Proc. Neutrino Astrophysics - MPP Theory Group

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known true source activity. The combined analysis of both series results in R = 0.93 ± 0.08,
clearly demonstrating the absence of large systematic errors which could account for the
observed 40% solar neutrino deficit.
A similar experiment has also been performed by the Sage collaboration. They got a
quantitative recovery of R = 0.95 ± 0.12 [6].
The fact that both experiments, though employing different chemistry, demonstrated in
these experiments full efficiency, proves the trustworthiness of the radiochemical method and
shows that the 40% solar neutrino deficit observed in the radiochemical gallium experiments
cannot be ascribed to unknown systematic errors. In particular, the 51 Cr neutrino energies
nicely accommodating those from the solar 7 Be-branch, the full efficiency of the gallium
experiments to 7 Be-neutrinos is demonstrated.
71 As experiments
However, though the 51 Cr source in Gallex did outperform the sun by more than a factor
15 after insertion into the target tank, the experiments still are low statistics, involving only
several dozens of neutrino produced 71 Ge atoms. Therefore, at the very end of Gallex the
collaboration has performed a large-scale test of potential effects of hot chemistry, which
might lead to a different chemical behaviour of 71 Ge produced in a nuclear reaction compared
to the stable Ge carrier isotope: The in-situ production of 71 Ge by β−decay of 71 As. A known
quantity of 71 As (O(10 5 ) atoms) has been added to the tank (t-sample), where it decayed with
T 1/2 = 2.9d to 71 Ge. Four runs have been made under different operating conditions (mixing,
carrier addition, standing time), cf. table 2. For every spike a reference sample (e-sample)
was kept aside, making possible to calculate the ratio of t- and e-sample which does not suffer
from most of the systematic uncertainties associated with 71 Ge-counting.
Table 2: Experimental conditions and results (ratio t-sample/e-sample) of the 71 As runs.
run mixing conditions Ge-carrier standing result
[h × m 3 /h] addition time (tank / external)
A1 22 × 5.5 with As 19.9 d 1.01 ± 0.03
+0.17 × 60
A2 6 × 5.5 no Ge-carrier 19.9 d 1.00 ± 0.03
B3-1 24 × 5.5 after As 2.0 d 1.01 ± 0.03
B3-2 — after As 22.0 d 1.00 ± 0.03
In all cases a quantitative recovery of 100% was achieved. This demonstrates on a 3%-level the
absence of withholding effects even under unfavourable conditions like carrier-free operation 1 .
Gallium Neutrino Observatory
With the 71 As-tests Gallex has completed its large-scale experimental program. However,
solar neutrino measurements with a gallium target at Gran Sasso will be re-commenced in
spring 1998 in the frame of the Gallium Neutrino Observatory (GNO) [2] which is designed for
1 The stable germanium carrier is not only used for determination of the extraction yield, but also plays the
role of an ’insurance’ to saturate potential trace impurities which might capture 71 Ge in non-volatile complexes.