Anexample is a known acility that perorms undeclaredactivities or in the case o nuclear explosionmonitoring, waveorm signals that indicate thelocation, depth and time o an explosion. It shouldbe noted that tele-seismic waveorm signals cannotdistinguish between a conventional chemical or anuclear explosion.

The case that the source location is unknown ismore complex, especially as the calculated possiblesource region (PSR) can be large (up to several tenthousand o km3). Methods to reduce the PSR aree.g.

calculating ratios i dierent radioxenon isotopeshave been measured and determine the time othe event or the possible source (e.g. a nuclearreactor in equilibrium emits dierent radioxenonratios than at start-up or shut-down or than a

nuclear explosion). Ratio calculations are,however, only possible or radioxenon isotopes imultiple isotopes are detected. This methoddoes now work with argon or krypton;

use inormation rom dierent monitoring stationswhich have measured a signal that could originaterom the same source;

use the dierent signals rom several days,measured at the same station.

It should be noted that the traces o certain

radioactive noble gases measured may have verydistinct history they can be released as a pu orover a longer time, they have travelled a while anddecayed during that time period. Further they dilutedin the atmosphere and probably were mixed with airmasses that might also contain noble gases romother sources. Also, since the samples are collectedin 8-24 hours shits, the exact time and durationwhen the measurement station is exposed topassing noble gas cloud is not known. All theseactors have to be taken into account when data are

interpreted. The more samples are measured with ashort time period each, the better the possibilitiesare to nd its source o origin, as the signal will notbe diluted with air that contains only backgroundactivity.

4.5. Noble gas measurement networks

The only treaty that currently uses environmentalmonitoring o atmospherically transportedsubstances or its verication is the CTBT, whichmonitors radionuclide particulates and radioxenonisotopes. All other networks are national or academicones.

4.5.1. Global Krypton-85 networks

Several networks monitor worldwide 85Kr. One othem is the German Integrated Measuring andInormation System (IMIS), which operates with 5stations worldwide [8]. The air is sampled at thestations or about one week with a collection volumeo 0.06 m3/h. Krypton is collected by adsorption onactive charcoal at -97 C (using liquid nitrogen)

and then shipped to Freiburg, Germany oranalysis.

4.5.2. The network o the International

Noble Gas Experiment

As described in 3.3., our automated systems weredeveloped in the ramework o INGE. In 2000 theseour systems were tested together or severalmonths in parallel in Freiburg, Germany [43].Aterwards, these our prototypes were installed atIMS sites in Guanghzou (China), Buenos Aires

(Argentina), Spitsbergen (Norway) and Papeete(Tahiti). In the meantime, industrial versions havebeen developed and more stations are now installedworldwide, as is shown in Table 4.


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The INGE network has some unique eatures:

high time resolution: previous measurementcampaigns sampled air or weeks or monthsbeore the noble gas samples were measured,whereas the INGE stations sample between 8and 24h per cycle. With such short sampling timeATM can be used to calculate possible sourceregions;

geo-resolution: the 40 stations (with the possibility

to be increased to 80 ater entry into orce o theCTBT) are distributed in such way that the traceso a kton nuclear explosion can be measuredwithin 2 weeks, which means that signals roman event can be monitored still at distances oseveral thousand o km.

very low detection capability: down to 0.2 mBq/m3 or 33Xe or a 2 hour measurement;

automated systems that can be deployed at veryremote places without the need o local technicallyhighly skilled personnel.

The Democratic Peopls Republic o Koreasannounced nuclear explosion was a good test casewhere a long distance radioxenon signal was

measured in one o the INGE stations, in Yellowkniein the north o Canada [44]. The IMS seismic signalgave a correct indication o the time and place theexplosion took place. The orward ATM calculationspredicted then, using the beore mentioned sourceterm or a kton nuclear explosion, two distinct33Xe signals in Yellowknie between two and threeweeks ater the explosion with very low concentration.This two peak signal was then indeed measured atthe predicted time and activity concentration.

Backtracking ATM calculations are consistent withthe assumption that the measured 33Xe signal couldhave originated rom the Korean peninsula.

5. Verifcation applications

This chapter will describe the applications o thediscussed noble gas measurements, in the light onon-prolieration treaties and weapons detection.

The goal o the Non-Prolieration Treaty (NPT),opened or signature on July 968, is to limit the

spread o nuclear weapons. The Euratom Treaty,signed in 957, established a nuclear materialcontrol system and assigned to the EuropeanCommission the responsibility o satisying itsel

Country Station location Start date System type

Argentina Buenos Aires June 2005 ARIX-II

Australia Darwin Sept. 2006 SAUNA-II

Brazil Rio dei Janeiro u.p.

Canada Ottawa Nov. 200 SPALAX

Canada St. Johns March 2006 SPALAX

Canada Yellowknie Aug. 2003 SPALAX

China Beijing Dec. 2006 SPALAX

China Guangzhou mid 2007 SAUNA-II

France Cayenne (French Guinea) mid 2007 SPALAX

France Papeete (Tahiti) May 2002 SPALAX

France Runion u.p.

Germany Schauinsland Feb. 2004 SPALAX

Japan Takasaki Nov. 2006 SAUNA-II

Mongolia Ulaanbaatar June 2006 SPALAX

New Zealand Chatham Island mid 2007 SAUNA-II

Norway Longyearbyen (Spitsbergen) Sep. 200 SAUNA-II

Panama Panama City Jan. 2007 SPALAX

Russian Federation Dubna Sept. 2006 ARIX-II

Russian Federation Ussuriysk u.p.

Sweden Stockholm Aug. 2005 SAUNA-II

U.S.A. Charlottesville mid 2007 SAUNA-II

Table 4: The INGE network radioxenon stations that are or will be operational in 2007.

The start date column provides the date when frst spectra were send to the PTS. Currently there are no

ARSA systems measuring in INGE. Ottawa is the only non-IMS operated station and is a national contribution

o Canada. u.p.: under procurement.


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that ssile nuclear materials (e.g. U and Pu) are notdiverted rom their intended use as declared by theusers. It doesnt oresee, however, environmentalsampling as a verication method. TheComprehensive Nuclear-Test-Ban Treaty orbids allnuclear explosions and the Fissile Material Cut-oTreaty would ban the production o ssile materialor nuclear weapons.

A non-treaty verication application o radioxenonmeasurements could be the monitoring o the noblegas activity concentration close to nuclearinstallations rom the point o view o quality osaety.

5.1. Non-Prolieration Treaty

The Treaty on the Non-Prolieration o NuclearWeapons (NPT) has three pillars: non-prolieration,

disarmament and the right to use nuclear technologypeaceully. It entrust the International Atomic EnergyAgency (IAEA) as its nuclear inspectorate.

The IAEA has specic roles, among them theinternational saeguards inspectorate which veriesthat member states are not diverting nuclear energyrom peaceul uses to nuclear weapons or othernuclear explosive devices, i.e. that they comply tothe NPT. Under a Comprehensive SaeguardsAgreement, which is signed with each member

state, the IAEA has the task to veriy the declarednuclear material and nuclear material relatedactivities o the country.

Iraqs clandestine nuclear weapons programme innon-declared acilities showed the limits o traditionalsaeguards. Thereore, member states added in997 measures to strengthen the IAEAs inspectioncapabilities. These are incorporated in the AdditionalProtocol to the NPT, which is a legal documentcomplementing comprehensive saeguardsagreements. The measures enable the IAEA not only

to veriy the non-diversion o declared nuclearmaterial but also to provide assurances as to theabsence o undeclared nuclear material and activitiesin a State. This Additional Protocol brought e.g. alegal basis or environmental sampling to veriycompliance with the NPT. It describes wide-areaenvironmental sampling (WAES), which is wouldallow the Agency to take samples also ar away romthe declared acilities. Its art. 9, however, stipulatesthat such techniques and the proceduralarrangements related to WAES have to be agreedrst by the IAEA Board o Governors.

Some possibilities where WAES could discoverillegal activities are e.g. detection o non-declared

reactor operations, o hidden reactor operationsand o stored ssile material.

Since 85Kr escape rom dissolved nuclear uel duringthe re-processing activities, it could serve a anindicator i a known reprocessing plant is stilloperational or not. Measurements in or close by the

plant can provide evidence o such activities. Mobileenvironmental monitoring o 85Kr can also identiyclandestine Pu separation acilities. The detectionprobability using remote environmentalmeasurements, according to a study made byKalinowski et al. [2004] near Karlsruhe, Germany o85Kr as an indicator or plutonium separation, oundthe ollowing detection rate or the separation o 4kg o plutonium per week:

80% to 90% at the distances < km

70% at 5 km

40% at 39 km

5% at 30 km

Currently, radioxenon monitoring is mainly used tomonitor nuclear explosions rom a long distance orto identiy traces o an underground nuclearexplosion by sampling underground gas during anon-site inspection, both to veriy the CTBT. However,as signals o radioxenon point to nuclear activities,other applications could be:

verication o known nuclear reactor operations:measuring the radioxenon isotopical ratios cangive inormation on specic reactor operations,e.g. the ratios change considerably during start-up and shut-down [33]. However, themeasurements are typical or each acility andhave to be perormed in the vicinity o the reactoras they as they depend e.g. on the presence oacility design and delaying the emissions to theenvironment. Further, the measured air shouldnot get mixed too much with the radioxenon

background, in order not to loose the uniquesignal;

detection o hidden reactor operations: the air inthe vicinity o nuclear power plants can have aconcentration o a ew Bq/m3 o 33Xe. I theregional radioxenon background is known,hidden reactor operations can be ound. However,the measured signals should be validated withATM to conrm that the signal was not broughtto the noble gas measurement station rom along distance. Measurements with a mobile

system, however, could be perormed at dierentlocations, to get certainty o the results. Oneshould also note that legitimate nuclear activitiesmay also cause emissions, thereore, the nuclear


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activities close to the inspection area should becharacterized and monitored in the same time.

A non-environmental measurement application oradioxenon noble gas could be the verication ostored ssile material via measurements oradioxenon isotopes rom spontaneous ssion in

the material. kg o weapons grade plutonium willhave in equilibrium an activity o around 2 kBq. Onlya part will be released in the air, which still can be inthe order o Bq, which is well above the normal localbackground.

5.2. Comprehensive Nuclear Test-Ban-Treaty

The CTBT is a key instrument o the internationalnuclear non-prolieration and disarmament regimebuilt around the non-prolieration o nuclearweapons. Its total ban o any nuclear weapon test

explosion will constrain the development andqualitative improvement o nuclear weapons andend the development o advanced new types othese weapons.

Its article points out the essence o the Treaty:

Each State Party undertakes not to carry out anynuclear weapon test explosion or any othernuclear explosion, and to prohibit and preventany such nuclear explosion at any place under itsjurisdiction or control.

Each State Party undertakes, urthermore, torerain rom causing, encouraging, or in any wayparticipating in the carrying out o any nuclearweapon test explosion or any other nuclearexplosion.

The objective o the IMS is, according to the CTBT:At least 90% detection capability within 4 daysater a nuclear explosion in the atmosphere,underwater or underground or a kton nuclearexplosion. A network is being build o waveormmonitoring stations (seismic (70 stations),hydroacoustic () and inrasound (60)) andradionuclide stations (radionuclide particulate (80),noble gas (40) and certied radionuclide laboratories(6)). The radionuclide sampling sites have beendened in an appendix o the Treaty.

In the past nuclear explosions in the atmosphere, orjust below the ground or water surace, were mostlydetected and identied via particulate radionuclidemonitoring. Under a CTBT one has to assume that apotential violator would try to avoid detection. Under

such evasive scenarios the most dicultradionuclides to contain are the noble gases as theydont stick to crack suraces or react with any othermaterials available. Based on characteristic radiation

and hal-lie there are our xenon isotopes that aremost suitable or verication: 3mXe, 33mXe, 33Xeand 35Xe. Thereore, one o the technologies toveriy the Treaty is the global environmentalmonitoring o these noble gases, as discussed in3.3.

I an event detected by one o the stations o theIMS (or by national technical means) raises concernsabout compliance with the basic obligations o theCTBT, an On Site Inspection (OSI) may be conductedto clariy whether a nuclear explosion has takenplace or not. Such an inspection could take placeonly ater entry into orce o the Treaty, and wouldrequire agreement by at least 30 o the 5 memberso the CTBTOs Executive Council. An inspectionarea o up to 000 square kilometres would besearched by a team o inspectors. The purpose o

an OSI would be to clariy whether a nuclearexplosion has been carried out in violation o theTreaty and to gather any inormation which mightassist in identiying the potential violator. The noblegases measured during an OSI are 37Ar andradioxenons, as described in 3.. and 3.3.

5.3. Fissile Material Cut-o Treaty

A Fissile Material Cut-o Treaty (FMCT) wouldstrengthen nuclear non-prolieration by adding abinding international commitment to existing

constraints on nuclear weapons-usable ssilematerial. It is proposed to negotiate such a treaty atthe Geneva based Conerence on Disarmament(CD), which would ban the production o ssilematerial or nuclear weapons or other nuclearexplosive devices. It would not apply to plutoniumand HEU or non-explosive purposes. It would alsonot apply to non-ssile materials, like tritium, and itwould not address existing stockpiles.

Monitoring 85Kr would be together with remotesensing methods a plausible verication techniqueor this possible FMCT [45].

6. Discussion and outlook

Techniques are available to measure the relevantnoble gas isotopes 37Ar, 85Kr, 3mXe, 33Xe, 33mXe,and 35Xe in laboratories at ultra-low level. For eacho these isotopes mobile sampling and measurementequipment has recently been developed. Currentlythe use o mobile equipment is studied and testedor the use o an onsite inspection under the CTBT,

but other applications are also possible. At presentthere is no mobile measurement technique availablethat measures all noble gas elements simultaneously;however, the collection and analysis o argon, xenon


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and krypton does have several things in common.Thereore we may see in the uture a measurementsystem that is capable to perorm thesemeasurements together.

The worldwide background or 37Ar and 85Kr is welldened. The background o radioxenon, however, is

not known accurately due to its regional variation. Itis still complicated to distinguish globally a legitimateradioxenon release rom a nuclear plant with thesignal rom a possible nuclear explosion. It dependsrom the source strength, the position o the station,the local background and the detection capability(MDC) o the measurement system used. A keyissue in nuclear explosion monitoring need to besolved: we have theoretically modelled the presenceo radioxenon in the world but the global backgroundhas not been veried with the measurements. This

makes the distinction between civil sources oradioxenon and a nuclear test dicult in many areas.Thereore, more background measurements have tobe perormed in regions where there are nuclearacilities but no radioactive noble gas data areavailable yet (e.g. South Arica, South Asia andPersian Gul region) to understand the absoluteactivity concentrations and the isotopic ratio atdierent places worldwide.

Technically, all global and national nuclear vericationnetworks could be joined together to learn global

backgrounds and to perorm verication. The use othe existing IMS radionuclide network or otherverication regimes like e.g. the WAES, seems to beobvious rom a nancial and scientic point o view.However, there are still political obstacles that donot allow dierent verication regimes to co-operatewith ull strength. The scientic community, however,can and should learn a lot rom each other, in orderto improve each system the best possible way, tomake the world a better place to live...

7. AcknowledgementsThe author would like to thank Pro. Martin Kalinowski(University o Hamburg), Dr. Lars-Erik De Geer(Swedish Deence Research Agency), Dr. MikaNikkinen (CTBTO) and Dr. Clemens Schlosser(German Federal Oce or Radiation Protection) orthe very interesting and ruitul discussions as wellas Dr. John Coyne (CTBTO) and Pro. Helmut Bck(Vienna University o Technology) or their permanentsupport. He urther would like to acknowledge allthe colleagues who have contributed to the

International Noble Gas Experiment (INGE) overmany years, as well rom the development site asthose operating the systems in the eld andmonitoring them at the headquarters in Vienna.

8. Disclaimer

The views expressed in this publication are those othe author and do not necessarily refect the viewso the Comprehensive Nuclear-Test-Ban TreatyOrganisation Preparatory Commission.

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Documents

Ultra-Low-Level Measurements of Argon, Kryptonand Radioxenon for Treaty Verification Purposes