Large Gaseous Detector of Ionizing Eradiation in Search for Coherent Neutrino-Nucleus Scattering
aa r X i v : . [ phy s i c s . i n s - d e t ] N ov Gaseous Detector of Ionizing Eradiation inSearch for Coherent Neutrino-NucleusScattering
A.V.Kopylov, I.V.Orekhov, V.V.Petukhov, A.E.Solomatin
Institute of Nuclear Research of Russian Academy of Sciences117312 Moscow, Prospect of 60th Anniversary of October Revolution 7A
Abstract
We propose to search for coherent neutrino-nucleus scattering (CNNS)by means of a triple-sectioned low background proportional counter.As a working medium we plan to use argon and xenon at about 1MPa. We have shown using bench-scale assembly, that pulse-shapediscrimination enables to effectively suppress noise pulses from elec-tromagnetic disturbances and microphonic effect in the energy regionwhere one expects signal from CNNS (from 20 eV to 100 eV) with afactor of about 10 . The calculation has been done of the backgroundfrom neutrons, generated by muons of cosmic rays. The experimentalsetup has been proposed. At small recoil energies when neutrino does not “see” nucleons constitut-ing a nucleus but rather scatters as a wave on a grid, the neutrino-nucleuselastic scattering by means of exchange of Z -bozon is coherent over the nu-cleons in the nucleus. Due to the coherence the cross section is proportionalto the square of the number of neutrons in a given nucleus (the contributionof protons is given in the expression for the cross section with the weight ofapproximately 0.08) and it can reach so big value that even for a mass of a tar-get of about 1 kg and a flux of antineutrinos from a reactor of 2 · nu/cm /s the count rate may reach the value of several events per day. This processhas been described in 70th of last century [1, 2], has been often discussedlater on [3]–[7] but has never been observed because of extremely small (lessthan 600 eV for reactor antineutrinos) kinetic energy of the recoiling nucleusand only small portion of this energy (about 15%) is transferred into theone of ionizing eradiation. The discovery of this process would be the great Corresponding author: Kopylov A.V., Institute for Nuclear Research of RussianAcademy of Sciences, Prospect of 60th Anniversary of October Revolution 7A, 117312,Moscow, Russia; telephone +7(495)8510961, e-mail: [email protected] > ).2. Possibility to use gas at relatively high pressure about 1 MPa to obtainthe mass sufficient for count rate of about 1 events per day.3. Good signature of the events by a pulse shape (very characteristic frontand tail of the pulses).4. The possibility to discriminate noise from electromagnetic disturbancesand microphonic effect.5. Availability of the efficient methods of gas purification.6. Detector can be fabricated only from very pure materials without PMTsas a possible source of ionizing eradiation etc.7. The possibility easily change the working gas (argon – xenon) notchanging the configuration, what is important to perform the com-parative measurements at the same site.We performed the measurements of the energy spectra of the pulses inargon using a small bench scale assembly. The calibration has been doneusing F e as a source of X-ray eradiation of 5.9 keV. Proportional counterhad 37 mm, the central wire of 20 mm in diameter and it was filled by argonand methane (10%) mixture by 100 and 300 kPa. The shapes of the pulsesfrom output of charge sensitive preamplifier of the sensitivity of about 0.4V/pC have been recorded by 8-bit digitizer. The shapes recorded duringcertain time were analysed in off-line. The aim was to see how efficient couldbe the pulse shape discrimination of the noise pulses from electromagneticdisturbances and microphonic effect in the region below 100 eV, i.e. wherethe main effect is expected from CNNS of reactor antineutrinos. In Fig.1 weshow the pulses observed during time interval 400 µs where one can see “true”pulse with correct signature from ionization process and “wrong” pulse fromelectromagnetic disturbances. 2
100 200 300 400-1012 mV s
Figure 1: The pulses on the output of charge sensitive preamplifier: Thepoint ionization (red) and from electromagnetic disturbances(blue). Thesensitivity of electronic channel 1 mV/10 eVElectromagnetic disturbances have usually non regular shape, the pulsesfrom “microphonic effect” have typically response in the audible range withthe shapes close to sinusoidal. The pulses from the point ionization in gashave typically a relatively short front edge (a few microseconds) correspond-ing to the time drift of positive ions to cathode and long (hundreds of mi-croseconds) tail corresponding to the time of the base line restoration of thecharge sensitive preamplifier. These events might be produced in our de-tector by internal radioactivity of the materials of the counter, by electronicemission from the walls of the counter and also by ionizing particles producedby cosmic rays. The amplitude of these events may be even smaller then anaverage energy to produce a single electron pulse because of the relativelybroad energy distribution in this case (Polia distribution). Using two peaksfrom F e calibration source (5.9 keV and 2.85 keV escape peak in argon)we observed relatively good linearity of the conversion energy – amplitudeand rather high 4 · gas amplification. In the range from 20 eV to 100 eV,where main effect from coherent scattering of reactor antineutrinos shouldbe observed, the pulse shape discrimination enabled to reduce the noise bya factor of about 10 . Thus we show that this range can be effectively used3or counting of the events from CNNS. The similar problem of counting theevents from very small energy release has been solved in a number of exper-iments with cryogenic detectors. In 1997 we together with the staff of thelaboratory of Professor Sandro Vitale in University of Genoa in Italy werefirst who succeeded in counting the pulses from peaks 57 eV and 112 eV fromthe decay of Be [8]. The energy threshold in this work was 40 eV. This wasachieved thanks to effective pulse shape discrimination of the noise pulsesfrom electromagnetic disturbances and “microphonic effect”. The count ratefrom CNNS of reactor antineutrinos is calculated to be a few events per dayper kg of argon in the energy range from 20 eV to 100 eV. To collect a massof argon of about 1 kg the detector should have the volume of about 50 literseven at the pressure 1 MPa. But to get the gas amplification higher 10 atHigh Voltage 3 kV the diameter of the cathode should be 40 mm, not more.To reconcile these conflicting demands we should use an array of countersand each counter should have a central, avalanche region with a small diame-ter of the cathode and external, drift region, separated from avalanche regionby a grid. The diameter of the drift region is taken to be 140 mm. Apartfrom this, there should be external cylindrical layer of counters working as anactive shielding and also as a passive one of the fluorescence from the wallsof the counter. All assembly is placed in a cylindrical body made of titaniumas a relatively pure on Ra material, as our previous measurements haveshown. In Fig.2 we show the general view of this counter.We plan to use an array of 16 similar counters, each working on separatecharge sensitive preamplifier and digitizing board. The counters will be as-sembled in 4 planes, each one having 4 counters. The size of the assemblywill be approximately 100x100x100 cm. To reduce the background from cos-mic rays, neutrons and gamma-rays the assembly will be placed in the boxmade of slabs of iron 30 cm thick, internal surfaces will be lined by boratedpolyethylene 20 cm thick. To shield from fast neutrons from the reactor weplan to use additional external layer of water 50 cm thick and on the outside– plastic scintillator as an active veto shield from ionizing particles of cosmicrays penetrating to the depth of about 16 m of water equivalent. The watershield reduces the background from fast neutrons by an order of magnitude,thus, it will be possible by comparing the data collected with and withoutwater to determine how large will be the contribution of reactor neutronsto the effect observed. All this assembly will be placed in a hermeticallysealed housing filled by argon purified of radon. We select this design ofshielding to reduce at most the background from gamma-quanta from ex-4igure 2: The detector before assemblingternal radioactivity and from neutrons, generated in iron by cosmic rays.Borated polyethylene 20 cm thick decreases approximately 10-fold the fluxof fast neutrons from iron. The slabs of iron 30 cm thick effectively absorbgamma-radiation from the walls. In Fig.3 we show the calculated effect fromCNNS and the background from neutrons, generated at 16 meters of waterequivalent for argon and xenon as a working medium of the detector.The energy spectrum of nuclear recoils presented on Fig.3 was taken from[9]. In the calculation of the background from scattering of neutrons, gen-erated by muons of cosmic rays, we used the data on the energy spectrumof neutrons from [10]. For precise interpretation of the effect from CNNSone needs an accurate, with the uncertainty of a few percents, measurementof the quenching factor in gaseous xenon and argon which is expected tobe approximately 10-15% at the energy of the recoiling nucleus lower then500 eV [11]. Further development of the technique described in this paper isneeded to accomplish the task within approximately 5 years to obtain somesignificant physical result. Acknowledgements.
We warmly acknowledge funding from the Pro-5igure 3: The energy spectrum of nuclear recoils from CNNS of reactor an-tineutrinos (1 – xenon, 2 – argon) and from scattering of neutrons, generatedby muons of cosmic rays (3 – xenon, 4 – argon)grams of support of leading schools of Russia (grant
References [1] D.Z.Freedman PRD 1974 B269
D68
C86 B398
PhD thesis, Department of Physics
Case Western Reserve Uni-versity 2002[11] K.W.Jones and H.W.Kraner Phys.Rev. 1975