# LENS

# Home page of the Experiment:http://lens.in2p3.fr

The primary objective of LENS is direct real-time measurement of the solar n e spectrum < 2 MeV, consisting of 7Be, the CNO and in particular, the basic pp ne's with energy < 0.42 MeV, via a ne capture reaction that can be specifically tagged by radiations that follow the capture. After studying several possible target elements (Gd, Yb), the focus of LENS now is on Indium as the target element of choice. Neutrino capture in 115In leads to a transition to the 613 keV isomeric state in 115Sn with the emission of a prompt electron e1. The capture transition can be tagged by the delayed coincidence with the cascade [(e/g)2 + g3](115 + 499) keV with a tag time of 4.76 m s. The In detector is particularly attractive because:

1) It offers the lowest ne capture threshold known yet, 114 keV , so that a nearly complete spectrum of the "standard candle" pp ne 's, indeed the total low energy solar ne spectrum containing the dominant flux can be observed;

2) a relatively high theoretical pp ne signal rate of ~1000 events /yr for a 10 ton In target is predicted;

3) a highly selective ne tag that offers a strong defense against the formidable backgrounds that are endemic in general to the very low-energy regime of the pp solar neutrino signals, and in particular in the case of indium, is created by the natural radioactivity of 115In (natural abundance=95.7%, half-life=6x1014 y);

4) the temporal and energy regimes of the tag are conducive to practical implementation.

Two possible In-LENS designs are under study: 1) a homogeneous design consisting only of In-containing modules arranged at high granularity (high number of parallelepiped cells) , and 2) a " hybrid" design in which In-containing cells at high granularity are interspersed with In-free detection volumes at low granularity.

The efforts of the collaboration in the years 2000-2003 concentrate on the following items: i) development of a liquid scintillator highly doped with In (previously Yb), having adequate light yields and transmittance to detect low energy events in a big detector, ii) installation undergound @LNGS of a shielded test facility to test module of detector, iii) Montecarlo computation of signal and noise (coming mainly from 115In b decay to 115Sn) to define granularity, tagging efficiency ecc.