Nuclear Physics B (Proc. Suppl.) 28A (1992) 425-429North-Holland

1. INTRODUCTION

Double beta decays are among the rarest pro-

cesses to be looked for since one expects only few

events in several months of running time . More-

over, the total energy available, Q,3d, ranges in the

1 to 3 MeV domain, i.e . in the energy domain of

the natural or man-made radioactivities. There-

fore, when located in underground laboratories, the

background components of double beta decay ex-

periments come mainly from the natural U and Thseries, and eventually from some radioactive con-

taminants like 'K, s°Co, '3'Cs, etc. . . . As a conse-

quence, any experimental set-up has to be design

only with ultra low radioactivity materials. For ex-

ample, the Bordeaux-Caen-Kiev-Orsay-Strasbourg

collaboration is designing a large wire chamber for

the tracking of electrons, associated to plastic scin-

tillators for electron energy measurements [1] . For

this detector to be performant, along term program

of material selection and control using the -y-ray de-

tection technique has been initiated since about 3

years in the Fréjus Underground Laboratory.

In parallel to this strong 2/3 activity, the lab-

oratory is Iso involved in other fields cf research

in which the background conditions require the use

of low activity materials. This include dark mat-

ter experiments (with scintillators or bolometers),

neutrino oscillation experiments and researches in

oceanography, environmental sciences or medecine

wid industrial applications .

0920-5632/'92/$05.00 0 1992 - Elsevier Science Publishers B.V

All rights reserved .

NUCLE,r,F. FHY~

SUPPLEMENTS®CEE I

ULTRA LOW RADIOACTIVITY MEASUREMENTS IN THE FREJUS UNDERGROUND LABORATORY

J. BITST02 , D. DASSIE', F. HUBERT' , Ph . HUBERT', M.C . ISAAC', C. IZAC', S. JüLLIAN2 ,F. LECCIA', P. MENNRATH'

1-Centre d'Etudes Nucléaires, IN2P3-CNRS et Université de Bordeaux, F-33170 Gradignan, France .2-Laboratoire de l'accélérateur Linéaire, IN2P3-CNRS et Université de Paris-Sud, F-91405 Orsay, France .

Using the performances of the low background Ge spectrometer, a long term program of low activity materialselection and control is being carried out in the Fréjus Underground Laboratory. Gamma-activities aremeasured with sensitivity down to 0.1 dpm/kg . Applications involve physics and astrophysics research, oceanand environmental sciences and industrial developments .

2. THE LOW BACKGROUND Ge DETECTOR.

As well known, there are mainly two origins for

the background . One is due to the natural ra-

dioactive chains, 238 U, 235U and 232Th, mid to '"K.The other comes from man-made mdioactivities,

mainly '3'Cs and 'Co and cosmogenic radioact .iv-

ities. Most of these radioactive isotopes are -)-rayemitters and so the use of very performant Ge spec-

trometers appears to be the appropriate technique

for material selection. Main advantages of this tech-

nique are : i)easy identification of the -y-ray emit-

ter due to the very good energy resolution, ii )non

destructive measurement and test of large amount

of material when available, iii) in only one mea-

surement, obtention of the non-equilibrium ratios

between the isotopes of a natural chain.

The main disadvantage is the rather low effi-

ciency, 1%.

For ultra low radioactivity measurements with

Ge spectrometers several conditions must be fid-

filled :

1. Location in an underground laboratory to

suppress any effect of the cosmic rays ( moons, neu-

trons and cosinogenic activities) .

2. Cryostat built. in a special configuration and

only from highly selected materials.

3. Heavy passive shielding, generally realized

with lead and OFHC copper, and airtight. to mini-

mize any effect of the radon gaz coming from out-

side .

426

4. No free space available between the detector,the shielding and the tested sample .

5. When feasible, use of a 47r or "Marinelli" ge-ometry of the sample, to increase the (efficiencyweight) factor or the sensitivity.

6 . High stability of the T!nalogic electronics forlong term measurements.

Following the above criteria, a prototype lowbackground Ge spectrometer, built in collaborationwith the Enertec-Intertechnique company, has beeninstalled in the Frijus Underground Laboratory. Itconsists of a 100 cm' high purity Ge crystal of type"p" . The choice of a type "p" crystal instead ofa type "n" has been done in order to mininuzethe effect of the "'Pb contamination of the elec-trical contacts, and so the background level up to5.3 MeV ;2j . It has, however, the disadvantage tostrongly reduce the y efficiency at low energy.

The cryostat, endcaps of the crystal and dif-ferent mechanical parts holding the assembly, weremade of a very pure Al-(4%)Si alloy, specially re-alized by the Pechiney company, with less than0.3 ppt (10-lzg/g) of U and Th isotopes . No -y-activities were detectable in this alloy correspond-ing to the Ra, Th and TI isotopes of the naturalchains, to the sensitivity level of 0.1 dpm/kg (de-cay per minute and per kilogramnne) . Moreover,no -y-activities were found for the most common ra-dioisotopes, namely 13'Cs (<0.1 dpm/kg), 'Co (<0.1 dpin/kg) and 'h (< 2.0 dpm/kg), and no cos-anogenic activities have yet been observed .

The cryostat has a J-type geometry, and the pre-auplifier is set outside the shielding (see fig . 1) .The first stage of the pre-amplifier, made with se-lected FEZ' and resistor and a homemade capacitor,is set inside the cryostat, 10 cm away from the Gecrystal . This has the advantage of keeping on thegood energy resolution of standard Ge detectors .The passive shielding consisted of 18 cm of OFHCcopper inside 15 cm of ordinary lead .

J. Busto et al. / Ultra-low radioactivity measurements

Figure 1 : Schematic drawing of the Ge detec-tor inside its shielding . A free space of 100 cin3 isavailable for samples in the "standard" geometry .For a "Marinelli" geometry the inside Cu shieldingis slightly modified .

Special cares are taken to avoid a possible con-tamination by the surrounding radon : no free spaceis left between the detector and the shielding, and aclosed plastic sheet surrounds the lead shielding . Inaddition, in the Fréjus Underground Laboratory ,a strong air ventilation keeps a constant. low Rnactivity of 50 Bq/m3 . Within these experimen-tal conditions, the background counting rate is 10counts/hour for energies > 30 keV .

The electronics consist. of a currently availablespectroscopic amplifier and a 8192 channel ADC.The energy calibration, adjusted with standardsources to 0.4 keV/channel, covers the energy range30 keV up to 3.5 MeV . The energy resolution(FWHM) is 1.8 keV on the 1332 keV line of 'Co,and reaches only 1.9 keV after 3 months of runningtime. This stability of the electronics is mainly aconsequence of the constant temperature ( :zz23°C)and the low hygrometric degree (==50%u) in the un-

derground laboratory .

Figure 2: Typical background spectrum recorded in 335h ofcounting rates (counts/hour) for some -y-ray lines .

Figure 2 shows a typical background spectrumobtained after 335 h . The remaining activities from212Pb, 214 Bi and 4°Ii are probably from the vi-ton vacuum O-ring and the first stage of the pre-amplifier, while the 13'Cs activity is presumed tobe a fallout contamination from Chernobyl . Workis presently being done to still reduce this level ofbackground and to improve the microphonic noiseat low energy (< 30 keV)

3. MEASUREMENT PROCEDURE ANDSENSITIVITY .For the different measurements, the samples are

generally directly placed on the top of the endcap ofthe detector . If a large amount of material is avail-able, the "Marinelli" geometry is preferred, sinceit leads to a better sensitivity. In all cases, spe-cial cares must. be taken during the measurementto remove the activity of the Rn gaz and its long-life daughters (x1apb TI/2 =27 m, 212 Pb T1 / x =10.611 ) which can be adsorbed on every surfaces, orhave been enclosed in the sample. Consequentlythe counting rates are periodically checked and themeasurement starts only when these counting ratesbecome constant . Most of the time, the first twoclays of the data taking are not used in the anal-ysis . For each measurement, the -y efficiencies arecalculated with a Monte-Carlo code using the exactgeometrical configuration and density of the sam-ple . Typical sensitivities are 1 dpm/kg in the stan-

J. Busto etaL J Ultra-low radioactivity measurements

ENERGY,keV

427

running time . Numbers in parenthesis give the

dard geometry and 0.1 dpm/kg in the "Marinelli"geometry, in few days cf running.

4 . RESULTSMore than 300 different samples have been mea-

sured until now, either for underground experi-ments, including low background Ge detector de-velopments, or for some environmental controls,lnedecine and industrial applications . These sam-ples are of many various types: metals, seluicon-ductors, insulators, scintillators, electronic and PNItube components, epoxy glue, et c . . . Table 1 givesthe radionuclide concentrations in some of the mostcommon materials used in low background experi-mental set-ups .

From our experience, we can say that at. thepresent sensitivity level of detection, very few mate-rials are really exempt of radiopurities . For a givenmaterial, the level of radiopurities may vary fromone company to another and even inside the samecompany. For example, all types of OFHC copperhave less than 1 dpm/kg in natural and man-madeactivities, and so, copper is largely used for detectorcomponents or for shieldings . However it. generallycontains non negligeable cosmogenic activities, suchaS s4Mn, ""'"Co, induced by energetic neutronswhen the sample is above ground . Brodzinski et

a1 . [3] have shown that this cosmogenic activity canstill be eliminated by electro-forming the differentcopper hardware components. A second example

428

is with the delrin material, generally considered asan insulator free of radioactivity. However in a re-cent measurement we observed slight contamina-tions in 13'Cs (0.012 dpm/kg), 137Cs (0.06 dpm/kg)and 'Co (0.04 dpm/kg). As a last example, wefound, within a lot of 50 FET transistors, oneof them and only one, highly contaminated with'Sr(T 1 1 2=65d. ) . Until now we have no any defini-tive explanation for this peculiar case, unless a ra-dioactive flag is used for counting the FET unitsduring production . All these example show thatno absolute recipie can be established in the choiceof the low radioactive materials needed in the lowbackground experiments . It is highly desirable tocontrol all used materials with the performant lowbackground Ge detectors now commercially avail-able .

!. Bustoet al /que-low radioactivùy measuremems

Table 1 : Radionuclide concentrations in some of the most common materials used in low background experi-ments . Activities are given in dpm/kg.

* Capacitors without ceramics, made with epoxy resins and metal plates .

If radiopure materials needed for experimentsare not available, it is necessary to proceed to aspecial development, and then to control the dif-ferent steps of this development . To illustrate thispoint, we will use the development carried out. incollaboration with the Climax company (Colorado,USA), of a 50p. natural foil of nlolybdenium, to beused in a 2/3 decay tracking experiment. [1] . Thestarting point is a very pure ammonium dinlolyb-date (ADM) powder which is first transformed bysome chemical process into a metallic Mo powder,then sintered into a metallic Mo bar which is thenrolled down into _foils . Fig . 3 shows the low energyparts of the -y-ray spectra registered after the fast.steps. The ADM (fig . 3a) and M6 (fig . 3b) pow-ders contain the same levels of impurities, namely 1dpm/kg in 214Pb or 214Bi and 0.1 dpm/kg in 211 pb.

Materials 214Bi 2UTl 137Cs 'K 'CoAl+(4%)Si alloy < 0.2 < 0.1 < 0.1 < 1.5 < 0.1OFHC copper < 0.3 <0.12 <0.2 <3 <0.2brass (screws) < 0.2 <0.1 <0.1 <3 <0.2titanium (screws) < 0.4 <0.4 <2 <4 <3stainless steel < 0.2 <0.1 <0.2 <3 2iron < 0.3 <0.1 <0.2 <4 <2nickel < 2 <1 <2 <40 <4lead < 0.2 <0.1 <0.1 <2 <0.1teflon < 0.5 <0.2 <0.2 <4 <0.3altuglas < 0.4 < 0.2 < 0.2 < 5 < 0.3delrin < 0.02 < 0.02 0.06 < 0.3 0.04nylon < 0.5 <0.2 <0.3 <6 <0.2epoxy < 1 <0.8 <1 <40 <2scint . NE110 2 < 0.4 < 0.4 < 10 < 0.4CsI (Na) < 2 <0.5 260 <20 <0.5Nal (TI) 20 2 3 2000 6Si(monocryst.) < 1 <0.3 <0.5 <10 <0.8saphir < 2 <0.8 <1 <20 <1.5resistors 50 20 3 800 < 1capacitors' < 1 <0.4 <1 <10 <2

This clearly demonstrates that the chemical processused for the transformation of the ADM powder tothe Mo powder does not added any extra pollution .On the contrary, the metallic Mo bar sample spec-trum (fig . 3c) shows rather strong 7-ray lines at63keV (234 Th), 93 keV ("Th), and 186 keV ("'U),clearly indicating an uranium pollution . A carefulanalysis of the intensities of the different lines, es-pecially of the [A(63keV) / A(93keV)I, allowed usto suspect a surface contamination which probablyoccurred when preparing the sample . We then pro-ceeded to a simple ultrasonic cleaning process ofthe Mo bars and recorded the spectrum shown infig.3d . As expected, the low energy uranium lineshave been supressed .

To conclude, it appears clearly that the use oflow background Ge spectrometers are essential inthe realization of low background experiments . Forthe Fréjus Underground Laboratory, we plan for thenear future to get into operation a larger volume(400 cm') detector. We will continue our effort todecrease the background rates and hope to siiii getan order of magnitude in the sensitivity level, i.e .,down to 10-2 dpm/kg, in a "Marinelli" geometryand in few days of statistics .

ACKNOWLEDGEMENTSThe authors would like to thank A. Guiral and

the Fréjus Underground Laboratory staff for theirtechnical assistance . They are also grateful to theIntertechnique Engineering and especially M. Berstfor their effort in construction of the low back-ground Ge detector . Discussions with J.L . Reyssare gratefully acknowledged.

REFERENCES[1] D . Dassi6 et al ., Nucl . Instr . and Meth . A,

in print, and D. Blum et al ., this volume .[2J Ph . Hubert et al., Nucl . Instr . and Meth . A252

(1986) 87.[3] R. L . Brodzinski et al ., Nucl . Instr. and Meth.

A292 (1990) 337 .

!. Busto et aL l Ultra-low radioactivity measurements

IA"IIiI I III I .

80 120 160 200 240 280

0

-40 80 120 160 200 240 280 320 360 400

ENERGY,keV

Figure 3 : .y-ray spectra recorded at three differ-ent steps during the realization of a 50p natural

Mo foil. a) Starting material, ammonium dimolyb-

date (ADM), running time t= 275h . b) Metallic

Mo powder, t_ = 262h. c) Metallic Mo bar sam-

ple (after sintering), t = 458h. d) Metallic Mo bar

sample after decontamination, t = 353h .