The HELLAZ solar neutrino experiment- the measurement of the spectrum of pp and Be

The HELLAZ solar neutrino experiment - the measurement of the spectrum of
pp
and
Be
Th. Patzak, on behalf of the HELLAZ collaboration Laboratoire de Physique Corpusculaire et Cosmologie, College de France, IN2P3-CNRS, 11, Place Marcelin Berthelot, F-75231 Paris, France, E-mail: Patzak@cdf.in2p3.fr
Figure 1: Typical MC event of a neutrino interaction resulting in a 250 keV recoil electron. The ionisation cloud after a drift length of 2.50 m left by the electron is represented by the small dots (representing the individual electrons). The continues line corresponds to the track of the recoil electron without drift.
, of recoil electrons from elastic neutrino - electron scattering. The two measured quantities are related to the neutrino energy in the following way:
A large time projection chamber (TPC), lled with cooled helium will serve as active target. The apparatus with proper shielding will be situated in the Gran Sasso underground laboratory in Italy. To obtain a reasonable event rate the detector is operated at low temperature and high pressure to produce 2 1030 target electrons in a 2000 m3 TPC. The kinetic energy of recoil electrons is measured by counting the individual electrons in an ionisation cloud generated by the energy loss of the recoil electron due to ionisation in the helium gas. A typical event is shown in gure 1.
1 Introduction
Data from all running solar neutrino experiments 1;2;3;4;5 indicate a large di erence between the ux of neutrinos calculated in the standard solar model 6 and the measured neutrino ux. To provide an essential contribution to solve the enigma of solar neutrinos a new generation experiment, HELLAZ (HELium at Liquid AZote - nitrogen - temperature), had been proposed in 1992 7;8 . The most interesting solar neutrinos are formed in the reaction p + p ?! 2 D + e+ + e with a continuous 1 spectrum and an end-point energy of 420 keV (pp - neutrinos). Furthermore, the mono-energetic 7 Be neutrinos produced in the reaction 7 Be + e? ?! 7 Li + e 3 4 are highly interesting because a best t to all data from experiments is compatible with zero ux for 7 Be neutrinos. Our main objective with the HELLAZ experiment is to measure the spectrum of pp - neutrinos and 7 Be neutrinos. Predictions from various solar models for the ux of 7 Be and 8 B - neutrinos show large di erences since the involved nuclear cross sections are not well determined in this low energy region. All existing model predictions for the ux of pp - neutrinos are in very good agreement. This comes from the fact that the ux of pp - neutrinos is strongly constrained by the solar luminosity and measurements from helio seismology. Therefore, any observed di erence in spectral shape, avour or ux from model predictions has to be interpreted in terms of new physics ( avour oscillations) or in drastic changes of the solar model, which is very di cult because of the above mentioned const
me
cos ? 1
(1)
2 The HELLAZ experiment
The concept of the HELLAZ experiment is based on the measurement of the solar neutrino energy, E , by measuring the kinetic energy, Te , and the scattering angle, 1
A new generation solar neutrino experiment, HELLAZ, has been proposed and is currently under development. The experiment is dedicated to measure the spectrum of neutrinos originating in the sun. In a rst period we will concentrate on the low energy pp-neutrinos and 7 Be-neutrinos. The pp neutrinos are the most important since they are at the origin of the nuclear fusion chain of the sun. Moreover, the ux of pp-neutrinos is well constrained by measurements of the solar luminosity and helio seismology. All model predictions agree on the ux of pp-neutrinos within a few percent. Hellaz is a real-time experiment measuring the neutrino energy in elastic neutrino-electron scattering reactions. The target consists of cooled helium contained in a 2000 m3 TPC.
Figure 2: A side view of the cylindrical time projection chamber of the HELLAZ experiment to measure the spectrum of solar neutrinos.
To reconstruct the scattering angle, , of the recoil electron the relative position in space of each individual ionisation electron is necessary in order to obtain the angular resolution we are aiming at. From the individual space points the center of gravity of the rst few mm (containing a few hundred electrons) of the track can be reconstructed. The individual coordinates are measured using a x,y detector covering the surface of each endcap of the TPC. The relative z coordinate is obtained by measuring the arrival time of the individual electrons using TDC's. A conceptional design of the HELLAZ experiment can be seen in gure 2. The described method allows a measurement of the kinetic energy of the recoil electron above 100 keV (this results in an energy threshold for the neutrino energy of 220 keV) with a precision of TT 3 %. An angular resolution of about 35 mrad will be achieved. Therefore, the energy of the incoming neutrino can be measured with a precision better then 10 %.