Line-spectroscopic approach to study of solar neutrinos by cryogenic bolometry of charged and neutral current reactions onLi7

Abstract

The astrophysics and the particle physics of solar neutrinos can be tested independently by two line signals induced by neutral (NC) and charged current (CO neutrino reactions on ^Li, in a highresolution, bolometric, 4-ton LiF detector. The NC 7-ray line signal measures the flux of neutrinos of all flavors with £'v> 0.478 MeV, setting a firm empirical basis for the model-dependent links of the solar reaction chains. The CC signal from the model-independent '"pep'" line is a "yes/no" signal for flavor conservation. The shape of the CCipep) line is a direct thermometric probe of the solar core.PACS numbers: 96.60. Kx, 25.30.Pt, 95.55.Vj The unresolved status of the solar neutrino problem despite data from four major experiments strongly encourages the development of detectors with which the underlying astrophysics and particle physics can be tested independently.In this spirit, we explore a new directcounting approach based on low-energy neutral (NC) and charged current (CC) transitions in ^Li [1], which promise information of unique importance because of their low transition thresholds and the basic difference in their flavor sensitivity. The NC channel in ^Li detects neutrinos of all flavors from all the major modeldependent solar reactions, thus providing a global test of astrophysical models of the Sun. The CC channel in ^Li is tuned only to electron neutrinos (v^) from the basic p-\-e-\-p reaction in the Sun, which produces a modelindependent, monoenergetic, ''standard" flux ideal for a basic general test of nonstandard neutrino models.In the proposed experiment, the two reactions produce sharp line signals, a feature of key importance for signal identification above background. Timely developments in cryogenic bolometry which demonstrate the feasibility of keV-class energy resolution in LiF crystals [2] offer a matching high-resolution detection technique. Preliminary signal/background considerations given here show that a relatively small 4-ton solar detector may yield decisive data in one year. A LiF detector will focus on three major results: the NC and the CC line signal rates and the line shape profile of the CC signal. We outline below the significance of these objectives and broadly identify basic design criteria for a LiF detector.The CC reaction Ve -^ ^Li-> e ~ 4-''Be produces e ~ signals reflecting the solar Ve spectrum. The only line signal (at 0.580 MeV) arises from the 1.442 MeV v^ line from the solar pep reaction, a simple variant of the basic pp reaction. The Ve (pep) flux at the Sun is determined (±5%) by basic nuclear physics [3], not by the astrophysical model details. Observation of a smaller CCipep) signal at the Earth can be caused only by nonconservation of Ve flavor by various means, especially conversion in solar matter or vacuum to another (active or sterile) flavor. In the parameter spaces currently allowed by experiment, all the proposed nonstandard neutrino models predict a zero or severely reduced Ve (pep) flux at the Earth. Thus, the CC(pep) line signal in ^Li is a unique "yes...

What can be learned by measuring the fluxes of the7Beand thepepsolar neutrino lines?

“…In this respect, neutral current excitations are similar to radiochemical solar neutrino experiments; they measure the sum of the reaction rates due to all the neutrino sources above the energy threshold. It seems likely [37], with our current expectations for the low energy fluxes of solar neutrinos (based upon the standard solar model and existing neutrino oscillation solutions), that the neutral current excitation of 7 Li is dominated by the higher-energy 7 Be branch. However, to verify or improve these expectations, additional observational information must be obtained from neutrino absorption experiments or from neutrino-electron scattering experiments that can identify the fluxes from individual neutrino sources.…”

“…As possible targets, they suggested 11 B, 40 Ar, and 35 Cl, all of which are sensitive to the continuum neutrinos from 8 B beta decay in the sun. In a more recent study, Raghavan, Raghavan, and Kovacs [37] proposed using a 4 ton LiF detector with potentially keV energy resolution to study neutral current and charged current solar neutrino reactions. Most recently, Bowles…”

“…The neutral current matrix element for reaction ( 9) is large and is known accurately [37] since the matrix element for reaction (9) is, by isotopic spin invariance, the same as the matrix element for the observed superallowed decay from the ground state of 7 Be to the first excited state of 7 Li.…”

“…As emphasized by previous authors [36,37], the principal role at present of a neutral current excitation experiment is to provide a measure of the total neutrino reaction rate, independent of neutrino flavor, for the entire solar neutrino spectrum. Since neutral current excitation experiments cannot be used at present to isolate the contribution of an individual line, we will not discuss these excitation experiments further in this paper.…”

“…The superallowed matrix element for this transition is large and is known accurately. Raghavan, Raghavan, and Kovacs[37] have suggested that a practical solar neutrino experiment could be carried out with a 4 ton LiF detector. The fluorine in a LiF detector could make possible a simultaneous study at high energy resolution of the higher-energy 8 B solar neutrinos[40].Table IV presents, for different neutrino oscillation scenarios, the best-estimates and the 95% C.L.…”