XENON 1T Demonstrator Report

Nathan Holman

December 26, 2011

Abstract

Observation suggests ∼80% of all the mass of the universe currently comes from a nonluminous, nonbaryonicform of matter known as dark matter. Currently, WIMPs are one of the most favored models to explainthe missing mass of the universe. XENON100, a detector experiment in Gran Sasso, has place the bestupper limits for the cross section at ∼ 10−44cm2 for a 50GeV/c2 mass WIMP. Since detection of the WIMPremains elusive the next generation LXe experiment, XENON1T, will have a 1 tonne fiducial volume andhas a projected sensitivity to ∼ 10−47cm2 for a 50GeV/c2 mass WIMP. In order for the next generationexperiment to run efficiently and to gain the required level of purity the current gas system is inadequate.Work on the demonstrator apparatus has made significant progress towards the development of a highlyefficient gas system for XENON1T. Through the chacterization of the internal pressures, addition of a heatexchanger, new liquid-gas phase bell design and automated data analysis necessary groundwork has beenlaid out for the next generation LXe dark matter experiment.

Contents

1 Introduction 31.1 Rotational Velocity Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2 Bullet Cluster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Current Status of XENON100 72.1 XENON100 Detector Phenomonology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Limits on WIMP Cross-Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Demonstrator: A XENON1T Apparatus 103.1 Heat Exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.1 Simple Theoretical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2 Recirculation System Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Liquid-Gas Phase Bell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.3 Slow Control and Pressure Analysis Computer Programs . . . . . . . . . . . . . . . . . . . . . 133.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2

Chapter 1

Observational Motivation for DarkMatter Searches

Historical development of dark matter searchestakes us back to 1933 with the publication: ”Spec-tral displacement of extra galactic nebulae” by FritzZwicky [1, 2]. Zwicky was not a mainstream as-tronomer and many of his ideas (e.g. dark matter,neutron stars and supernovae) were not taken seri-ously at first but are now well established theories.It wasn’t until Vera Rubin’s [3] measurement of theflat rotational velocity curve of the M31 galaxy (SeeSection 1.1) that dark matter came into mainstreamscientific research. Evidence based on WMAP con-strained the matter density, Ωmh

, and the baryonicmatter density, Ωbh

to the current limits [4]:

Ωmh = 0.1358+0.0037

−0.0036 (1.1)

Ωbh = 0.02267+0.00058

−0.00059 (1.2)

These constraints imply that only16.7% of the mat-ter density in the universe comes from baryonic mat-ter (i.e. atoms and other hardrons). So what is theother 83.3% ? Leptons like the neutrino certainlycontribute as well as mesons but current constraintssuggest that the total mass of the neutrino mass-eigenstates is: Σmν ≤ 0.28eV [6]. From this theupper limit for the neutrino mass density, Ωνh

, canbe determined as [5]:

Ωνh =

Σmν

93.14eV∼ 0.0030 (1.3)

It follows then that neutrinos only contribute an ad-ditional 2.2% of the total mass density of the uni-

verse. The natural question becomes where is theother 81.1% of the mass density of the universe com-ing from? Two possible explanations for this appar-ent discrepancy are the following: either the currentunderstanding of gravity is incomplete or there ismuch more mass in galactic formations than is sug-gested by the electromagnetic radiation.

The following chapter will proceed as such: the firstsection will develop the observational phenomenonknown as the missing mass problem. Section twowill provide some evidence for why dark matter is afavorable explanation over modified gravity.

1.1 Rotational Velocity Curves

Based on existing frameworks there are two possibleexplanations for the flat rotational curves: dark mat-ter of Tensor-Vector-Scalar TeV eS Gravity. Bothexplainations attempt to reconcile observation andtheory either by altering gravitational physics or byadding missing gravitational mass. The measure-ment of rotational velocity curves of spiral galaxiesby Vera Rubin was the first compelling evidence fornew physics .

In a first-order approximation, a star rotatinggalaxy can be treated as a Newtonian System withtwo point masses: m1 and m2 where m1 is the massof the center of the galaxy and m2 is the mass of astar. We will assume m2 m1 for the time being.From classical mechanics the equation of motion for

3

a point particle in a gravitational potential is:

r(φ) =`

µGm1(1 + εcos(φ))(1.4)

Where ` is the angular momentum, µ is the reducedmass for the system, G is the gravitational constantand ε is the eccentricity of the orbit for a given semi-major axis of magnitude ’a’ where:

ε =`

µ

√1

aGm1(1.5)

Using Newton’s laws, it can be shown that the rota-tional velocity of a star in a Keplerian orbit is:

v(r) =

√Gm1

r(1.6)

Since the above model assumes a point mass for thetotal galactic mass m1 it causes the velocity to blowup to infinity as we take the limit r → 0 To improvethe accuracy of this model we can replace m1 withthe integral:

m1 =

∫ρ(r)dV (1.7)

Where ρ(r) is the mass density and dV is the vol-ume element. Consider the mass density ρ(r) can bedescribed as a constant density up to some criticalradius rc(e.g. the galactic bulge):

ρ(r) =

γ if 0 ≤ r ≤ rc0 if r > rc.

(1.8)

Using this one can show that when r < rc the gravi-tational potential, Φg(r) is:

Φg(r) = −Gm1

2r3c3r2c − r2 (1.9)

Where m1 is (using equations 1.7 and 1.8 and inte-grating over all space):

m1 = 4πγ

∫ rc

0

r2dr =4πγr3c

3(1.10)

Outside the galactic bulge, if we assume that the den-sity of matter is zero, the gravitational potential willonce again be the Newtonian potential:

Φg(r > rc) = −Gm1m2

r(1.11)

This is a result of Gauss’s law for gravity which statesthat for a spherically symmetric mass, the gravita-tional attraction for all external objects behaves asthough all the mass were concentrated in a singlepoint at the center.

Gauss’ Shell Theorem: A spherically symmetricbody will produce a gravitatonal attraction for allexternal objects as if the mass were concentrated ata point at the mass’ center.

Proof: From Gauss’ law for gravity we can writethe surface integral for the vector field g:∫

S

g · dS = −4πGm1 (1.12)

Let n be the unit normal vector to the sphericallysymmetric surface, S. We can then write:

g = g(r)n (1.13)

and: ∫S

g · dS =

∫S

g · ndS (1.14)

Combining these two and noting that n · n = 1 wenow have:∫

S

g · dS = g(r)

∫S

dS = g(r)4πr2 (1.15)

The 4πr2 comes from performing the surface integral,which for a spherically symmetric mass, will be thesurface area of a sphere with radius r. So it nowfollows that gravitational force is:

g(r) = −Gm1

r2(1.16)

The potential for this force is:

Φg(r) = −Gm1

r(1.17)

Which is what we were trying to prove. Q.E.D.Using the above theorem we can now write the

gravitational potnetial on a particle as the piecewisefunction:

Φg(r) =

−Gm1

2r3c3r2c − r2 if 0 ≤ r ≤ rc

−Gm1

r if r > rc.

(1.18)

4

As we can see in Figure 1.1 the velocity distributionno longer has the aphysical behavior of ν(r)→∞ asr → 0.

Figure 1.1: For a spherically symmetric distributionof mass the predicted rotational velocities are plotedas a function of distance from the radius. While in-side the uniform mass the rotational velocities riselinearly with a slope of Gm1

2r3cuntil the cutoff radius

rc. After which the velocity drops of as a function of1√r.

While this theoretical model is perfectly acceptableas a second order approximation in the Newtonianregime, it is at odds with observational data [3]. InFigure 1.2 [7] we can see that spiral galaxies haverelatively flat rotation curves for radii past the galac-tic center. There are two possible ways to resolvethis issue, either our understanding of gravity at longranges is inaccurate(TeVeS gravity), or the apparentlight halo does not account for all the mass (i.e. thereexists dark matter). Galaxy rotation curves in andof themselves are not enough to differentiate whichtheory is accurate as both can explain the apparentphenomenon.

Figure 1.2: Rotation curves of spiral galaxies using aCO-line telescope [7]. Here it is easy to see that therotation curves do not obey the Newtowian law v ∝1√r. Rather we see the rotational velocity for stars

outside the galactic bulge is approximately constant.

1.2 Bullet Cluster

The debate between TeVeS gravity and dark matteris unresolvable through the study of rotation curvesalone. Since both theories can describe the flat ro-tation curve effect other predictions must be probedin order to differentiate which theory is truly cor-rect. One such measure can come from the study ofcolliding galaxies. If TeVeS is true the gravitationallensing should be centered at the luminous portionof the galaxy as the effects of TeVeS only flatten therotation curve at sufficently large radial distances.On the other hand if dark matter is the more accu-rate picture then the gravtational lensing should oc-cur past the luminous center (assuming the collisionhappened long enough ago). This is a result fromthe property that dark matter is inert to the elec-tromagnetic interaction, and as such, feels virtuallyno frictional force. So when two clouds of dark mat-ter pass through each other they continue relativelyunimpeded, whereas luminous matter will slow downdue to friction with interstellar and galactic gases.

A bimodal weak lensing effect outside the luminouscenter was indeed observed in a cluster known as thebullet cluster in 2006 [8]. At the time the paperwas published ”any non-standard gravitational forcethat scales with baryonic mass” fails to reproduce the

5

observed lensing effects. Currently, there exist someTeVeS corrections that explain for this observed be-havior [9]. It is the opinion of the author of thispaper that given two theories it may be possible tomodify one theory to fit an observation but the the-ory that predicts an observation will be the strongerof the two since its accuracy comes from a priori con-siderations. As such dark matter can be considered,although not definitively, the more accurate theoryto date.

Figure 1.3: Mass contours extrapolated from weaklensing effects show two mass density peaks for thiscluster are located on opposite sides of the luminouscenter [8].

6

Chapter 2

Current Status of XENON100

XENON100 is a liquid noble gas dark matter ex-periment with a fiducial volume target of xenon. Cur-rently, XENON100 is collecting data but has releasedsome publications on the status of their search asthe most sensitive detector for WIMPs in the world.Using 100kg fiducial mass target, XENON100 em-ploys the expected scintillation and ionization froma WIMP nuclear recoil to characterize the expectedsignal and discriminate a significant amount of back-ground events.

2.1 XENON100 Detector Phe-nomonology

Direct Detection(DD) dark matter phenomenologyhas been well studied [10, 11]. In particular, thereare two modes of scattering in direct detection exper-iments: spin-dependent and spin-independent. It willbe the focus of this section to discuss the cross sec-tion for WIMPs based on theoretical considerationsand how XENON100 measures rare WIMP-nucleoninteractions. While spin-dependent interactions willbe considered they are less likely to be detected com-pared to spin-independent interactions [12, 13].

The rate of interaction of galactic dark matter, as-suming no lower energy threshold, can be written as:

R =∑i

Ninχ〈σiχν〉 (2.1)

where Ni are the number of nuclei for each species iin the detector, nχ is the number density of galacticdark matter and 〈σiχν〉 is the WIMP-nucleus cross

section averaged over the relative WIMP speed,ν, tothe detector. In the case of XENON we can furtherreduce this formulation by making the assumptionthat there is only one specie of nuclei in the detector(i.e. no impurities in the fiducial volume). Thenequation 2.1 becomes:

R ' Nnχ〈σχν〉 (2.2)

The total WIMP-nucleus elastic cross section can beformulated as [13]:

σχ =1

πν2

∫ 4µ2ν2

0

f2nA2XeF

2S(q2) + 4Λ2F 2(q2)f2adq2

(2.3)where µ is the center of mass reference frame, A2 isthe number of nucleons, q is the momentum trans-fer from the WIMP to the nucleus, fn is the nucleonform factor, fa is the axial current form factor, andF 2(q2) comes from the Fourier transform of the nu-cleon density, ρ(~r) written as:

F (q) =

∫ρ(~r)ei~q·~rd3~r (2.4)

F (q) is called the form factor and describes how co-herent scattering off a nucleus is affected by the geo-metric configuration of the nucleons in the nucleus.

If we look at the spin independent term in equation2.3, we can write the cross section as just:

σχ =1

πν2

∫ 4µ2ν2

0

f2nA2F 2

S(q2)dq2 (2.5)

7

Equation 2.5 shows that for coherent scattering, thecross section is proportional to the number of nu-cleons squared. So, heavy nuclei such as Xenon(A ' 131) and germanium (A ' 69) are naturalchoices for coherent scattering experiments.

Figure 2.1: A schematic diagram of electronic andnuclear recoils.

XENON100 uses a liquid-gas time projectionchamber (TPC) to characterize events inside the de-tector. There are two types of interactions that theXENON100 detector is sensitive to: electronic re-coils (ER) and nuclear recoils (NR). Electronic re-coils are the primary source of background noise inthe XENON100 detector but are well studied [14].Nuclear recoils can come from neutrons and are alsobelieved to come from coherent WIMPs scattering offthe nucleus. Both cases produce scintillation (calledS1) and ionization from which XENON100 extractsits’ data. An electromagnetic field is placed along thelength of the TPC causing ionized electrons that donot recombine with xenon to drift inside the cham-ber. When the electrons cross the liquid-gas phasea second burst of scintillation (called S2) occurs dueto an acceleration caused by a rapid change in themean free path. The ratio of S2 to S1 can be usedwith greater than 99% accuracy to discriminate be-tween electronic recoils and nuclear recoils [15]. Inparticular: (

S2

S1

)WIMP

(S2

S1

)γ

(2.6)

2.2 Limits on WIMP Cross-Section

Recently, there has been some discussion by CoGeNTfor a possible ”light WIMP” signal with a mass of∼ 7GeV/c2 [16]. Furthermore, they claim to haveobserved an annual modulation [17] with a 2.8σ sig-nificance. While this observation is a far cry fromdiscovery much work has been done to try and un-derstand these results. In particular, some theoristsalong with CoGeNT have argued that their resultcoincides with an earlier, even more dubious claimmade by the DAMA/LIBRA group [18]. Numeroustheoretical studies have been done to see if their re-sults are valid [19, 20, 21] but most have cast seriousdoubt based on a theoretical and statistical basis.

Figure 2.2: Current experimental limits for WIMPcross-section vs mass, each at the 90% CL [15, 16,18]. Note that for the XENON100 Curve the 1σ and2σ sensitivities are indicated. The grey regions arethe expected parameter space predicted by CMSSMis indicated at 68% (dark grey) and 95% (light grey)CL [23].

CoGeNT’s claims grow weaker with the inclusion ofother experiment’s results. In particular, XENON100and CDMSII have both placed limits on the cross sec-tion that effectively put Cogent’s result in the exclu-sion curve [15, 22]. XENON100 current reports anupper limit of the cross section for WIMP-Nucleoncross section of ∼ 10−44cm2 for a 50 GeV/c2. Fig-ure 2.2 shows the projected XENON100 cross sec-tion sensitivity. When XENON1T comes into opera-

8

tion the projected cross-section sensitivity will be to∼ 10−46cm2 for a 50GeV/c2 WIMP.

9

Chapter 3

Demonstrator: A XENON1TApparatus

Since the most recent results of XENON100’sWIMP search [15] have had null result it becomesimportant to look to the future generation WIMPsearches. For the XENON Collaboration this meanswork has begun on building XENON1T (where 1Trefers to the mass of the fiducial volume in kg).Recent development of an improved recirculation-purification system (Demonstrator) that is capableof recirculation rates of 100 SLPM has provided nu-merous insights into how XENON1T will operate, in-cluding which systems require further upgrades or in-vestigation. The following sections cover recent mod-els and results into the efficiency of a new, largerheat exchanger, Slow Control, and analysis codingused to understand the operation of the Demonstra-tor recirculation-purification system.

3.1 Heat Exchanger

In order to build an efficient recirculation-purificationsupport system for XENON1T optimization of thecooling of gaseous xenon from the Getter back intoliquid-phase is absolutely necessary to maintain highflow rates. The use of a commercial parallel plateheat exchanger was previously found to be 96

3.1.1 Simple Theoretical Model

The second law of thermodynamics, in one form, de-fines heat as the exchange of thermal energy acrossa temperature gradient. The most recognizable formof this law comes from basic chemistry: Q = mc∆T .This however poses a problem as heat exchangerswhich, in order to have more than a modest efficiencyrequires zero temperature difference. Specifically, atthe liquid-gas phase transition the heat exchangerwould have to have a heat flow with zero temperaturedifference.

Considering the latent and specific heat of xenon inaddition to the temperature of the gas coming fromthe Getter is 300K, the liquid-gas phase transitionis at 161K and the Pulse Tube Refrigerator (PTR)keeps liquid xenon at 161K in liquid phase only (e.g.no latent heat in the fluid). Using basic thermody-namics one can find for 1kg of xenon the latent heatrequired would be 93kJ whereas the heat required towarm the gas (where a non-zero temperature differ-ence could occur) would only require 23.6kJ. Giventhis factor of 5 difference in heating requirements theheat exchanger could at best have an efficiency of 20

At first glance heat exchange with a zero temper-ature gradient may appear to be a physical impossi-bility, as it would violate the second law of thermo-dynamics. This would be true, if both systems wereidentical in every way. Stepping back from the prob-lem for a moment if we consider the phase diagram for

10

phase d iagram of xenon

Input in terpre ta tion :

xenon phase d iagram

Phase d iagram :

gas

supercriticalregion

solid

liquid

sub limation cu rve

satu ration cu rve

melting cu rve

critical poin t

trip le poin tSTP

0 100 200 300 400 500 600104

103

102

101

1

10

10 2

103

temperature K

pre

ssureMPa

Genera ted by Wolfram |Alpha (www.wolfram alpha.com ) on Ju ly 18 , 2011 from Cham paign, IL.

© Wolfram Alpha LLC—A Wol fram Research Com pany1

Figure 3.1: Phase Diagram of Xenon

xenon (Figure 3.1). As the gas flows from the getterthe KNF diaphragm pump provides additional pres-sure. This means the xenon from the getter is at alocation higher on the coexistence curve than that ofthe xenon from the TPC column. As a result, heatflows between the two substances due to an entropicdifference resulting in condensation of the gas phasefrom the getter and evaporation of the liquid fromthe TPC column.

3.1.2 Recirculation System Charac-terization

XENON1T requires numerous modifications to thecurrent recirculation system and as a result a fullcharacterization of a the system is needed to ensureit operates as desired. Perhaps one of the most im-portant aspects of the system is the internal pressure.This is because if the pressure gets to high there arenumerous risks including: KNF diaphragm recircula-tion pump failure, venting of xenon due to burstingof the failsafe disk and explosion of a portion of therecirculation system. As such we measured the pres-sure drops across the getter, heat exchanger, massflow controller and KNF pump as shown in Figure3.3.

Figure 3.2: Measurements of the pressure dropsacross the KNF recirculation pump, getter, heat ex-changer and mass flow controller.

Notably, the pressure drop across the KNF di-aphragm pump is the largest of the four measuredsystems. Since the pump must perform work to pushthe xenon through the entire system it is reasonablethat it would create the highest-pressure drop dueto the impedance of flow through the pump itself.The next largest system impedance comes from thechemical getter; the remaining systems do not con-tribute significantly to resistance of flow. The demon-strator’s heat exchanger is designed to save coolingpower for the PTR and as such one can quantify thisthrough an efficiency measurement. To maintain anequilibrium temperature for the xenon the PTR mustexpend 10.6 W

SLPM . With the addition of a heat ex-changer, merely by measuring the power cost for thePTR can we gain an idea of the efficiency of the heatexchanger for saving cooling power. Using the em-pirically derived formula the efficiency, λ is:

λ = 1− ∆P

10.6(3.1)

Where ∆P is slope of the power versus flow rate

11

weighted defined as:

∆P =Pf − Pi

flowf − flowi(3.2)

Figure 3.3: Heat Exchanger Efficiency

The measurement of the efficiency shows that ithovers at a relatively constant ∼ 90% as seen in Fig-ure 3.3. Notably, the efficiency for larger flow ratesdoes begin to decline around 75 SLPM. This is mostlikely due to two limiting factors: an upper limit toheat exchange and radiative heating from the vacuumcontainer for the recirculation system. Since the en-tire system is in an insulation vacuum convective andcontact heating is negligible. Using Stefan-BoltzmanLaw we estimated a lower limit for the radiativeheating power to be 19.41 Watts. Adding super-insulation, a multilayered reflective material that re-flects thermal photons, is currently being added tothe system to minimize the radiative heating.

3.2 Liquid-Gas Phase Bell

Another important change from XENON100 toXENON1T is a major modification of the detector

chamber itself. Currently, the LXe detectors oper-ate on a liquid-gas phase time projection chamberto veto electronic and nuclear recoils from radioac-tive sources. In particular, XENON100 uses gas flowfrom the recirculation system to maintain a constantlevel of gas inside the chamber. Since XENON1T willmake use of a parallel plate heat exchanger the gasnormally used from recirculation will be convertedinto liquid and will no longer be available to stabilizethe liquid-gas TPC. To account for the change in therecirculation system’s design a new means of creat-ing a stable gas level inside the bell was developed.A small heater can be used to boil the LXe at the topof the bell thus creating a gas layer inside the bell.With this design we decouple the gas level from therecirculation rate allowing for independent control ofeach.

Figure 3.4: An overlay image of the Liquid-gas phasebell. Near the top of the interior of the bell sits a 25W(50Ω) which is used to boil off the liquid interior ofthe bell. A cylinder capacitor is used to determinethe level as a function of capacitance.

After designing and proper cleaning of the bell wetested it inside LXe in the demonstrator. During thistime we determined quickly that radiative heating ofthe flange at the bottom of the TPC chamber wasboiling LXe and causing the inside of the bell to be

12

pressurized. This was measured using a cylindricalcapacitor. Since the capacitance for any capacitordepends on the dielectric constant, which is phasedependent, we can determine the level inside the bellby measuring the capacitance. Specifically, we notethat liquids typically have a higher dielectric constantthan gases and as such when the bell is full of LXe thecapacitance is maximal. Using this knowledge onecan easily make an empirical formula for the heightas a function of capacitance. In Figure 3.5 we can seethe capacitance as a function of time decrease whenthe heater is off indicating that the bell is filling withgas due to radiative heating of the flange. Further-more, in liquid nitrogen testing we can see the heateras the slope steepens between∼77pF and ∼81.6pF.Since the heater takes up a significant fraction of thevolume the rate at which the gas level increases isfaster since there is less volume of liquid nitrogen todisplace.

Time

05:15 05:30 05:45 06:00 06:15 06:30 06:45 07:00

Le

ve

l

60

65

70

75

80

85

90

95

100

Heater Off Heater On

Figure 3.5: Plot of the level (capacitance in pF) ver-sus time of the liquid-gas phase bell in liquid nitro-gen. The first three trials were done with the heateroff. The other trials were with the heater at variouspowers.

In spite the instability (i.e. constant filling of gasinside the bell) of the liquid-gas bell work was doneto determine the filling rate as a function of heat-ing power. Averaging over several runs we were ableto determine, in liquid nitrogen, that the gas levelincreased (i.e. capacitance decreased) with a mag-

nitude of 1.61 pFmin . After the offset value was deter-

mined we began measuring the gas-filling rate as afunction of power the results of which can be seenin Figure 3.6. Notably, if we look to Figure 3.5 wecan see that the capacitance drops rapidly until itreaches the bottom of the heater and then rapidlydecelerates. This is due to the liquid nitrogen flashboiling off the heater due to direct contact with theheater. Once the gas level pushes below the heaterthe rate at which the capacitance decreases is not aresult from direct thermal contact but rather throughconvection of the nitrogen gas inside the bell.

Figure 3.6: Change in capacitance (i.e. gas fillingrate) versus heating power. A linear relationshipwas observed between the change in capacitance andheating power.

3.3 Slow Control and PressureAnalysis Computer Pro-grams

In order to maintain smooth operation of the demon-strator apparatus constant monitoring of variouspressures, temperatures, voltages, resistances, power

13

consumption and recirculation rates is vital. At highrecirculation rates the internal pressures of systemcan cause pump failure and possible explosion. Asa safety measure a break disk was installed that canwithstand ∼ 3.8 bars gauge until it vents the xenoninto the atmosphere. Since xenon is approximately$1000 per kg a system that contains ∼ 6.45kg asignificant financial loss can occur if system failureoccurs. In order to help monitor the system forovernight/weekend runs a warning system is in on-going development to monitor temperature fluctua-tions.

Time (s)0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Flo

w R

ate

(L

/s)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Graph

Figure 3.7: The collected flow rate data from arecouperation run plotted versus the time. Usingflow.cpp the total recoupered xenon was found to be1097.2 litres or ∼6.46 kg.

One important aspect of preventing system fail-ures is to accurately know how much xenon is putinside the system. A mass flow meter was placed aton the junction for output into the system and re-cuperation gas lines in order to know the fill rate instandard litres per minute (SLPM). Originally, thesewould be taken by hand every minute and used togain a rough estimate of the total volume input forthe system. This method was highly inconsistent asthe flow rate could fluctuate several SLPMs within agiven minute. As such a program called flow.cpp wasdeveloped to take computer-collected data placed in atext file and use Riemannian-sums integration to findthe total volume and mass of xenon placed in or recu-perated from the system. The following figure showsa plot of the flow rate versus time for a rocouperation

run from which we determined the mass of the xenoncurrently used in the system was ∼6.46kg.

3.4 Conclusions

The development of a new gas system for the nextupgrade of the XENON dark matter experiment hasseen notable development in the last few months.The internal pressures of the system, including theimpedances to flow have been recorded for recircula-tion rates up to 80SLPM. Furthermore, the additionof a heat exchanger has lowered the power require-ments of the PTR allowing for 90% efficiency in savedpower consumption. Higher recirculation rates havean observable drop in efficiency, which may be due toradiative heating or flow impedance across the heatexchanger. Inclusion of super-insulation is currentlyin place and the efficiency measurements of the newheat exchanger are ongoing. Automation of the massflow measurement along with plotting software forpower consumption was developed to speed up theanalysis process. Lastly, the development of an auto-mated warning system has had some progress and inongoing development and debugging.

14

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