the status of gallex

Nuclear Physics B (Proc. Suppl.) 19 (1991) 77-83 North-Holland

77

THE STATUS OF GALLEX

Till KIRSTEN

Max-Planck-Institut fiir Kernphysik, P.O. Box 103 980, D-6900 Heidelberg

for the GALLEX Collaboration *)

A radiochemical Gallium-Solar-Neutrino Detector has been set up at the Gra~ Sasso Underground Laboratory. . All. . ma'or installations, are implemented and the target, 53.5 m ~ of salliumchloride. solution contaimng ~0.3 tons of gallium, has been brought underground after rem0ymg cosmogemc Ge-isotopes through nitrogen purge. This final implementation status and the achieved parameters characterizing the experiment are described in this report. Full scale performance tests and optimalization of the operating conditions of the extraction system are presently under way.

1. INTRODUCTION

The motivations for employing a radiochemical Gallium detector to measure very low energy solar neutrinos have been described in many previous articles 1-8 re- porting on the installation and implementation of the GALLEX experiment at the Gran Sasso Underground Laboratory (LNGS) between 1985 and 1990. In short, it is mainly the potential to probe manifestations of non-zero neutrino masses down to - 1 0 6 e V / c 2, many orders of magnitude below the range otherwise experimentally

accessible. In view of the deficit of 8B-solar neutrinos established with the Home, stake Chlorine detector 9 and confirmed with the Kamiokande Cerenkov detector l°,ll, it is quite likely that matter enhanced neutrino oscillations CMSW"-effect 12) are operative, yet astrophysical causes can still not be ruled out. A measurement of the very low energy pp-neutrinos will allow to decide between these two possibilities, since the expected pp-neutrino flux is tied to the solar luminos- ity and therefore hardly affected by not too unreasonable stellar structure modifications. In addition, actual values for neutrino mass

*)GALLEX COLLABORATION: ~ora tor i Nazionali del Gran Sasso: E. Bellotti; Max-Planck-lnstitut fa'r Kernphysik, Heidelberg: P. Anselmann, M. Breitenbach, W. Hampel, G. Heusser, J. Kiko, T. Kirsten,# A. Lenzing, E. Pernicka, R. Plaga, B. Povh, C. Schlosser, H. VSlk, R. Wink, M. W6jcik ; Kernforschungszentrum Karlsruhe-KFK: R. v.Ammon, M. Balata, K. Ebert, T. Fritsch, K. Hellriegel, E. Henrich, L. Stieglitz, F. Weyrich; Dip. ai Fisica dell'Universit~ Milano: O. Cremonesi, E. Fiorini, S. Ragazzi, L. Zanotti; Physik Dept. EIS, Technische Universitat Manchen: F. v.Feilitzsch, R. M6Bbauer, U. Schanda; Universite ite Nice-Observatoire: G. Be:,homieu, E. Schatzmann; Weizmann Institute of Sciences, Rehovot: I. Carmi, I. Dostrovsky; ll-Universit~ di Roma: S. d Angelo, C. Bacci, P. l~elli, R. Bernabei, L. Paoluzi; DPhPE CEN-Saclay: M. Cribier, G. Duoont, L. Gosset, B. Pichard 1, J. Rich, M. Spiro, T. Stolarczyk, C. Tao, D. Vignaud; Brookhaven National Laboratory, Upton, N.Y.: G. Friedlander, R.L. Hahn, F.X. Hartmann, J.K. Rowley, R.W. Stoenner, J. Weneser.

~p ermanent address: Phys.Inst., Jagellonian University, Krak6w, Poland deceased

0920-5632/91/$3.50 © Elsevier Science Publishers B.V. (North-llolland)

78 T. Kirsten, GALLEX Collaboration/The status of GALLEX

differences and mixing angles can be deter- mined by combining pp- and 8B-neutrino data due to the distinct spectral response in the MSW-mechanism.

At the time of this conference, all experimental installations are completed and operative (except for the artificial 51Cr-neutrino source, which will be employed at a later stage). It is this status which is described in this communication. Mean- while, we have entered the test phase to characterize the target purity and the blank level in full scale experiments under variable experimental conditions.

2. SYNOPSIS OF GALLEX Reaction: 71Ga (Pc, e') 71Go

The energy threshold is 233 keV. The Standard Solar Model (SSM) predicts a production rate of (132 +20, -17) SNU* (3 e) 13, out of which 74 SNU are due to pp- (and pep)-neutrinos. The isotopic abundance of natural 71Ga is 39.6%, the half life of 71Go is 11.43 d.

Target: 101 t of aqueous galliumchloride solution containing 30.3 tons of gallium, exposed in one single tank in the LNGS Underground Laboratory.

Production rate: 1.1 captures per day (SSM) Production background:

Muons: < 1.5% of SSM-signal fast neutrons: < 0.5% of SSM-signal target impurities: < 0.2% of SSM-signal

Extraction: purging of volatile GeCI 4 with 3000 m 3 of nitrogen during 20 hours.

Conversion: reduction to GeH 4 with subse- quent gas chromatographic purification.

*) SNU = solar neutrino unit, 10 "36 captures per atom and second

Counting: Low-Level proportional counting in miniaturized Suprasil counters with 30% GeH4-70% Xe gas mixture. Active and passive shielding, pulse shape analysis for background reduction.

Sequence: 20 days exposure, 1 day extraction: repeat:: 30+ runs in first 2 yrs.

Calibration (projected) 1 MCi 51Cr-source

3. GALLIUMCHLORIDE TARGET SOLUTION We use an 8.13 normal aqueous gallium-

chloride solution to facilitate Go-extraction as volatile GeCI 4 by nitrogen purge of the target. 30.3 tons of Ga are corttained in 101 t of solution. The solution was produced by P.&6ne Poulenc in Southern France and con- ditioned to a CI/Ga ratio of 3.24, the opti- mum for Go-extractions according to our extraction test experiments. The Ga-content is 30.0%, and the density is 1.89 g/cm 3. All purity specifications are easily met by the delivered product (Table 1). This was checked by neutron activation analysis (U, Th, Fe, As, So, Ba) and by radon-determinations using proportional counting. Addi- tional Go-extraction tests with spiked product solutions also gave satisfactory results. It is particularly remarkable that the critical 226Ra-concentration is more than one order of magnitude below specification, making background-71Ge production through actinide impurities completely negligible. We consider this to be a result of our intense guidance of the producer and our continuous analytical production control. The solution was received in a Technical Outside Facility CTOF") erected 1 km from the tunnel entrance. Here it was transferred into a

T. Kirsten, GALLEX Collaboration / The status of GALLEX 79

10 m3-mixing and transport tank equipped for fast truck transport into the tunnel. It was then purged with nitrogen to reduce the cos- mogenic 68,69,71Ge to less than .1% of the initial concentration. At the end of the purge, it was quickly (within 2 hrs) transported underground.

Table 1: Key properties of the target solution measured specification

Ga (g/l) 567 CI (g/l) 937 CI/Ga (mol/mol) 3.24 density (g/ml) 1.887 226Ra (,pCi/kg) 0.04

U (ppb) <0.04 Th (ppb) < 0.04 Zn (ppb) -20 Se (ppb) .2 As (ppb) .6 Ba (ppb) - 15 Ge (ppb) < .5

565 + 10

>3.15

<0.50*) <25*) <2*)

Colour water clear with a slight yellowish shade

These specifications would lead to a 1%- contribution to the standard solar model production rate for each individual background source.

4. GALLEX UNDERGROUND LABORATORY FACILITIES The Gallex facilities are located in the

eastern wing of hall A in the Gran Sasso Underground Laboratory 14. Access is by 6.3 km highway from the tunnel entrance. The LNGS infra-structure includes a 40 t hall crane, a number of auxiliary cranes as well as phone- and computer links to the outside institute.

The 3-storey process building accommo- dates the tank room and the Ge-extraction

facilities, such as absorber columns and control room. In smaller labs it houses the Ge-synthesis- and counter filling stations, and the facifities for the calcium-nitrate neutron monitor. Plastic spill trays with the capacity to take up all the target solution plus diluting water in case of an emergency, such as e.g. caused by an earthquake, are installed below the side access road next to the tank room.

The 2-storey counting building conta~ the counting station in a Faraday cage (Ground Floor Lab, "GFL") and - connected via optical fiber - the First Floor Lab ("FFL"). The latter houses the electronics beyond stiff counter signals, the computer facilities, and a small auxiliary counting station.

5. TARGET TANKS AND GERMANIUM EXTRACTION The 53.5 m 3 of 8.13 molar gallium-

chloride solution are exposed to solar neutrinos in one single 70 m3-tank. This facilitates extraction and a later artificial neutrino exposure with a 51Cr-Megacurie neutrino source inserted into a central thimble. The latter can also take up the Ca(NO3) 2 vessel for internal neutron monitoring (see 6.)

We have installed two equal tanks for redundancy, safety, and eventual later tank inspections; yet only one tank is used at a time. The tanks are made by Plastilon Ltd (Finland) from PALATAL (brand name) un- saturated polyester reinforced by special low - U/Th- (Coming "$2") glass fibre. They are lined with PVDF. The same material is also used for the sparging system within the tanks, consisting mainly of 4 concentric sparger rings near the tanks bottom and punched with 200 holes.

80 T. Kirsten, GALLEX Collaboration / The status of GALLEX

Operating conditions and equipment design were defined and optimized with the help of a pilot facility, sealed 1:9, which is operative at Karlsruhe since 2 years.

The extraction is performed by use of 3000 m 3 of nitrogen during 16-20 hours in the "onee-through"-mode, yielding > 98% recovery of the Ge-earrier (typically 1 rag). Instead of inconvenient and noise generating compressors, we use a liquid nitrogen evaporator. The volatile germaniumchloride taken over by the sparging nitrogen is ab- sorbed in 3 large absorber columns (3.1 m length, 25 em diameter), filled with pyrex helices and counter current water circulation ( -40 litre per column). Finally, the Ge is contained in about one litre of water. After solvent extraction into - 1 0 0 cc CC14, the GeC14 is baekextraeted into - 1 0 ee of tritium-free water and then reduced through the reaction GeCI 4 + NaBH 4 + 3 H 2 0 - , GeH 4 + H3BO 3 + 3HCI + NaCI.

Two GeH 4 prepzration lines and two counter filling lines are newly built from scratch for the Gran Sasso Lab as duplica- tion of respective lines having been operating at MPI and BNL since many years. Tritium- free reagents and gasehromatographie purification assure the required purity. Conversion yields are 98+ %; for yield determinations we use an atomic absorption speetro- photometer. Extraction yields can, in addition, be monitored by isotope dilution tech- nique using off-site thermion-mass spectro- metry at Karlsruhe. This will imply spiking with separated germanium isotopes, presently we are still using hyperpure natural Ge-carrier prepared from a Ge single crystal and checked for absence of radioactivity by low level counting.

6. CALCIUM NITRATE EXPERIMENT AND SIDE REACTIONS Unwanted 71Ge production is largely

dominated by 71Ga(p,n)71Ge whereby the protons are secondaries from either natural radioactivity in the target or from the resid- ual cosmic ray muon flux at the underground site (- 15/m2,d) 15. A redetermination of the relevant 71Ge production rate in GaCl3-solution at the CERN muon beam 16 led to an estimate of the cosmic ray-induced 71Ge production rate in the target geometry of the experiment equivalent to 1.5 + .4 SNU (1.1% of the SSM production rate).

Contributions from the target impurities are negligible (see Table 1). The environ- mental fast neutron flux at the experiment site is low" (~n(>2.5 MeV)=.23 x 10"6/era 2, s 17.

In order to also account for the neutrons originating from the tank walls and for the actual moderation conditions, in situ measurements of the 71Ge production rate via fast neutrons have been performed with the help of an elongated 470 1 vessel containing Ca(NO3) 2 solution. It is inserted into the thimble of the target tank. From the 37Ar produced via 4°Ca(n,o037Ar in the Ca(NO3) 2 solution, the 71Ge production rate can be scaled. From this we estimate that the 71Ge production rate, due to fast neutrons, is equivalent to .4+.2 SNU (.3% of the SSM- signal).

7. LOW LEVEL COUNTING After conversion of the extracted germa-

nium into gaseous GeH4 (german), low-level detection of 71Ge is performed using miniaturized gas proportional counters located in a detector tank consisting of coinei-

T. Kirsten, GALLEX Collaboration/The status of GALLEX 81

dence/anticoincidence devices (well-type NaI crystal pair) and passive shields (lead, copper, iron). 71Oe (electron capture) decays result in ionizing events at -10.4 keV (K- peak) and - 1.2 keV (L-peak) due to Auger electrons and/or stopped X-rays. Using a fast transient digitizer, the fully registered pulse shape serves to discriminate against backgrounds (especially Compton-electrons). The fast wave form analyzer together with the superior properties of a newly developed preamplifier have led to a remarkable resolution power, up to the identification of individual features within a single primary ion- ization event.

In five years of counter development we have continuously refined and optimized the counter performance with respect to efficiency and background. Our final design (counter type "HD2-Fe") is made from hyperpure Suprasil, has a solid Fe- (or Si)- cathode, and a directly sealed 13 #m-tung- sten anode wire. Its key features are minimal dead volume (volume efficiency > 93%), low capacity (< lpF), hence high signal/noise ratio and low background rates (see below). 20 identical counters of this type alone are available. Using special quartz technology, their dimensions have been standardized (7 mm diameter, 26 mm length, 1 cc active volume).

Counters are supported in specially tailored Cu/low-activity Pb housings, into which the preamplifier is also integrated. They are positioned within a combined active and passive shield. The shield tank features: - 8 counter positions within a NaI pair

spectrometer with the option to detect also 69Ge and 68Ga (positron emitters) in the coincidence mode

- 24 positions within a low radioactivity copper block (opposite end)

- high purity Cu-shield next to the Nal - outer steel vessel filled with low radio-

activity lead - two sliding end doors also filled with

lead - air lock design with glove boxes to en-

able counter change without venting the low radon atmosphere inside

- radon removal and --'--';',-i-;,,,, • - v . . . . 6 s y s ~ m

The device is installed inside the Faraday cage in the GFL of the GALLEX counting building. The analog part of the electronics installed in the GFL is electrically decoupled from the MicroVax and its periphery installed above the FFL via an optical fibre link. The data are supplied by the CAMAC oriented electronics to condensed disk with shared on-line access and permanent tape storage. The software for on- and off-line data analysis is readily developed. The full system ~,~ operating and performs well.

Systematic studies of the counting properties of potential gas mixtures have re- vealed the general suitability of GeH4-Xe mixtures for Xe-proportions from 0-95% at pressures up to 2 atmospheres. These studies also gave guidance for optimizing the trade- off among individual parameters such as slow drift velocity (low Xe, good rejection efficiency), good energy resolution (high Xe, low gas pressure), K/L peak efficiency ratio, amount of Ge-carrier which can be accom- modated (order of 1 mg), convenience of energy and rise time calibration, and overall background performaace. Our favoured gas composition ("Gallex-gas") is a 30% GeH4/70% Xe-mixture at 1.1 arm., operating with - 1600 Volt. The overall efficiency

82 T. Kirsten, GALLEX Collaboration~The status of GALLEX

is 72% (42% K, 30% L), the energy resolution 16% (K) and 42 % (L).

The neutrino exposure time of an individual run is 20 days. Counting is performed for periods of > 90 days, hence normally 5 or more multiplexed counters operate simul- taneously. Calibration is performed with Ce X-rays through a 30 #m Suprasil end win- dow, exciting Xe escape lines at 1.1, 5, and 10 keV.

8. COUNTER BACKGROUNDS Counter background measurements on

the various counter types under development had been performed before at the Heidelberg Low-Level Counting Laboratory, with full veto power to reject the frequent cosmic ray related events at shallow depth. Neverthe- less, a reduction of the integral background rate (by a factor 2-3) was obtained after moving into the Gran Sasso tunnel. Mea- surements performed first in a preliminary

("Bypass") station, then in the FFL and, more recently, in the GFL-configuration gave good results. Employing pulse shape discrimination using a fast Tektronix transient digitizer, the results obtained for these counters are shown in Table 2. The mean background rates are .13 epd (L-peak) and .02 epd (K-peak). These values allow to measure a 90 SNU-produetion rate of 71Ge in a 4-year experiment with a relative error of 8%.

9. ARTIFICIAL 51Cr-NEUTRINO SOURCE All technical provisions for inserting an

artificial calibration source 18 into the target tank have been installed (thimble, source crane, mechanical structure adaption of the roof above the tank). The source experiment is presently planned in 1992, after about 18 months of observing solar neutrinos.

Table 2: Counter backgrounds measured at Gran Sasso

Number of Counting gas Location Total counters counting

time [d]

Bkg count rate [counts/d]

Integral L peak

Total Fast

K peak

Total Fast

3 GeH4 Bypass 179.5 1.05 a) 0.31 0.15 0.09 0.01 FFL

2 Xe-GeH4 GFL 67.6 1.57 b) 0.35 e) 0.06 e) 0.19 0.04 7 Xe-GeH4 GFL 44.4 0.91 a) 0.36 0.11 0.09 <0.03 1 Xe-GeI-I4 d) GFL 73.0 1.94 0.44 0.130 0.11 0.05

a) >0.5 keY; b) > 1.2 keY; c) 1.2-1.9 keV; d) GeH4 from synthesis line; e) >0.3 keV; f) unvetoed: 0.08

T. Kirsten, GALLEX Collaboration~The status of GALLEX 83

This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract number 06HD554L This work has been funded by INFN (Italy), CEA (France), KFK Karlsruhe (Germany), KRUPP Foundation (Germany), and others.

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