CsI(Tl)-photodiode detectors for spectroscopy at low radiation levels

Nuclear Instruments and Methods in Physics Research A329 (1993) 433-439 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

Csl(Tl)- photodiode detectors for spectroscopy at low radiation levels Rainer Kotthaus

Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, P. O. Box 40 12 12, Munich, Germany

Received 23 November 1992 Some aspects of CsI(TI)-photodiode detectors relevant for applications in low level radiation spectroscopy in the MeV energy range have been studied with a view to a sensitive search for the lepton number violating neutrinoless double beta decay of °° Mo . Energy resolutions of slightly less than 100 keV (FWHM), limited by electronic noise, have been obtained for crystal masses up to more than 3 kg . The improved energy resolution facilitates a very sensitive investigation of crystal contaminations with nuclides of the Th and U chains by spectroscopic analysis of a-decays. The cleanest crystal was free of U and Th contaminations at the detection limit of 0 .27 a-decays per kg and h, a radiopurity well sufficient for applications in low level spectroscopy. 1. Introduction Recently, there has been an increasing need for the detection of very low radiation levels. Prominent examples are spectroscopic analysis of trace impurities, searches for very rare nuclear processes, like the double beta (pp) decay, dark matter searches, as well as the detection of solar neutrinos. In a previous study [1] we had investigated the applicability of large CsI(Tl)-photodiode detectors for spectroscopic measurements at very low radiation levels. The specific interest was to study the feasibility of a search for the neutrinoless pp-decay of 1°°Mo, which requires extreme background suppression at energies around the decay energy of 1°0Mo, i .e . 3 MeV . This comparatively high energy release implies substantial advantages of '°° Mo over other pp-decay candidate isotopes both for decay rate and background discrimination . The conventional 2vßß-decay of 1°°Mo has been observed in three different experiments [2-4] at half-lives around 10 19 years . Present best lower half-life limits on the lepton number violating Ov mode are a few times 10 21 yr [2,5]. In this paper results of more sensitive background studies are presented. Improvements are primarily due to enhanced energy resolution of kg size detectors and reduced cosmic ray induced backgrounds. The energy resolution has been improved substantially compared to our previous measurements by optimizing the photodiode readout and by applying more refined crystal surface treatments . Cosmic ray background reduction was achieved by moving the experiment to an underground site shielded by a minimal overburden of 42 kg/cm2 of rock. Backgrounds caused by crystal con-

taminations with natural radioactivity from the U and Th decay chains were studied for various large CsI(T1) crystals from different manufacturers by probing for a-activity . This very sensitive method to study radiopurity by searching for monochromatic lines from asources immersed in the detector material was facilitated by the relatively strong contamination of some crystals with ...Ra and its a-active progeny which served very useful calibration purposes . In section 2 we briefly describe the experimental setup. For details we refer to ref. [1]. In section 3 optimization and measurements of energy resolution for -y-rays are discussed. Results on crystal radioactivities are presented in section 4. In section 5 the residual background spectrum around 3 MeV, the region of interest for '°° Mo pp-decay, is discussed. In section 6 we summarize our results and draw conclusions .

2. Experimental setup and procedures Thallium doped CsI-crystals from different manufacturers [6] were used as scintillators . Most measurements were done with cylindrical crystals (diameter: 7.5 cm, height : 5 .0 cm, mass : 1 kg) and a truncated square pyramid (cross section: 5 x 5 to 4 x 4 cm', height : 34 cm, mass: 3.13 kg). A special geometry ("open well" detector) was a hollow cylinder (bore diameter : 2.5 cm, mass : 0.89 kg). TI concentrations are in the order of 10 -3 (molar fraction). In this range dopant variations have little effect on scintillation light yields [7]. The scintillation light was detected with large (active area : 18 x 18 mm 2) PIN silicon photodiodes (PD)

0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B .V . All rights reserved

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[8] directly attached to the flat crystal front face using optical couplant [9]. The PD are made of 300 g.m thick wafers of high resistivity Si and combine small junction capacitance per unit area (43 pF/cm2 at full depletion) with low dark current (typically : 6 nA at T = 25°C) and minimal series resistance . These PD characteristics are the key to large signal to noise ratios and lead to improved energy resolution of kg size detectors (see section 3) . Substantial care was taken to optimize the light collection efficiency and uniformity [1]. Crystal surfaces were polished (usually with A1 203 powder immersed in ethylene glycol) and wrapped with several layers of white teflon tape [10] for diffuse light reflection . For mechanical protection and electrical shielding the detectors were housed in thin-walled Al cans or Cu containers . We used standard commercial nuclear spectroscopy electronics consisting of a low noise charge sensitive preamplifier, a main amplifier with adjustable shaping time and an ADC operated in peak sensing mode . The PD bias voltage V,, was set by minimizing the electronic noise as measured by the width of a precision charge signal from a test pulse generator injected into the preamplifier via a calibration capacitor. Optimal noise reduction was typically obtained at Vb = 60 V. Signal to noise ratios were maximal for unipolar pulse shaping with one common differentiating and integrating time constant of 2 or 3 ws . The amplifier contribution to the electronic noise was about 350 e - Urns). For absolute charge calibration the PD was directly used as semiconductor detector for low energy -y-rays from 241Am and 57CO sources. The known energy loss per electron-hole pair in Si (3 .62 eV) allows to assign amounts of deposited charge to the photopeaks observed in the ADC spectra. For photon energy calibration of the scintillator-PD assembly we used photopeaks from standard -y-calibration sources or y-ray backgrounds (4° K, 2°8T1, 214130 . Scintillation light yields for a-particles depend on their specific energy loss dE/dx ("Birks law") [111 and are much smaller than for 1'-rays (typically L,IL y = 60 to 70%) . To calibrate the detector response to a-particles we used inherent a-activity from nuclides of the Th decay chain which was observed in some of our crystals (see section 4). The procedure is described in detail in ref. [1]. Background measurements were carried out at a surface laboratory at the MPI and at an underground location in a salt mine [12] covered with a minimal overburden of 42 kg/cm2 of rock. The flux of penetrating cosmic rays at the underground site is reduced by an approximate factor of 10 3 compared to sea level. Ambient -y-activity due to 4°K and the Th and U descendents (as measured by the 214Bi (1764) and 208Th (2614) photopeaks) is higher at the salt mine than at the surface laboratory by factors between 3 (U

chain) and 7 (4°K) #1 . This ambient y-ray background was shielded against by 20 cm of ordinary lead not specially selected for radiopurity . Radon contained in the mine air leads to typical radiation levels of up to 500 Bq/m 3 depending on the mine ventilation [14] . To keep Rn from penetrating the detector shield, the outside of the Pb brick-wall was hermetically sealed with plastic foil . 3. Energy resolution for 1'-rays The energy resolution obtained with kg size CsI-PD detectors in the MeV range is dominated by electronic noise, as analyzed in ref. [1]. In this case the fractional energy resolution T/Ey varies as Ey 1 and, to some crude approximation, should merely be determined by the crystal surface area and not depend on the total sensitive area of the PD used to detect the scintillation light ("Groom's Theorem" [15]) #2 . The assumption behind this theorem is that both signal and noise are proportional to the total PD area . This is not exactly true . The total noise, though dominated by the PD capacitance, i.e . the active area, is offset by the amplifier noise and the signal, i.e . the fraction of scintillation light converted to photoelectrons, is affected in a nonlinear way by multiple reflexions at the crystal surface and by absorption losses . We studied the influence of such effects on the energy resolution by equipping the same crystal (1 kg cylinder) in turn with 1, - - -, 5 PD and by measuring total noise, photoelectron yield and energy resolution for each detector configuration . The result is shown in fig. 1. The noise increases approximately linearly with the number of PD but shows a significant offset (360 e -). The photoelectron yield (Ne/keV) increases less steeply than linearly as a result of the combined effects of light reflection and absorption at the crystal surface and in the passive part of the PD surface (ceramic substrate) . The resulting energy resolution as measured by the FWHM T898 of the "Y photopeak at 898 keV shows a shallow optimum for 2 PD . The measurements with different PD arrangements implied frequent reassembling of the detector and were thus done with nonoptimum surface conditions . After a careful preparation of the crystal surface and the diffuse light reflector the optimum resolution of T898 = 95 keV was obThe natural radioactivity due to 4°K, 232Th and 238U at the salt mine is quantitatively similar to radiation levels measured at the Gran Sasso Underground Laboratory in Italy [131. #2 Options to circumvent the limitations formulated in "Groom's Theorem" are to enhance light collection efficiencies with fluorescent flux concentrators [16,17] or to use nearly capacitance-less drift PD [18] . a1

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Fig. 1 . Measurements of electronic noise, photoelectron yield and energy resolution for a 1 kg Csl(Tl) cylinder equipped in turn with 1, - - -, 5 photodiodes. The energy resolution is measured by the FWHM x898 of the 88 Y photopeak at 898 keV. The arrow marks the amplifier noise contribution (360 e rms). The square symbol represents the final resolution (T898 = 95 keV) obtained after carefully optimizing the crystal surface and the diffuse light reflector. tained (square symbol in fig. 1) . Fig. 2 shows the fractional energy precision I'/E, in the range 300
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EY [keV] Fig. 2. Energy dependence of the fractional resolution as measured by the FWHM T of photopeaks observed for various y-calibration sources. The insert shows the spectrum measured for 60 Co. The broken line is a fit of the data to a power law (T/E Y = 9.94%-EY [MeVI-° 91).

4. Radioactive crystal impurities To understand and to suppress inherent detector radioactivity is of utmost importance for low level spectroscopy . Whereas there are well developed techniques to shield a low level counting facility against ambient radioactivity and cosmic ray related background, inherent activity caused by radioactive contaminants or by'activation of source and detector materials will unavoidably lead to background in the measured energy spectra and thus - in the region of interest has to be suppressed as much as possible by material selection and proper fabrication procedures . In previous studies [1] we had found contaminations of CsI(Tl) crystals with radionuclides of the Th chain as identified by monochromatic lines due to sequential (228 Th, 224R a 22°Rn, 216po and 212Bi) a-decays ("a" in fig. 3a). In addition, 212Bi [3-decay (branching fraction : 64 .6%) leads to a characteristic continuum ("a + R" in fig. 3a) caused by pileup with the a-decay of the short-lived daughter 21 2 p o (half-life : 300 ns), which is not time resolved . While the measured radioactivity is too high for low level counting applications the immersed a-source inside the contaminated crystals served very conveniently to calibrate the crystal response to a-particles (see section 2 and ref. [1]) . acalibration of scintillators with external sources is notoriously difficult because "dead", i.e . nonscintillating surface layers of sometimes substantial thickness (several p,m) may cause variable and usually unknown energy losses (see ref. [1] and references therein) . We have extended our measurements [1] of inherent Csl(TI) a-activity with improved sensitivity to crystals from different manufacturers . Fig. 3 shows examples of background energy spectra taken with unshielded detectors. In all cases prominent photopeaks (4° K(146I), 214 Bi(1764, 2204), 208TI(2614)) due to ambient y-activity are seen . In addition, the crystal used in fig. 3a shows internal a- and aß-activity from Th chain nuclides which is not seen in spectra (b) and (c) taken with a 3.13 kg crystal (truncated square pyramid) from a different supplier [20] . In particular, spectrum (c), taken at the salt mine, allows one to probe very stringent upper limits on Th and U impurities using the a-calibration established with the resolved a-lines in spectrum (a). Background levels in the a-search region (3 .5 < EY < 5 MeV) of spectra taken in the salt mine are typically 80 counts/keV kg yr. The upper limit on the total a-activity from Th chain elements as given by a 3o- background fluctuation is 0.27 counts/kg h. This

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Fig. 4. Time dependence of a-activity from Th chain nuclides for two different CsI(TO crystals. The broken lines show the expected time dependence for contaminating the crystal mate rials with 228Ra in some incidence four years before the measurements were started.

Fig. 3. Background energy spectra measured with unshielded Csl(Tl)-PD detectors. (a) 1 kg cylinder, (b) and (c) 3 .13 kg truncated pyramid (crystal from a different manufacturer) . Spectra (a) and (b) were taken at the MPI, (c) was measured at the salt mine (for the structures observed see text) . translates into an upper limit on the 228 Th concentration of 5 .7 X 10-22 g/g #3 which is more than three orders of magnitude below the levels measured for various 1 kg cylinders [1]. Similar limits apply for U chain radionuclides as evaluated from the nonobservation of 22'Ra and 214po a-lines. The fact, that in the Th contaminated 1 kg crystals 232Th, the long-lived parent nuclide of the Th sequence, is not seen, poses interesting questions concerning the provenance of the observed radionuclides . #3 Here it is assumed that 228Th is in transient equilibrium with its progeny. This is true in cases, where Th a-activity is seen (fig . 3a). The equivalent limit on 232Th assuming secular equilibrium of the entire decay chain, which is definitely not the case in fig. 3a, would be 4.2X 10 -12 g/g .

In principle, they may have entered the crystal materials as a fragment of the Th decay chain (fractured either at the ß-emitter 22sRa or at 228 Th) or may have built up after initial contaminations with 228Ra (T112 = 5.6 yr) or 228Th (T112 = 1.91 yr). The various possibilities can be differentiated by measuring the time dependence of the a-activity (fig . 4). The only hypothesis consistent with the almost constant activity observed over a period of two years is the buildup of daughter activity after initial contamination of the detector materials with 228Ra . The measurements are best fit by curves expected for a contamination incidence approximately four years before the measurements started (in accord with the purchasing date). 5. Backgrounds in the region of interest around 3 MeV The 3.13 kg CsI(TI) crystal free of detectable Th or U traces, was used to probe residual backgrounds in the region around 3 MeV, the decay energy of 1°°Mo. These measurements were done at the salt mine with the crystal inside a box made of low radioactivity OFHC Cu (wall thickness: 1 mm) and the whole detector, i.e . CsI(Tl) crystal with two attached PD and preamplifier, surrounded by a tight, hermetic shield of 20 cm of Pb . At the outside the shield was sealed with plastic foil to keep Rn contained in the mine air from penetrating the Pb brick-wall . A spectrum covering the energy range from 2 to 4 MeV measured in a 250 h exposure is shown in fig. 5. A weak residual 208 TI photopeak at 2614 keV is seen . The spectrum below is dominated by residual ambient 1'-activity. Above the TI peak the background spectrum falls off very gradually. Compared to spectra taken with the unshielded detector (see fig. 3c) the 208TI(2614) line is suppressed by

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Fig. 5. 250 h measurement of residual background in the salt mine (overburden > 42 kg/cm2) taken with a shielded 3.13 kg Csl(TI) crystal equipped with two photodiodes . The weak, but statistically significant 2 ° 8T1(2614) photopeak signals the presence of Th containing materials inside the detector shield of 20 cm of ordinary lead . The background level around 3 MeV, the decay energy of 1°°Mo, is 20 counts/keV kg yr . about 10 3. The residual activity is mostly due to Th containing materials inside the Pb shield close to (but not inside) the Csl(TI) crystal. Candidate sources are the PD . According to ref. [21] PD of the type used for these measurements show Th activity at a level of 1 pCi per nuclide of the Th chain. This would explain more than half of the observed 208T1(2614) signal . The continuous background in the region of interest for 1OOMo 00-decay around 3 MeV is 20 counts/keV kg y. Contributions in this energy range are expected both from the Th and U chains, in particular from 0yand yy-coincidences from 2°8 T1 (Qa = 4.99 MeV) and 214Bi (Qa = 3.27 MeV) decays . Cosmic ray induced background is suppressed by the overburden of the mine location to a level much below residual ambient radioactivity of materials inside the detector shield . 6. Summary and conclusions Some aspects of CsI(Tl)-PD scintillation detectors relevant for applications in low level spectroscopy, notably Ov 00-decay of "Mo, have been studied with crystals of masses up to more than 3 kg . By use of large area (18 x 18 mm 2) low capacitance (43 pF/cm2 at full depletion) Si PIN PD and by carefully optimizing the scintillation light collection an absolute energy resolution of slightly less than 100 keV has been obtained nearly energy independent in the range above 300 keV. This precision is sufficient for any 00-experiment with sources external to the detector as it matches the energy straggling even in very thin sources. The use of two PD per scintillator optimizes the signal to noise ratio for all crystal masses (0 .89 to 3.13 kg) which were

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used . The resolution is limited by electronic noise. Further improvement would require a smaller PD capacitance per unit active area . A promising route in this direction is the development of nearly capacitanceless drift photodiodes [18] . Less effective is the choice of thicker Si wafers #4 implying larger bulk currents per unit area [1]. To reduce junction capacitance by choice of thicker PD (up to a few mm) without increasing bulk currents correspondingly would require semiinsulating diode materials, e.g . HgI2 with a band gap of 2.13 eV. The spectral sensitivity of HgI2 is well matched to the emission spectrum of CsI(Tl) [22]. -y-energy resolutions measured with a small Csl(Tl)-HgI 2 detector [23] are competitive with Si PD results. Another option to further improve the signal to noise ratio is to use Si avalanche PD which provide internal charge multiplication by typical factors of a few hundred. Initial results on -y-spectroscopy with small Csl(Tl) scintillators read out by 1 cm 2 avalanche PD [24] are promising, but do not yet improve upon results obtained with conventional Si PIN PD . The good energy resolution facilitated an extremely sensitive search for trace contaminations of the CsI(Tl) crystals with natural radioactivity from the Th and U sequences by probing for monochromatic lines from a-sources immersed in the crystal. The method was helped by the availability of crystals contaminated with Th chain radionuclides to a level such that a precise calibration of the energy response to a-particles was possible without having to rely on external a-sources [1]. Five Th descendent radionuclides (228Th, 224Ra, 220Rn, 216p o, 212Bi) were clearly identified by their a-decays (for details see ref. [1]) . The characteristic a/0 branching of 212Bi is also seen . The superposition of the 212Bi 0-decay and the a-decay of the short-lived daughter 212 Po leads to a continuum above the 212po

a-energy. The observed nuclides are in equilibrium with each other, but not in secular equilibrium with the long-lived parent nuclide 232Th, which is not seen at a detection limit about three orders of magnitude below the observed daughter activity . From monitoring the time dependence of the a-activity over a period of two years we have strong evidence that the crystal material must have been contaminated with 228Ra at an incidence about four years before the measurements started. The cleanest crystal (3 .13 kg truncated pyramid) did not show any Th or U trace contaminants at the 3o, sensitivity limit of 0.27 a-decays/kg h. This limit corresponds to 228Th concentrations of less than 5 .7 x 10 -22 g/g, equivalent to 390 228Th nuclei/mole of CsI. This #4

The PD type used in these studies is also available as a 500 Wm thick version (Hamamatsu type : S 3204-05, C = 25 pF/cm2 at full depletion) .

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search for inherent a-activity is much more sensitive (by a factor of 3500) than a recent study of trace impurities of Th or U descendents in CSI(TI) by probing for y-activity with high resolution Ge detectors [21] . From our a-activity measurements we conclude that CsI(Tl) crystals can be grown free of Th and U contaminations at levels required for applications in low radioactivity measurements . The limit on the 228Th concentration measured for our 3 kg crystal can e.g . be converted to an effective half-life detection limit Tij2 in an experiment on toOMo pp-decay : too CMo ) ~228 Th ~ ÉRR BR-1 Tij2 = T1/2 ~ (asTh NN bg where: N = number of respective nuclei, BR = fraction of 228Th decays leading to background in the region of

interest, ERP(Ebg) = detection efficiency for pp-decays (background events). For a realistic experiment N( to0 Mo)/N(Csl) > 5% . BR is difficult to quantify. From the observed a-line intensity of contaminated crystals (fig . 3a) we estimate BR < 10 -3 . We conclude Ti~2 [yr] > 1 .2 X 10 23 (E pß /E bg )~' where the efficiency ratio depends on details of the experiment, but will certainly be substantially larger than unity . Crystal contamination will thus limit a search for to0Mo pp-decay at a level which is at least two orders of magnitude beyond the best existing limits [2,5]. The residual background in the region of interest for neutrinoless pp-decay of to'Mo (2.9 to 3.1 MeV) so far could be suppressed to 20 counts/keV kg yr and is due to ambient y- and possibly py-activity from materials inside the detector shielding but, as concluded from the absence of a-activity, outside the CsI(Tl) crystal. Good candidate sources are the PD, known to show U and to a lesser extent Th activity [18]. Also Rn penetrating the plastic lining of the detector shield and the shielding material itself (ordinary Pb, not selected for radiopurity) could contribute . Energy depositions above the 208T1 photopeak at 2614 keV are mostly due to coincidence events ( ,y-y-, py-, o y-, apy-coincidences). Therefore pattern recognition by measuring e.g . separate energy depositions and delayed coincidences with a segmented multicrystal detector is a powerful method to discriminate between pp-decays and various backgrounds leading to the same total energy . Pattern recognition techniques have proved to be very effective for background rejection in recent and ongoing experiments on "Mo pp-decay [3,4,2527]. Extrapolating from such experience, it is entirely conceivable to reduce the presently purely spectroscopically analyzed background to a level well below 1 count/keV kg yr, which represents the state of the art in the best ongoing experiments on 76 Ge [28] . A seg-

mented CsI(Ti)-PD spectrometer of several 100 kg of detector material certainly has the potential to improve the best existing limits [2,5] on the neutrinoless pp-decay of to0 Mo substantially. Of crucial importance would be the availability of several kg of enriched radiopure too Mo. Acknowledgements I wish to thank Prof. G. Buschhorn for encouragement, support and valuable suggestions, Dr . E. Lorenz for discussions and the loan of equipment, and Dipl . Phys . U. Kilgus for his careful reading of the manuscript . The generous support of the measurements in the salt mine by the Salzbergwerk Berchtesgaden is gratefully acknowledged . In particular, I like to thank Dr . P. Ambatiello and Dipl . Geol . S. Kellerbauer for their hospitality and continuous efforts. References [1] U. Kilgus, R. Kotthaus and E. Lange, Nucl. Instr. and Meth . A297 (1990) 425. [2] H. Ejiri et al ., Phys . Lett . B258 (1991) 17 . [3] S.R . Elliott et al ., J. Phys . G17 (1991) S145 . [4] A.A. Klimenko et al ., Proc . 24th Rencontre de Moriond, January, 1989, Les Arcs, Savoie, France . [5] M. Alston-Garniost et al., Phys . Rev. Lett . 63 (1989) 1671 . [6] BDH Chemicals, Ltd. (England), Quartz et Silice (France) . [7] B.C. Grabmaier, IEEE Trans. Nucl . Sci. NS-31 (1984) 372. [8] Hamamatsu Coorp. (Japan), type : S 3204-03. [9] Dow Corning (USA), type : Q2-3067. [10] PTFE Gaflon, supplied by: Plastic Omnium GmbH (Germany). [11] J.B . Birks, The Theory and Practice of Scintillation Counting (Pergamon, Oxford, 1964). [12] Salzbergwerk Berchtesgaden, D-8240 Berchtesgaden, Germany. [13] C. Arpesella, S. Latorre and P.P . Sverzellati, Report LNGS-92/35 (1992) (unpublished). [14] Measurements made in 1989 by Dr . R. Fritsche (private communication by Dip] . Geol . S. Kellerbauer, Salzbergwerk Berchtesgaden). [15] D.E . Groom, Nucl. Instr. and Meth . 219 (1984) 141. [16] E. Aker et al., Nucl . Instr. and Meth . A321 (1992) 64 . [17] M. Suffert, Report CERN-PPE/91-222 (1991) (unpublished). [18] B. Avset et al., Nucl . Instr. and Meth . A288 (1990) 131 . [19] S. Gunji et al ., Nucl . Instr. and Meth . A295 (1990) 400. [20] Quartz et Silice (France) . [21] N. Kamikubota et al ., KEK Preprint 90-149 (1990) (unpublished) and private communication by N. Kamikubota. [22] J .M . Markakis, IEEE Trans . Nucl . Sci. NS-32 (1985) 559. [23] J.M . Markakis, IEEE Trans. Nucl . Sci. NS-35 (1988) 356.

R. Kotthaus / Photodiode detectors at low radiation levels [24] R. Farrell et al ., Nucl . Instr. and Meth . A288 (1990) 137. [25] M. Alston-Garnjost et al ., Nucl . Instr. and Meth . A271 (1988) 475. [26] H. Ejin et al ., Nucl. Instr. and Meth . A302 (1991) 304. [27] D. Dassie et al ., Nucl . Instr. and Meth . A309 (1991) 465 and contribution to 12th Moriond Workshop on Massive

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Neutrinos, Jan. 25-Feb . 01, 1992, Les Arcs, Savoie, France . [28] Proceedings of 14th Europhysics Conference on Nuclear Physics: Rare Nuclear Decays and Fundamental Processes, 1990, Bratislava, Czechoslovakia .