Half-life measurements of lutetium-176 using undergroundHPGe-detectors

Mikael Hult a,n, Tim Vidmar b, Ulf Rosengård a, Gerd Marissens a,Guillaume Lutter a, Namik Sahin c

a European Commission, Joint Research Centre, Institute for Reference Materials and Measurements (IRMM), Retieseweg 111, B-2440 Geel, Belgiumb SCK �CEN, Boeretang 200, B-2400 Mol, Belgiumc TAEK-SANAEM, Ankara, Turkey

H I G H L I G H T S

� The half-life of 176Lu was determined using the sum-peak method.� Three different HPGe-detectors located 225 m underground were employed.� The average massic 176Lu-activity in the metallic Lu-foils was 52.6170.36 Bq g�1.� With an isotopic abundance of 2.5970.01%, the half-life is (3.72270.029)�1010a.

a r t i c l e i n f o

Available online 1 December 2013

Keywords:Sum-peak methodUnderground laboratoryLutetium-176

a b s t r a c t

The half-life of 176Lu was determined by measuring the 176Lu activity in metallic lutetium foils.Three different HPGe-detectors located 225 m underground were employed for the study. Measurements using the sum-peak method were performed and resulted in an average massic activity of(52.6170.36) Bq g�1. The foils were of natural isotopic abundance so using the massic activity andthe value of the natural isotopic abundance of (2.5970.01)%, a half-life of (3.72270.029)�1010a couldbe calculated.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Naturally occurring 176Lu decays by β� decay to 176Hf asdepicted in Fig. 1. With a half-life of �1010 years, this radioactivedecay provides an important isotopic clock for studying theevolution of the earth. Several studies concerning the age ofminerals and the earth itself rely on a precise value of the 176Ludecay constant, λ-176. However, numerous γ-counting studies haveyielded disparate values of the λ-176 decay constant, indicatingunresolved analytical problems in γ-counting techniques, and donot constrain the λ-176 decay constant with enough precision andaccuracy for geochronological applications. Furthermore 176Lu isan important radionuclide in stellar research as it can be used asan s-process thermometer (Heil et al., 2008).

Over the years, several measurements of the 176Lu half-life havebeen made utilising different methods, see Fig. 2. The first measure-ments were done in 1930s using beta-counting. Since then a majority

of the research groups have used γ-counting techniques. A firstestimate of the half-life was given by Heyden andWefelmeier (1938).The most recent measurement was performed at PTB using liquidscintillation counting (LSC) (Kossert et al., 2013). In the most recentevaluation (Chechev, 2011) the evaluated half-life is (3.7670.08)�1010a (relative uncertainty 2.1%), which is a too high uncertainty forcertain datings and astrophysical problems.

We note that in several of the previous studies certain factorshave not been considered like e.g. stability of the source, appro-priateness of a method, angular correlations of gamma-rays,impurities and cosmogenic background. In this paper we seek tomeasure the 176Lu half-life with a primary method, the sum-peakmethod, using high purity metallic samples instead of saltsor solutions. In addition we performed the measurements 225 munderground in order to avoid cosmogenic activation of thesamples and to obtain a background reduction with almost4 orders of magnitude compared to above ground. Furthermore,data were analysed using a conventional gamma-ray spectrometryapproach to make a consistency check.

Except for rare decays like double beta decay or rare decaybranches (Andreotti et al., 2011), underground laboratories havenot been used a lot for measuring decay data. In the case of 176Lu,

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0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apradiso.2013.11.071

n Corresponding author. Tel.: þ32 14 571 269; fax: þ32 14 584 273.E-mail address: mikael.hult@ec.europa.eu (M. Hult).

Applied Radiation and Isotopes 87 (2014) 112–117

this is the first underground measurement and there are severaladvantages when performing the measurements underground(Hult et al., 2006):

(1) For the sum-peak method to perform well, the source shouldbe as small and point like as possible. Since it is difficult(expensive) and introduces some drawbacks to obtain lute-tium enriched in 176Lu, the activity of a very small Lu-sampleof natural isotopic abundance must necessarily be very low.

(2) The sum-peak at 509 keV is important to include in thecalculations when using the sum-peak method. Above groundthe count rate in the 511-keV peak is significant and willinterfere with the 509 keV peak at least when dealing with thelow activities necessary for the sum-peak method.

(3) When measuring low activity samples, cosmogenic activationcan potentially be a problem. In case of these lutetiumsamples, we note that a potential problem may come from175Lu(n,γ)176Lum (T1/2¼3.6 h), which also produces an 88 keV

gamma-ray. This problem is solved by performing the mea-surement underground.

(4) The underground measurement is also able to deliver data onpossible gamma-ray emitting impurities in the sample andenables correction for this.

2. Materials and methods

2.1. Samples

Six samples composed of natural 176Lu were used. Five of thesamples were in the form of rectangular metallic foils with an areaof 25�25 mm2. The thicknesses of the five samples were 25, 50,100, 200 and 1000 μm. The sample with thickness 50 μm was at alater stage of this study cut into a circular foil with a diameter of10 mm. In this paper we refer to this sample as the “10 mm foil”.The chemical purity of the samples stated by the provider Good-fellow Ltd. was 99.9%. The purity of the samples was additionallychecked by the Jožef Stefan Institute in Ljubljana (Slovenia) usingk0-neutron activation analysis (k0-NAA) and X-ray FluorescenceAnalysis (XRF). The k0-NAA showed the presence of Mn (44 mg/kg),Th (191 mg/kg), Hf (23 mg/kg) and Ta (24,400 mg/kg). The XRFanalysis showed the presence of Fe (12,200 mg/kg), Ce (195 mg/kg),La (945 mg/kg), Th (218 mg/kg) and Ta (20,300 mg/kg). Combiningthese data leads to a purity by mass of (97.5070.29)%.

Lutetium is known to be hygroscopic. However, metallic lute-tium is quite stable. In the first gamma-ray measurements, asample was placed in a specially designed sealed Plexiglasss

container under a dry nitrogen atmosphere in order to avoidproblems caused by its hygroscopic nature. At the same time, testsin laboratory air of a sample were carried out. In this test the masswas measured and the count rate in a germanium detectorrecorded as a function of time. Initially data points were recordedevery 15 s for 5 min and after that with increasing time intervals.After 1 year of measurements there was no significant change inthese parameters. There was also no visible change in the samples.Therefore we focus our analysis using the sum-peak method inthis paper on the measurements carried out using metallicsamples placed directly on the endcap of HPGe-detectors withouthaving them inside a special container. All the thin samples

2+

0+

176Yb

82.13

082.13

7-176Lu

0

6+

2+

4+

0+176Hf

596.82

0

88.351

290.18

306.78 E2

201.83 E2

88.34 E2

8+ 997.74

99.66%0.34%

400.99 E2

1.4 ns

Fig. 1. The Lu-176 decay scheme. The EC-branch, which is indicated, has neverbeen detected although several attempts have been reported.

2.9

3.1

3.3

3.5

3.7

3.9

4.1

4.3

4.5

4.7

Heyden, 1938

Libby, 1939

Arnold, 1954

Dixon, 1954

Glover, 1957

McN

air, 1961

Donhoffer, 1964

Brinkm

an, 1965

Sakamoto, 1967

Prodi, 1969

Boudin, 1970

Kom

ura, 1972

Patchett, 1980

Norm

an, 1980

Sguinga, 1982

Sato, 1983

Gehrke, 1990

Dalm

asso, 1992

Nir-El, 1998

Scherer, 2001

Grinyer, 2003

Bizzaro, 2003

Nir-El, 2003

Luo, 2005

Am

elin, 2005

Am

elin, 2005

Albarede, 2006

Albarede, 2006

Kossert, 2013

This work

LSC

2.18

(0.0

6)

Hal

f-life

/ 10

10a

Gam

ma

coun

ting

2.10

(0.

2) Beta-counting

Gamma Sum-peak

Gamma-counting

LSC

Age comparison

“Radiogenic”

MCP-ICP-MS

Gamma-gamma coincidence

Fig. 2. Overview of 176Lu half-life values reported in literature. The horizontal grey lines show the evaluation by Chechev (2011). The uncertainties are reported as standarduncertainties (1s).

M. Hult et al. / Applied Radiation and Isotopes 87 (2014) 112–117 113

(25, 50, 100 and 200 μm) were taken underground immediatelyafter delivery in October 2011. The 1 mm thick sample wasoccasionally used above ground but always kept inside a sealedPlexiglasss container.

2.2. Experimental

The activity measurements were performed in the 225 m deepunderground laboratory HADES (Andreotti et al., 2010). Comparedto ground level, the muon flux in HADES is reduced by a factor of5000 and the neutron flux reduction is of the same order ofmagnitude. This makes environmental activation of the samplesan issue that can be neglected. Furthermore, the backgroundcounting rate is reduced by about 4 orders of magnitude and theimportant background peak at 511 keV has a count rate of about1 count per day in the detectors that were employed here. Threedetectors were employed in this study, Ge-3 (Manufacturer:Eurisys; Relative efficiency: 60%; crystal configuration: coaxial),Ge-4 (Canberra, 106%; coaxial so-called XtRa) and Ge-8 (Canberra,19%; planar, so-called BEGe). They all have an endcap of highpurity aluminium but otherwise are quite different, a diversitywhich we considered important for this study. Ge-4 and Ge-8 havesubmicron top deadlayers and almost no bulletization, whilethe top deadlayer of Ge-3 is about 0.85 mm and the radius ofthe bulletization about 7.2 mm. In addition to the 225 m over-burden, the detectors are shielded by about 15 cm of old lead and10–15 cm of freshly produced electrolytic copper.

The samples were placed directly on the endcap of thedetectors. Measurements were also made with two samples ofthe same size placed on top of each other. The results (in Bq/g)were consistent with the results from measurements of a singlesample. Therefore, by combining several samples the followingsample thicknesses (in μm) could also be studied: 75 (25þ50), 125(100þ25), 150 (100þ50), 175 (100þ50þ25). It was found impor-tant that the samples overlapped exactly or else it would influencethe results. It such cases a small piece of Teflon was placed on topof the samples in order to better keep them in place.

Data were collected using the Genie2000 system and theinherent deadtime correction was used. The highest deadtimewas recorded for the thickest sample (1 mm) on detector Ge4 andwas 0.54%.

In order to study the influence of X-rays on the measurements,a 1 mm thick absorber made of electrolytic copper was used forsome measurements. This copper absorber had been kept under-ground for at least 5 years prior to the measurements to minimisethe contribution of activation products.

2.3. Gamma-ray spectrometry

At first, the data were evaluated using a conventional gamma-ray spectrometry approach. The experimental equipment men-tioned in Section 2.2 was employed. The samples were measuredboth on the endcap and sitting on a special sample holder 6.3,7.3 and 12.3 cm (depending on detector) above the detectorendcap. Efficiency curves based on point sources were made andthen efficiency transfer and coincidence summing corrections(including the X-rays) were calculated using the Monte Carlocode EGS4.

2.4. Sum-peak method

Given that the samples were very chemically pure and con-tained only 1 radionuclide (except for small amounts of impu-rities), the data collected as described above could be evaluatedusing a primary technique called the sum-peak method. The sum-peak method for activity standardisation is applied to point

sources measured in a close geometry, which not only improvesthe statistics of the sum-peak area, but also eliminates or reducesthe need to take into account the angular correlation effects.It is, nevertheless, desirable to be able to standardise extendedsources by means of the sum-peak method as well. Examples aremeasurements aimed at the half-life determination of 176Lu byGehrke et al., (1990), and more recently the method was appliedfor standardisation of extended sources of Co-60 (Vidmar et al., 2009).For radionuclides with two cascading gamma-ray, like 60Co, thesum-peak formula is

A¼N1N2=N12þT ð1Þwhere A is the activity of the source, N1 and N2 are the count ratesin the two gamma-ray peaks, N12 is the count rate in their sumpeak, and T is the total count rate in the spectrum. To derive theformula for the calculations of 176Lu activity, a simplified decayscheme of the 176Lu decay was adopted with only the three majorgamma-ray lines (88, 202 and 307 keV) present and followingeach other in a triple cascade with a 100% feeding of the upper-most level. The values of the total conversion coefficients werekept unchanged, but no emission of X-rays was considered. It caneasily be shown that in this case the usual Brinkman formula(Brinkman et al., 1963; Brinkman and Aten Jr., 1963, 1965) appliesto any of the three possible pairs of gamma rays. Since we aredealing with three different pairs of gamma-rays, their respectiveformulae can be combined to yield an expression for the activitywhich uses the information contained in all the main peaks in thespectrum at once

A¼ ½ðN1N2N3Þ2=ðN12N23N13Þ�1=3þT ð2Þhere A is the activity of the source, N1, N2 and N3 are the countrates in the three gamma-ray peaks, N12, N23 and N13 are the countrates in their respective sum peak, and T is the total count rate inthe spectrum.

The 176Lu sources in this study cannot be considered simplypoint sources and the effects of the spatial variation of the full-energy peak and the total efficiency across each sample's volumecannot be ignored. This makes, strictly speaking, the originalformulae by Brinkman invalid, as demonstrated by Sutherlandand Buchanan (1967), who analysed measurements of extendedsources 125I of various sizes. This is why we decided to applya correction factor to the Brinkman formula. It is determinedwith the help of Monte Carlo calculations. The advantage of thisapproach is that the experimental procedure and the formularemain, apart from the correction factor, in their usual form andthat good accuracy can be obtained.

To take into account the effects of (i) the finite size of thesample, (ii) the self-absorption effects, (iii) the angular correlationsbetween the gamma rays and (iv) the emission of X-rays duringthe decay of the parent nucleus, as well as (v) the presence of thefourth gamma ray and the level associated with it in the real decayscheme, the formula was modified by introducing a correctionfactor C such that

A¼ Cð½ðN1N2N3Þ2=ðN12N23N13Þ�1=3þTÞ ð3ÞThe correction factor was derived fromMonte Carlo simulations, inwhich the complete decay scheme of 176Lu, as shown in Fig. 1, wassimulated, along with a realistic model of the detector and thesample. The GEANT 3.21 package (Brun et al., 1987) was used forthis purpose. The relative statistical uncertainties of the calculatedcorrection factors were kept lower than 0.1% on average. In such asimulation, the activity A is known and the simulated spectrumcan be analysed to yield the peak count rates and the total countrate and with them the value of C. The correction factors haveproven very close to unity in all cases except for the thickest,1 mm, sample and robust with respect to the imperfections of the

M. Hult et al. / Applied Radiation and Isotopes 87 (2014) 112–117114

detector and sample models, which also justifies the use of thesum-peak method in this particular case. This robustness can bewitnessed from the fact that even for two completely differentdetector models, Ge3 and Ge4, the two related correction factors Cfor a given gamma-ray pair do not differ by more than 1% (except,again, for the thickest, 1 mm foil).

Triple angular correlations between the emitted gamma raysshould have in principle been taken into account in the simulation,but the related expressions proved exceedingly complicated andcostly to compute, so three pair-wise correlations were introducedinstead. These turn out to have exactly the same form as those ofCo-60 and its well-known associated double cascade (Siegbahn,1955). Before the application of the individual correction factors inthe calculations, the regularity of their dependence on the foilthickness was exploited to smooth it, as depicted in Fig. 3. Theaverage relative difference between a data point and the fittedcurves in Fig. 3 amounts to 0.01%.

To study the robustness of the simulation method in some detailand to arrive at a reliable assessment of the uncertainty associatedwith the calculated correction factor C, the case of the 0.025 mmthick 176Lu foil measured on detector Ge3 without an absorber wasstudied by varying the most important parameters of the detectorand sample model and recording the resulting changes in thecorrection factor (a so-called sensitivity analysis). Radiographs ofthe detector were available, so most of the Ge-crystal parameterswere known with very good precision. In particular, the estimateduncertainties of the diameter and length of the detector crystal,respectively, were so small that their contribution to the uncertaintyof the correction factor was negligible. The same is true of thebulletization radius of the crystal. The size of the gap between thecrystal and the detector window, on the other hand, could not beestimated with equal precision, because the window was slightlydepressed in the middle. The resulting contribution to the uncer-tainty of the correction factors amounts to 0.2%. For the top deadlayerthickness we assumed an uncertainty of 0.15 mm, while the defaultvalue was 0.85 mm. Still, no significant effect on the value of thecorrection factor could be observed. The largest relative uncertaintyof all the parameters was the one associated with the foil thickness,reported by the manufacturer to be 20%, but this one as welltranslated into an insignificant uncertainty of the final result. Finally,the effect of the angular correlations was completely switched off inthe simulation to assess its importance. The resulting relative changein the value of the correction factor was 0.3%. This figure was ofcourse not included into the uncertainty budget, but it does provide ameasure of the robustness of the calculations.

Based on these simulation results we believe that it is fair toestimate the relative uncertainty of the calculated correctionfactors at no more than 0.3% by combining their statisticaluncertainty with the contributions from the sensitivity analysis.Variations in the absorber thickness were not studied since it wasassumed that it was well defined and since effects would besimilar to, but less pronounced than, variations in the thickness ofthe top deadlayer.

No attempt was made to study the switching-off of theemission of X-rays in the simulation, either. However, it can safelybe concluded that the effect of excluding them would have beenvery limited, since a very small difference is observed between thecorrection factor values obtained with and without the thickcopper absorber for a given detector-sample configuration. Therestricted impact of the X-rays can also be linked to their some-what limited abundance and to the fact that the most abundantX-ray emitted comes from a converted 88 keV transition and hasthe energy of 57 keV. The detector efficiency, especially for adetector with a thin top deadlayer, does not vary rapidly in thisenergy region, and so detecting the 88 keV gamma-ray or an X-rayresults in similar summing-out effects in coincidence with othergamma rays. The other two transitions are much less convertedthan the 88 keV one.

3. Results

Fig. 4 shows a spectrum collected using the thinnest sample(25 μm) placed on the smallest detector (Ge-8). As a contrast, Fig. 4also shows the thickest sample (1 mm) with a 1 mm Cu absorbermeasured on the detector with the thickest deadlayer (Ge-3). Thethree main Lu-176 gamma-rays (plus the 401 keV line) and theirmajor sum-peaks are indicated in the spectra. There is a markeddifference between the spectra caused by the presence of X-raysand associated sum-peaks in the spectrum from Ge-8. TheTh-impurity is easy to detect via 228Ac and 208Tl and some of thepeaks associated with the 232Th decay chain are indicated inthe plot. Above 1194 keV (the total β� decay energy of 176Lu) thereshould be no contribution in the spectrum from Lu-176 exceptfor pile-up, which is negligible with the low count rates usedhere. Fig. 5 shows the background spectrum of Ge3 together withtwo lutetium measurements with Ge3 in the region around theannihilation peak. In addition, the background spectrum of a low-background detector located above ground is shown. It highlightsthe usefulness of the underground measurement when measuringsmall amounts of lutetium.

1.010

1.005Ge8

1.000 Ge3

0.995

0.990

0.985

0.9800.15 0.200.00 0.05 0.10 0.25

Sample thickness / mm

Cor

rect

ion

fact

or

Ge4

Fig. 3. Correction factor for Eq. (3) as function of sample thickness. The curves arepolynomial fits to the data including the 1 mm thick sample (outside the range ofthis figure).

0

1

10

100

1000

10000

100000

0 200 400 600 800 1000 1200

Cou

nt ra

te (k

eV-1

d-1)

Energy (keV)

307

202

88509

401

290

395

Background

X-rays 202+X307+X

509+X

88+X

ThTh

ThTh

597

1 mm Lu on Ge3 with 1 mm Cu absorber

0.025 mm Lu on Ge8

0.1

Fig. 4. Gamma-ray spectra in HADES of two lutetium samples (25�25 mm2) andthe background. The numbers in the plot are the energies in keV. Some of the peaksassociated with the decay chain of 232Th are indicated with “Th”.

M. Hult et al. / Applied Radiation and Isotopes 87 (2014) 112–117 115

For the 7 different thicknesses 25, 50, 75 (25þ50), 100, 125(100þ25), 150 (100þ50) and 200 μm, the massic activities (in Bq/gof lutetium) obtained from applying the formula (3) agreed nicelyand no trend could be discerned. However in our continued analyseswe exclude the “stacked” samples (75, 125 and 150 μm) as wenoticed that it is quite difficult to place the two foils exactly on topof each other. We also exclude the 1 mm sample from the followinganalysis in order to keep the correction factor very close to unity. Forseveral of the foils the measurements were carried out on all threedetectors using the 1 mm thick copper absorber between the sampleand the detector. The results with and without absorber agreed witheach other. In the following analysis the absorber measurements arenot included because the counting statistics were poorer.

The standard deviation of the massic activity results for the4 remaining foils (25, 50, 100 and 200 μm) measured withoutabsorber was 0.14%, 0.18% and 0.34% for Ge3, Ge4 and Ge8,respectively. Furthermore the massic activity values obtained bycalculating a mean value for each detector showed good agree-ment. Ge-3: (52.28770.236) Bq/g, Ge-4: (52.60470.236) Bq/gand Ge-8 (52.62370.236) Bq/g. In addition, a longer measure-ment (16 days) of a circular foil of 10 mm diameter and 50 μmthick (mass: 35.2 mg) was carried out on Ge-3. The massic activityobtained was (52.95570.310) Bq/g. Table 1 gives the uncertaintybudget for the massic activity from a single measurement. Due tocorrelations, the uncertainty of the counting statistics as displayedfor a typical measurement in Table 1, was simply that of the peakwith the lowest number of counts. A final value for the massicactivity was obtained by calculating a weighted mean value of thethree values for the big foils and the one value of the small foil.In the weighted mean only the uncorrelated quantities like the

counting statistics were included. After performing the weighting,the uncertainties of the correction factor (0.3%) and the purity(0.3%) were added in quadrature with the uncertainty of theweighted mean. In addition, the standard deviation between the4 values (0.5%) was added in quadrature. The value that wasobtained in this way was (52.60870.356) Bq/g (relative uncer-tainty: 0.68%).

This value can be converted into a half-life, T1/2, assuming thatthe number of 176Lu atoms in the sample is known. We assume allsamples were of natural isotopic abundance. In their paper from2013, Kossert et al. report a new measurement of this value, θ witha very low uncertainty, (2.5970.01)%, which we chose to use forthe calculation using the following equation:

T1=2 ¼NAθ lnð2Þ

aMAð4Þ

where a (Bq/g) is the massic activity, MA is the atomic massof 176Lu, 174.967(1) g mol�1, and NA is the Avogadro number,6.02214179(30)�1023 mol�1. Using this formula and the defini-tion of a year being 365.2422 days, the half-life becomes(3.72270.029)�1010a. The main uncertainty components wereisotopic abundance, 0.4%, sample-purity, 0.3%, correction factor,0.3%, and standard deviation of measurements, 0.5%.

As mentioned above, we did evaluate the data collected in thisstudy using the normal procedure for gamma-ray spectrometry.At IRMM, this procedure normally results in a precision of about3% for well-defined samples, which has been confirmed throughparticipation in several proficiency testing schemes. However, inthe case of these Lu-samples the spread in result is about 3.4%which indicates some systematic errors that are not fully under-stood. This could be due, for example, to inadequate coincidencesumming corrections, inappropriate transfer of efficiency from apoint source or that certain decay data parameters are not knownprecisely enough.

The only gamma-ray emitting impurities found were those inthe 232Th decay chain. The Th-concentration in these samples wasabout 200 mg/kg. There were no serious interferences betweenpeaks from the Th-decay chain and the 176Lu-peaks of interest.However, as the total count rate is included in Eq. (3) the totalnumber of counts in the spectrum was determined by performingsimulations of the 232Th-decay chain using EGS4 and normalisingthe results to the observed Th-peaks. For Ge4 this resulted in atotal count rate from Th-impurities per gram of Lu-sample ofabout 0.25 s�1 g�1. Since the total count rate from Lu-samplesis in the order of 25 s�1 g�1, the effect was marginal but wascorrected for in all samples.

Although 176mLu has a short half-life (3.6 h) and would only beof concern above ground, we also considered the cosmogenicactivation as a possible source of error for above ground measure-ments. The 175Lu(n,γ)176mLu reaction has both a high thermal crosssection (26 b) and a resonance integral (646 b). However, in theenvironmental neutron fluxes expected in most above-groundlaboratories this would only have a very limited impact. Of mainconcern is the contribution to the 88 keV peak, but it results ina count rate almost five orders of magnitude lower than thatfor 176Lu.

Seeing that tantalumwas a major impurity (around 2.2%) it wasimportant also to check the contribution from the 181Ta(n,γ)182Tareaction which also has a high thermal cross section (21 b) andresonance integral (739 b). Again, the calculations show that innormal environmental neutron fluxes (above ground) the effectcan be neglected. However care should be taken when performingsimilar measurements in areas close to neutron sources. In thisstudy no peaks from 182Ta were detected in any spectra that werecollected.

Fig. 5. Background spectra and spectra from lutetium measurements in the regionaround the sum-peak at 509 keV and the annihilation peak at 511 keV.

Table 1Uncertainty budget for one single measurement on Ge-3 of the massic activity.

Entity Relative standarduncertainty (%)

Counting statistics 0.58Total count rate including extrapolation to zeroenergy 0.046

Background subtraction o0.01Subtraction of contribution fromradioimpurities 0.01

Sample mass o0.01Sample purity 0.31Live time o0.01Correction for angular correlation and self-attenuation 0.31

Total 0.73

M. Hult et al. / Applied Radiation and Isotopes 87 (2014) 112–117116

Finally the following reactions were considered: 176Lu(n,γ)177Lu,175Lu(n,p)175Yb, 175Lu(n,α)172Tm, 175Lu(n,2n)174Lu. It was found thatin a normal above-ground environmental neutron field theycontribute even less than the previously mentioned two reactions.

4. Discussion

The many different measurements of the 176Lu half-life madewith various different methods constitute an interesting data set,which could be analysed further to better understand shortcom-ings and benefits of specific methods. Beta counting techniquesthat are based exclusively on the detection of the negatrons arequite sensitive to the sample composition. Liquid ScintillationCounting is a beta counting technique but it includes also detec-tion of other types of radiation and has the advantage of being a4π-technique. The drawback is that it is sensitive to possibleimpurities in the cocktail and that it relies on good knowledge ofdecay parameters like the branching ratio. Several other techni-ques also suffer from problems due to unaccounted impurities.In this study the purity of the samples was less than what themanufacturer claimed. It could possibly have been the case insome previous measurements as well. The sum-peak methodseems to be very robust and well suited to the 176Lu decay. Thedata in this study were also analysed using Eq. (1) with all the3 combinations of paired peaks and including a correction factorcalculated analogue to the correction factor in Eq. (3). The resultsagreed well with the results of Eq. (3). Furthermore the triplecoincidence peak (88þ202þ307 keV) was also used and again aresult similar to the other values was obtained. This study alsoshows the shortcoming of conventional gamma-ray spectrometrywhen it comes to accuracy below a per cent. A possible reason fordiscrepant results using gamma-ray spectrometry is underesti-mated uncertainties for coincidence summing corrections (Lépyet al., 2012).

It is of interest to use even smaller sized foils to reach as point-like samples as possible. Our study shows that it can possibly bedone using samples of thicknesses up to 200 μm, which isnecessary to obtain a reasonable count rate. In undergroundlaboratories it is in principle possible to have very long measure-ment times since the background is so low, but it is difficultto occupy a detector for as long a time as would be necessary tomeasure, say, a 3 mm diameter sample of 50 μm, which wouldrequire half a year on detector Ge3. However, bigger detectorsexist, where the efficiency is doubled and the muon background isreduced further using an active shield. Another alternative thatcould be investigated is to use the underground well-detector thatwas recently installed in HADES.

5. Conclusions

The relative simplicity of the sum-peak method is quite beautifulandmakes the results robust. This has been shown by obtaining good

agreement (better than 0.5%) from measurements of differentsamples on different detectors. Assuming that the samples in thisstudy were of natural isotopic abundance, the combined final valuefor the 176Lu half-life is (3.72270.029)�1010a.

Acknowledgements

The work done by EURIDICE and the HADES team of SCK CEN inMol, Belgium, is gratefully acknowledged. Thanks to RadojkoJaćimović and Peter Kump at JSI in Ljubljana for k0-NAA and XRFanalyses.

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