Lightweek (vol.1 Issue 4- Sept.22-30, 2009)

Vol. I

September 22nd – 30th, 2009

Issue 4

THE GALLIUM NUTRINO OBSERVATORY – WHAT’S IN STORE? [PART-III] Introduction Together with the increase of the target mass, the crucial point in the GNO project is to obtain a substantial reduction of systematic error. Thus presently most of the R&D activity has as its final aim the reduction of the various components of the systematic error. To understand the situation we can refer to the table below, where the different sources of systematic error for the gallium detector are listed together. A comparison of the GALLEX results with the ©The Online Physics Club

GNO expected performance shows the following situation: • Uncertainty on the background from side 71 reactions producing Ge: it will remain unchanged, even with increased target mass, assuming the same purity of the additional gallium; • Uncertainty on the background from cosmogenic 68 Ge produced in the gallium solution before transportation underground: it is presently (GNO30) zero because all the activity has decayed away during the 5 years of operation http://theonlinephysicsclub.clubdiscussion.net

of GALLEX. Concerning GNO66, thanks to the experience gained with GALLEX, we are confident

that it will be possible to clean the freshly produced gallium from 68Ge produced by cosmic rays.

• Presence of Rn in the counters: one of the most dangerous background sources is given by the possible presence of radon inside the counters: the experience with GALLEX has shown that a few Rn atoms are sometime introduced in the counters during the synthesis and counter filling. The decays of Rn and its daughters can produce events which cannot be distinguished from real 71 Ge events, and are time dependent. For that reason an Rn cut was introduced in the GALLEX data analysis, by defining a dead time for each detected Rn decay chain. The efficiency of this cut was evaluated to be (91 ± 5)% and represents one of the major systematic errors in the experiment. An Rn test could lower this error to about 1% or

even less. A further reduction will come in GNO66 from the enlargement of the target mass: in fact the presence of Rn in the counters is related to the filling procedure and not to the target solution, so that its weight is inversely proportional to the target mass.

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• Uncertainty in the 71Ge detection efficiency: the main systematic uncertainty in GALLEX comes from the error in the counting efficiency. The latter is the probability that a 71Ge decay inside a proportional counter is detected and recognized as a good event by the analysis. • The remaining source of error (target mass and chemical yield) is expected to remain constant.

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SOURCE

Background from side reactions ( muons, Actinides, 69 Ge) Backgound from 68 Ge Background from Rn in the counter Counting efficiency (including energy and pulse shape cuts) Chemical yield and target size TOTAL

GALLEX

GNO30

GNO66

%

(expected) %

(expected) %

1.6

1.6

1.6

+0.9 −2.6 1.5

0

0

1.0

0.5

4.5

3.0

2.0

2.2

2.2

2.2

5.9

4.2

3.4

So what is the Research and the Development? Keeping in mind the situation outlined above, one can understand the importance of the following R&D topics, presently under development: 1. New X-ray calibration source. The energy- and rise-time calibration of the proportional counters is performed with cerium X-rays which are generated by the aid of a ©The Online Physics Club

radioactive 153Gd source and a Ce target. Since about 2 years 153 Gd sources are no longer commercially available at the needed activity (GBq) and purity level; moreover the half-life of 239.5 d requires frequent replacement of the 153 Gd sources. A solution to this problem is a low power xray tube in place of the Gdsource. Fortunately a commercial generator X-ray tube combination became available which is compact enough as to fit in the shield of the proportional counter http://theonlinephysicsclub.clubdiscussion.net

spectrometer. A system with two stepping motors has been designed and is now under construction which allows the moving of the tube inside the shield so that each proportional counter can be calibrated in its normal counting position on the passive side of the shield. The Gd source calibration was only possible when the counter was removed from its counting position and when the shield was open. Thus, the new procedure matches better the true measuring condition and allows more frequent calibrations at a much higher comfort. The calibration procedure will be fully software controlled. During normal counting periods the system is completely switched off from electric power in order to maintain the low electronic noise level. 2. 71Ge calibrations and Rn tests In the next future we plan to perform at LNGS some calibrations with counters filled with a known amount of ©The Online Physics Club

active 71Ge . This will allow the direct measuring of the efficiencies for the selection criteria to be applied for the data analysis, and hopefully to reduce the uncertainty on the counting efficiency. More measurements with 71Ge will be necessary after the new calibration system will have been installed. An Rn test measurement is also planned, to reduce the systematic error induced by the Rn-cut. 3. Enlargement counting system.

of

the

In the present configuration the electronic chain can support up to 8 counters. We plan to buy the electronics necessary to deal with at least 12 independent lines. This is needed to perform several tests without interfering with the solar neutrino observations: in fact assuming one extraction per month and 6 months counting time per counter, 6 lines are permanently occupied by the solar runs. Supplementary lines are needed for the Rn 71 and Ge calibrations, http://theonlinephysicsclub.clubdiscussion.net

background measurements for new counters, blanks etc. 4. New counters.

proportional

So far, all the proportional counters used in GALLEX and in GNO30 are individually glass-blown, and therefore their 71Ge detection efficiencies differ slightly from counter to counter, leading to a systematic error of the order of 4.5 % in the efficiency determination. In order to reduce the error we have started to investigate a different counter construction procedure. The idea is to fully fabricate the counters mechanically (no glassblowing involved) so that a mass production is possible. With these counters a systematic error on the counting efficiency not much in excess of 2% should be achievable. 5. Development of cryogenic detectors At TUM an R&D project is ongoing to test the feasibility ©The Online Physics Club

of low temperature calorimeters as detectors for measuring the 71Ge-decay in a future phase of GNO and optimize these devices with respect to the technical requirements for implementation into the largescale experiment at Gran Sasso. The cryogenic calorimetric detectors are made from a superconducting phase transition thermometer evaporated directly onto a dielectric crystal which acts as an absorber for particles and radiation. The energy deposited in this crystal is measured via the resulting temperature rise in the thermometer. The device is operated in the transition region of the superconducting to the normal conducting state of the thermometer, where a small temperature rise ∆T of the thermometer leads to a large increase ∆R of its resistance. The readout is done with a Squid-based DAQsystem. First results obtained with test detectors did already show an improvement by about a factor of five in energy resolution, compared http://theonlinephysicsclub.clubdiscussion.net

to miniaturized low background proportional counters. In 1998 a compound detector setup consisting of two thermally separated sapphire absorber crystals, each of them equipped with a superconducting phase transition thermometer, and

the 71Ge-activity deposited in between has been measured. The next steps are improvements in the Gedeposition technique, tests of absorber materials other than sapphire, and a several weeks prototype run at TUM.

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