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To test the sensitivity of using atmospheric (85)Kr to detect undeclared separation of plutonium from irradiated nuclear-reactor fuel, measurements of atmospheric (85)Kr taken in Tsukuba, Japan are analyzed to determine: (1) a lower limit of detection for discovering anthropogenic (85)Kr emissions, (2) the probability of detecting plutonium separation at the Tokai Reprocessing Plant, and (3) the extent to which these results can be generalized to other sites. A LLD of at least 3.4 sigma=0.14 Bq/m(3) with a theoretical false-positive rate of 0.05% is recommended for safeguards' purposes. At this threshold, the continuous separation of 100, 300, and 900 g equivalent weapon-grade plutonium per day was found to correspond to 10%, 50%, and 80% probability of detection, respectively. The smallest detected concentration was for the continuous separation of 45 g/day, with a probability of detection of about 0.6%. It was found that the detection rate is determined predominantly by the weather.

After 5.5 years of data taking, GALLEX ended its experimental phase. The solar neutrino production rate, 76.4 ± 8 SNU, represents about 55% of the predicted rate. The As tests prove, at the 1% level, the reliability of the technique and the detection of 51Cr neutrino, with the nominal efficiency (93.0±8%), control the response of the detector to neutrinos in the solar energy range.

The production of germanium radioisotopes by high energy muons in GaCl3 has been measured in a tank containing 851 of a GaCl3HCl solution, irradiated in the CERN muon beam. From the measured cross-section, the background induced by cosmic muons in the GALLEX experiment in the Gran Sasso Underground Laboratory has been determined to be equivalent to (2.8 ± 0.6) SNU 3 . This represents about 4% of the solar neutrino signal measured by the GALLEX experiment.

We describe the production of a 62 PBq (62 × 1015 Bq) source of ∼ 750 keV neutrinos coming from the electron capture decay of 51Cr. This is the first time that such a high-intensity, low-energy neutrino source has been produced. The rationale for having such a source is to check the overall procedures of the radiochemical solar neutrino experiment, GALLEX. The source was obtained by neutron activation of 36 kg of enriched chromium at the Siloe reactor at Grenoble. The enriched chromium (containing 38.6% of 50Cr compared to 4.35% for natural chromium) was produced by the Kurchatov Institute in Moscow, in the form of CrO3. It was then electrolyzed in Saclay to obtain chromium metal tubes, which were subsequently broken into coarse chips of typically 1 mm3 volume. The chromium chips were put inside 12 special zircalloy irradiation cells that were then placed around the Siloe reactor core, which had been specially reconfigured for this irradiation. The irradiation lasted for 23.8 days. Then the activated chromium was transferred in a stainless-steel container into a sealed tungsten shield having a wall thickness of 8.5 cm. The 51Cr activity at the end of the irradiation (EOB) has been measured to have a mean value of (62.5 ± 0.4) PBq, using several techniques: by neutronics and gamma scanning in the reactor shortly after EOB, with an ionization chamber, by calorimetry, and, after a considerable decay period, by gamma-ray spectroscopy and measurement of the non-radioactive 51V daughter. The source resided in the center of the GALLEX detector at the Gran Sasso Underground Laboratory between June 23, 1994 and October 10, 1994, irradiating the gallium target with a neutrino intensity well above the solar neutrino background. The results of this full-scale test are in good agreement with the expected efficiency of the entire GALLEX experiment calculated as a product of the known efficiencies of the various parts of the experimental procedure.

Since May 1991, the GALLEX experiment — installed in the Gran Sasso Underground Laboratory — is continuously monitoring the solar neutrino signal by measuring the 71 Ge production rate in a target of 30.3 tons of nat Ga. The measured signal 77.1 ± 8.5 (stat) +4.4 – 5.4 SNU (1 σ ) [1] is well below the Standard Solar Model (SSM) prediction: ⋍ 120–130 SNU. To exclude unknown systematic effects as responsible for the observed deficit, an experiment with a (62.5 ± 0.4) PBq 51 Cr neutrino source has been performed 2. , 3. . The ratio R=0.97 ± 0.11 between measured and expected Cr-produced 71 Ge demonstrates that the deficit cannot be ascribed to unknown experimental systematics. Although astrophysical reasons could still remain as a possible explanation of the “solar neutrino puzzle”, we recall that the GALLEX result, when considered together with the results of the other solar neutrino experiments, would allow to strongly constrain the neutrino masses and mixing angles in the framework of the MSW effect.

The miniaturized proportional counters used for the detection of 71 Ge in the solar neutrino project GALLEX are characterized. We also report on the construction techniques applied to build these counters and to achieve the described performance. A very low counting background is achieved by, among other things, careful selection of the materials used for construction. The total 71 Ge detection efficiency after applying cuts to reduce the background is about 66%.

We report the GALLEX solar neutrino results for the measuring period GALLEX III, the period from 12 October 1994-4 October 1995. Counting for these runs was completed on 29 March 1996. The GALLEX III result (14 runs) is [53.9 ± 10.6(stat.) ± 3.1 (syst.)] SNU (1σ). This is 15.8 SNU below but statistically compatible with the new combined result for GALLEX (I+II+III) (53 runs), which is [69.7 ± 6.7(stat.) −4.5 +3.9 (syst.)] SNU (1σ) or (69.7 −8.1 +7.8 ) SNU with errors quadratically added. We also give the preliminary result from our second 51 Cr-source experiment: the measured detector response is 83±10% of expectation. The combined result from both GALLEX 51 Cr-source experiments is 92±8% of expectation.

The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with an intense man-made 51 Cr neutrino source (61.9 ± 1.2 PBq). The source, produced via neutron irradiation of ≈ 36 kg of chromium enriched in 50 Cr, primarily emits 746 keV neutrinos. It was placed for a period of 3.5 months in the reentrant tube in the GALLEX tank, to expose the gallium chloride target to a known neutrino flux. This experiment provides the ratio, R , of the production rate of Cr-produced 71 Ge measured in these source exposures to the rate expected from the known source activity: R = 1.04 ± 0.12. This result not only constitutes the first observation of low-energy neutrinos from a terrestrial source, but also (a) provides an overall check of GALLEX, indicating that there are no significant experimental artifacts or unknown errors at the 10% level that are comparable to the 40% deficit in observed solar neutrino signal, and (b) directly demonstrates for the first time, using a man-made neutrino source, the validity of the basic principles of radiochemical methods used to detect rare events (at the level of 10 atoms or less). Because of the close similarity in neutrino energy spectra from 51 Cr and from the solar 7 Be branch, this source experiment also shows that the gallium detector is sensitive to 7 Be neutrinos with full efficiency.

The galliumchloride detector operated by the GALLEX-collaboration in the Gran Sasso Underground Laboratory responds primarily to pp-neutrinos. They are produced in the primary fusion reaction of hydrogen into deuterium and directly coupled to the solar luminosity. Standard Solar Models predict ca. 58% of the total signal expected in GALLEX (123-132 SNU) to be due to pp-neutrinos. The relative pp-neutrino dominance becomes even larger if the deficit of higher energy neutrinos (as observed in the Homestake- and Kamiokande experiments) is considered. During the first data taking period, 15 solar runs had been performed within the exposure period 14.5.1991 - 29.4.1992. The result, 81 {+-} 17 {+-} 9 SNU provided the first experimental evidence for pp-neutrinos from the Sun. At the same time, it confirmed the depression of higher energy neutrino fluxes relative to the model predictions. Here the authors report the results of 15 more solar neutrino runs, covering the period 19.8.92 - 13.10.93. They obtain 78 {+-} 13 {+-} 5 SNU. Evaluated together, the result for all 30 runs is 79 {+-} 10 {+-} 6 SNU. While the SNU rate of GALLEX I is well reproduced the statistical error has been reduced so substantially that a value of signal +2{sigma}more » is required to accommodate not only pp-and pep- but also the {sup 7}Be-neutrino induced {sup 71}Ge-production. Contrary, the fate of {sup 8}B-neutrinos has only little discernible effect on the GALLEX data. In conclusion, with the present errors GALLEX constitutes a 2.5 {sigma} problem for {sup 7}Be neutrinos within the frame of {open_quotes}astrophysical{close_quotes} solutions. Alternatively, the particle physics solution (MSW-effect) can consistently explain all available solar neutrino results, leading to a most probable mass scale with the muon-neutrino at approximately 3 meV (milli-eV). However, since the GALLEX result allows the presence of pp and pep neutrinos at full strength. the latter explanation of the data is not forced.« less

We report the solar neutrino results from the complete set of runs in the exposure period, GALLEX II, from 19 August 1992 - 23 June 1994. Counting for these runs was completed on 10 December 1994. The GALLEX II result (24 runs) is [75.2±9.7 (stat) −4.6+4.1 (syst)] SNU (1 σ). After three years of recording the solar neutrino flux with the GALLEX detector, the combined result from the 39 completed solar runs (GALLEX I+II) is [77.1±8.5 (stat) −5.4+4.4 (syst) SNU (1 σ) or 77.1 −10.1+9.6 SNU with errors combined in quadrature. The combined error (± 13%) has now approached a level where the limits on the derived contribution of 7Be neutrinos to the GALLEX signal confront the predictions of solar models.

The first period (GALLEX I) of data taking in the GALLEX solar neutrino experiment has been completed. From 14 May 1991 to 29 April 1992, the experiment observed the solar neutrino flux using neutrino capture by 71 Ga to form 71 Ge. Counting ended on 2 November 1992. The final result from the 15 runs of this periodpis (81±17 [ stat .]±9 [ syst .]) SNU (1 σ ). The first 6 runs of GALLEX II recorded the neutrino signal from 19 August 1992 to 3 February 1993. The preliminary result for this period is (97±23 [ stat .]±7 [ syst .]) SNU (1 σ ). With counting data considered until 29 April 1993, the joint result for all 21 runs is (87±14 [ stat .]±7 [ syst .]) SNU (1 σ ). The present neutrino recording period GALLEX II is continuing with one solar exposure every four weeks.

International audience; We have measured the rate of production of $^{71}$Ge from $^{71}$Ga by solar neutrinos. The target consists of 30.3 t of gallium in the form of 8.13 M aqueous gallium chloride solution (101 t), shielded by $\approx$ 3300 m water equivalent of standard rock in the Gran Sasso Underground Laboratory (Italy). In nearly one year of operation, 14 measurements of the production rate of $^{71}$Ge were carried out to give, after corrections for side reactions and other backgrounds, an average value of 83$\pm$19 (stat.)$\pm$8 (syst.) SNU (l$\sigma$) due to solar neutrinos. This conclusion constitutes the first observation of solar pp neutrinos. Our result is consistent with the presence of the full pp neutrino flux expected according to the "standard solar model" together with a reduced flux of $^8$B+$^7$Be neutrinos as observed in the Homestake and Kamiokande experiments. Astrophysical reasons remain as a possible explanation of the solar neutrino problem. On the other hand, if the result is to be interpreted in terms of the MSW effect, it would fix neutrino masses and mixing angles within a very restricted range.

The GALLEX collaboration is performing an experiment for the detection of neutrinos coming principally from the p-p fusion reaction in the Sun, via the reaction ve +71Ga →71Ge + e-. The experiment is running in the Gran Sasso Underground Laboratory of INFN, using as a target 30.3 tons of gallium in the form of 8.13 molar aqueous GaCl3 solution. A report is given of the status of GALLEX after the end of the operations devoted to the removal from the solution of the cosmogenically formed Ge isotopes, completed in the middle of 1991. The experiment is now collecting data on solar neutrinos and the data analysis is in progress. Preliminary results concerning the first year of measurement are presented.

The GALLEX result 83 ± 19 (stat.) ± 8 (syst.) SNU is two standard deviations below the predictions of stellar model calculations (124–132 SNU). To fit this result together with those of the chlorine and Kamiokande experiments requires severe stretching of solar models but does not rule out such a procedure, leaving the possibility of massless neutrinos. It clearly implies that the pp neutrinos have been detected. The Mikheyev-Smirnov-Wolfenstein (MSW) mechanism provides a good fit, and the GALLEX result fixes the Δm 2 and sin 2 2 θ parameters in two very confined ranges (around Δm 2 = 6 × 10 −6 eV 2 and sin 2 2 θ = 7 × 10 −3 and around Δm 2 = 8 × 10 −6 eV 2 and sin 2 2 θ = 0.6). Explanations of the solar neutrino problems based on the decay or magnetic interactions of neutrinos are disfavoured.

A radiochemical Gallium-Solar-Neutrino Detector has been set up at the Gran Sasso Underground Laboratory. All major installations are implemented and the target, 53.5 m3 of galliumchloride solution containing 30.3 tons of gallium, has been brought underground after removing cosmogenic 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.

The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with two intense 51Crneutrino sources (> 60 PBq) that were produced in the Siloe nuclear reactor and used at the Gran Sasso National Laboratory, one between June and October 1994, and the second between October 95 and February 96. The ratio, R, of the the neutrino source strength derived from the measured rate of 71Geproduction, divided by the directly determined source strength is R=1.01 +0.12−0.11 for the first source and R=0.84 +0.12−0.11 for the second one. The combined value of R for the two source experiments is R=0.93 ± 0.08. It shows that the > 40 % deficit of solar neutrino flux observed by GALLEX cannot be attributed to experimental artifacts and demonstrates the absence of any significant unexpected systematic errors at the 10 % level.