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New Ways for Real Time Detection of Low Energy Solar Neutrinos and Other Crucial Experiments in Nuclear and Particle Physics

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New Ways for Real Time Detection of Low Energy Solar Neutrinos and Other Crucial Experiments in Nuclear and Particle Physics Workshop, January 1992 LAPP-EXP-92-01 LPC 92 - 18 NEW WAYS FOR REAL TIME DETECTION OF LOW ENERGY SOLAR NEUTRINOS AND OTHER CRUCIAL EXPERIMENTS IN NUCLEAR AND PARTICLE PHYSICS L. GONZALEZ-MESTRES L.A.P.P. Annecy and L.P.C. Collège de France, Paris Abstract Simultaneous detection of light and phonons or heat (the luminescent bolometer), was proposed in 1988 in view of dark matter detection [1] and considered later [2] for double beta experiments. Fast single crystal scintillators made of indium compounds have been developed [3-6] for neutrino physics, and in most cases can in principle ope- rate at low temperature. Combining both ideas, simultaneous detection of light and phonons in a scintillating single crystal made of an indium compound and cooled to very low temperature, may reach three basic performances: a) better effective segmentation through digital analysis of the phonon pulse read on each crystal face; b) fast timing from the light strobe; c) good energy resolution, from combined analysis of light and phonon pulses. Such performances are crucial for background rejection in any experiment (solar neutrinos or neutrino-antineutrino oscillations) involving an indium target. Dark matter and double beta remain the main applications of the luminescent bolometer, but other uses were proposed in [6]: spectroscopy with particle identifi- cation, thermal neutron detection with a 6Li target, neutrino experiments based on nucleus recoil... A new possibility is application to heavy ion physics [7], where energy resolution can be combined with fast timing from scintillation and space resolution from the phonon pulses. Simultaneous detection of light and phonons provides equally a new way for basic studies of relaxation phenomena of excitations in solids. Photosensitive devices based on superconducting tunnel junctions appear as a suit- able read out, able to collect the light pulse followed by the delayed front of phonons. They can be operated at 3He temperatures (T > 300 mK) with excellent sensitivity [8]. Simultaneous detection of ionization and phonons may be an alternative to the lu- minescent bolometer for experiments with an indium target (InP, InSb...). Superheated superconducting granules with the avalanche effect are not ruled out for this purpose. 1. PHYSICS POTENTIALITIES WITH CRYOGENIC DETECTORS Real time solar neutrino detectors are an urgent need, and must be sensitive to the pp part of the spectrum. Dark matter experiments based on nucleus recoil should reach event rates as low as 10~2 kg"1 day"1 . Double beta experimental programs are pursued with success, but target diversification is required to obviate nuclear physics uncertain- ties [9]. Neutrino oscillations remain an unsolved theoretical and experimental problem in elementary particle physics. Heavy ion physics in the energy range 1 to 40 MeV per nucléon, will require exceptional detector performances in experiments at the next generation of heavy ion facilities [10], [11], [7]. Spectroscopy is searching for ways to improve energy resolution, as compared to semiconductor devices. In any underground experiment, searching for rare events, a crucial issue is the reduction and rejection of radioactive background. Low temperature detectors provide technical solutions to such challenges (high sensitivity, low thermal noise). Working at low T allows to detect sev- eral components of the signal which can be used for particle identification: ionization + phonons [12], light + phonons [6]; in both cases, fast timing can be preserved. Cryogenic devices allow for exceptional energy resolution from thermal signals [9], and good space resolution inside a crystal using ballistic or scattered phonons [13]. Solid state physics and chemistry will also benefit from such a progress in instrumen- tation. By simultaneous detection of ionization or light arid phonons at low temperature, with high sensitivity and low excitation energy, it will be possible to uncover new fea- tures of crystal structure and electronic energy levels, usually masked by thermal noise, nonlinearity and lack of complete information on relaxation processes. As compared to ionization, where carrier collection produces extra phonons, luminescence presents the advantage that the sensor does not disturb the crystal under study. Simultaneous detection of quasiparticles and phonons with superconducting tunnel junctions, allows to investigate the properties of nonequilibrium superconductors [14]. The measurement of the relative strength of radiative and nonradiative processes at T = 1.5 K was used [15] to study energy transfer in GaAs p-n junctions. Low temperature fluorescence is a classical tool [16] to explore electronic energy levels and electron-lattice coupling, as well as the structure and behaviour of molecular complexes. Bolometers allow to measure time-dependent specific heats, which are often missing in solid state physics littérature. The appearance of powerful computers and digital signal processors allows for an approach based on a detailed and exhaustive study, event by event, of energy degrada- tion in crystal detectors. In some cases, it is even possible to perform on-line digital analysis. Cryogenic particle detectors, but also solid state experiments, should serioulsy benefit of this impressive development of real-time and computing facilities. The original proposal [1] to use simultaneous detection of light and heat for particle identification (the luminescent bolometer) has been presented at previous Moriond Workshops [17], [2]. The development of scintillating single crystal detectors made of indium compounds was also dealt with in these contributions. We report on new results and ideas on both topics, and suggest to combine them in a neutrino detector. 2. THE INDIUM SOLAR NEUTRINO DETECTOR Some time ago [3], it was proposed to use single crystal scintillators made of indium compounds to detect solar neutrinos through Raghavan's reaction [18]. With 128 keV threshold, an inverse beta reaction on 115In creates an excited state of tin which decays with a lifetime of 3.3 //s emitting two 7 rays (116 keV and 497 keV ). The delayed coincidence would allow for a clean signature, if the detector is fast and if the two delayed -)'s are absorbed in two different cells. A planar array of Sr 104 long crystals (20 cm x 3 cm x3 cm ) read by photomultipliers (PM), or a cube read by avalanche photodiodes (APD) were foreseen [I]. The second solution allows for a compact detector without dead volume and radioactivity from the read-out. Transparent single crystals of terbium-doped I11BO3 (Fig. 1) were grown [4] . Tb3+ fluorescence is too slow, and doping with cerium was attempted. However, it is known [19], [20] that very often Ce3+ does not stabilize in trivalent indium sites. Attempts 3+ to make scintillating InBO3:Oe powder failed. A chemical way out was found [5]: 3 given an indium compound InR , where R is some radical (e.g. BO3 ), find another compound, XR , such that XR and InR have the same crystal structure and the ion X3+ allows for cerium doping. Then, it is possible to obtain compounds of the form 3+ 3+ IUj-X^xR , and dope with Ce the X sites. The feasibility of this procedure was checked with powders, in compounds incorporating up to 50% of indium in volume. 3+ Scintillating single crystals of In/Sc BO3:Ce were grown (Fig. 2). Current goals are: a) compounds allowing for growth of large crystals at a reasonable cost (e.g. fluorides [21]); b) the best possible light yield (the most performant cerium scintillator seems to 3 be YA1O3:(Y ^ , with % 5% of the energy converted into light [22]). Background turned out to be very difficult, to handle, as 1 event/day is expected with 1 tons of indium and 115In is radioactive (0.2 event g"1 s~' ). The /•» spectrum of 115In decay goes up to 490 keV in energy. It overlays the inverse fl spectrum from pp solar neutrinos, and its tail lies close to the 497 keV 7 . Main sources of background are: a) 3 ,i coincidences; b) delayed coincidences between a /? from 115In decay and a 7 from ambient radioactivity which would fake the two 7's of 115Sn" decay. It seems unrealistic to expect radioactivity rates better than 10~h Bq g"1 in the region E "> 500 keV . To reject background, three basic conditions arise [IS]: a) extremely good segmentation [^ \0b elementary cells); good energy resolution (~ 10% FWHM at 100 keV ); reasonably fast timing ( ^ 100 ns ). We recently proposed [6] cryogenic detection as a solution. (V3+ doping seems appropriate for this purpose, as trivalent cerium fluorescence remains fast (T • ()0 ns) and can improve in light yield as temperature goes down [23]. Eu2+ is an alternative, with slower luminescence signal (r 5: 900 ns [24]). Work on superconducting tunnel junctions (STJ) by the Oxford group [8] was pre- sented at the previous January Moriond Workshop [25]. The results obtained with a 432 STJ array on a 1 cm2 thin absorber, where the possibility to reach a threshold below 1 keV clearly emerges, are particularly relevant to the indium solar neutrino detector. At low temperature, a STJ-based device on the surface of a scintillating crystal can read the fast light strobe followed by the phonon pulse. Arrays of STJ in series provide excellent results at 3He temperatures and are well suited for large detectors. With 40% of In in weight, and a density of 5 g/cc , a; 30000 4x4x4 cm3 crystals could be used, with a STJ device per crystal face (ss 2 XlO5 electronic channels). After a trigger based on a 10 /is gate, digital analysis of the full event would extract good energy resolution and position information ( % 1 cm FWHM ). Effective segmentation would be % 2 x 106 elementary cells. Read-out design requires a compromise between the requirements of light collection, energy resolution and timing. Blackened STJ or a thin black absorber can improve light collection. Photosensitive STJ are an important research subject [26].
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