Theory for the direct detection of solar axions by coherent Primakoff conversion in germanium detectors

Abstract

It is assumed that axions exist and are created in the sun by Primakoff conversion of photons in the Coulomb fields of nuclei. Detection rates are calculated in germanium detectors due to the coherent conversion of axions to photons in the lattice when the incident angle fulfills the Bragg condition for a given crystalline plane. The rates are correlated with the relative positions of the sun and detector yielding a characteristic recognizable sub-diurnal temporal pattern. A major experiment is proposed based on a large detector array.

CUORE: a cryogenic underground observatory for rare events

“…Concerning solar axion detection the sensitivity of CUORE will depend strongly on the threshold reached. In the case of a 5 keV threshold and a 2 keV FWHM low energy resolution the g lim γγ sensitivity is of about 0.7 10 −9 GeV −1 [26].…”

“…Crystals of T eO 2 have a tetragonal < 110 > structure, and are grown along the (001) axis. The two axes normal to this axis are crystallographically equivalent, a fact relevant to their use in the search for solar axions discussed later [26]. The surface hardness is not the same for all sides, which complicates the crystal polishing.…”

Particle dark matter and solar axion searches with a small germanium detector at the Canfranc Underground Laboratory

“…In the process the energy of the outgoing photon is equal to that of the incoming axion. The axion production rate in the Sun -through Primakoff conversion of the blackbody photons in the solar plasma-can be easily estimated [18,8] within the standard solar model, resulting in an axion flux of an average energy of about 4 keV that can produce detectable X-rays when reconverted again in a crystal detector. Depending on the direction of the incoming axion flux with respect to the planes of the crystal lattice, a coherent effect can be produced when the Bragg condition is fulfilled, with the ensuing strong enhancement of the signal.…”

“…2, as well as the cross-section of the process and appropriate crystallographic information, we have calculated the expected axion-to-photon conversion count rate R(t) in a germanium detector (See ref. [19,8] for further details). An example of the expected count rate is shown in figure 3 as a function of time during one day.…”

The CUORICINO and CUORE experiments

“…Detectors of single crystals provide a simple mechanism for solar axion detection based on the Bragg coherent Primakoff conversion [28]. This method has already been employed in two germanium experiments [29] and by the DAMA collaboration [30].…”