Characteristics of the Gallex Spectrometer

Abstract

A description is given of the spectrometer used for the detection of ⁷¹Ge in the Solar Neutrino experiment GALLEX being performed in the Gran Sasso Underground Laboratory. The spectrometer consists of miniaturized proportional counters and a shield with a large well-type NaI pair (Tl) detector (active side) and an inner pure copper shield (passive side). Very careful material selection for the proportional counter- and shield-construction and radon suppression resulted in total background rates (>0.5 keV) between 0.4 and 1 count per day for many proportional counters. With energy and rise time cuts, the average rates for the relevant L- and K-peak of the ⁷¹Ge spectrum are 0.1 cpd and 0.03 cpd, respectively and, thus, are far below the signal predicted by the Standard Solar Model. Eight counter positions within the NaI pair detector have the option to detect also ⁶⁹Ge and ⁶⁸Ga (positron emitters) in the coincidence mode, though with slightly higher background for the ⁷¹Ge decay mode. An analysis of the different background components cannot fully account for the measured background of the proportional counters so that presumably a part of it is due to contamination during the assembling process. Here is a potential for further background reduction. After introduction, the basic concept of the experiment and the present status as of December 1991 are briefly outlined.

For the GALLEX-COLLABORATION: Heidelberg, Max-Planck-Institut für Kernphysik: P. Anselmann, W. Hampel, G. Heusser, J. Kiko, T. Kirsten, G. Monninger, E. Pernicka, R. Plaga, B. Povh, U. Roenn, M. Sann, C. Schlosser, H. Völk, R. Wink, M. Wójcik¹; Karlsruhe, Kernforschungszentrum — KfK: R. v. Ammon, K. Ebert, T. Fritsch, K. Hellriegel, E. Henrich, L. Stieglitz; L’Aquila, Laboratori Nazionali del Gran Sasso: M. Balata, E. Bellotti, C. Cattadori, N. Ferraris, H. Lalla, S. Pezzoni, T. Stolarczyk; Milano,Dip. di Fisica dell’Università — INFN: O. Cremonesi, E. Fiorini, S. Ragazzi, L. Zanotti; München, Physik Dept. E15 — Technische Universität: F. v.Feilitzsch, R. Mößbauer, U. Schanda; Nice, Université de Nice Observatoire: G. Berthomieu, E. Schatzmann; Rehovot, Weizmann Institute of Sciences: I. Carmi, I. Dostrovsky; Roma, II-Università — INFN: S. d’Angelo, C. Bacci, P. Belli, R. Bernabei, L. Paoluzi; Saclay, DPhPE CEN: S. Charbit, M. Cribier, G. Dupont, L. Gosset, J. Rich, M. Spiro, C. Chao, D. Vignaud; Upton, N.Y., Brookhaven Nat. Laboratory: R.L. Hahn, F.X. Hartmann, J.K. Rowley, R.W. Stoenner, J. Weneser