Design for a practical laboratory detector for solar axions

Abstract

We present a practical design for a detector sensitive to axions and other light particles with a two-photon interaction vertex. Such particles would be produced in the solar interior by Primakoff conversion of blackbody photons and could be detected by their reconversion into x rays (average energy about 4 keV) in a strong laboratory magnetic field. An existing large superconducting magnet would be suitable for this purpose. The transition rate is enhanced by filling the conversion region with a buffer gas (H, or He). This induces an effective photon mass (plasma frequency) which can be adjusted to equal the axion mass being searched for. Axion-photon conversion is then coherent throughout the detector volume for all axion energies. Axions with mass in the range 0.1 eV 5 m , 5 5 eV can be detected using gas pressures of 0.1-300 atm. Axions with the standard coupling strength to photons would give counting rates of lo-'-10 s e c ' over this mass range. The search would definitively test one of the only two regions of axion parameters not excluded by astrophysical constraints.fa which arises as the scale at which the global chiral U(1) symmetry postulated by Peccei and Quinn is spontaneously broken. Although it was originally thought that fa should be identified with the electroweak scale fweak =250 GeV (standard axions), the axion decay constant can take on, in principle, any value between the GeV range and the Planck scale of 1019 GeV. Since the interaction strength of axions with matter and radiation scales as 1 /f,, axion models with f, >> f are generically referred to as "invisible a~i o n s . ' '~-~ Depending upon the assumed value for f,, the existence of axions would lead to a startling variety of phenomenological consequences in particle physics, astrophysics, and cosmology. The combined evidence from these different fields now excludes large ranges of fa values, leaving only two rather narrow windows in which axions might still exist. Thus it has become a compelling task to attempt the detection of axions in these remaining ranges of parameters, or to exclude the Peccei-Quinn mechanism once and for all as not being realized in nature. 2089

Detecting Solar Axions Using Earth’s Magnetic Field

“…Solar axions have an average energy ω a ≃ 4 keV [10], which upon conversion in the magnetosphere will turn into x-ray photons of the same energy. The absorption length λ x for 4-keV x-rays in the Earth's atmosphere is about 10 cm at sea-level [11].…”

“…For this plot we integrated the conversion probability as given in Eq. (4) folded with solar axion spectrum [5,10] over axion energies from 1 − 10 keV.…”

“…The axion to photon conversion probability, in a transverse magnetic field of strength B, is given by [10,12] …”

Search for 14.4 keV solar axions emitted in the M1-transition of57Fe nuclei with CAST

“…This restricts the sensitivity of a helioscope to a specific range of axion masses, e.g., for a 10 m long magnet and axion energy of ∼10 keV the coherence condition sets the limit of m a ∼ < 0.03 eV on the axion mass, up to which such an experiment is sensitive. However, coherence can be maintained for higher axion masses if the conversion volume is filled with a buffer gas such as helium [13]. In this case, photons acquire an effective mass m γ whose value is determined by the gas pressure, and the axion-photon momentum difference becomes q = m 2 γ − m 2 a /2E a .…”

“…Such axions would have a continuous energy spectrum peaked near the mean energy of 4.2 keV and dying off above ∼10 keV. Most of the experiments that have been designed to search for these axions are based on the coherent axion-to-photon reconversion in a laboratory transverse magnetic field (the axion helioscope method [11][12][13][14][15][16][17][18][19][20][21]), or in the intense Coulomb field of nuclei in a crystal lattice of the detector (the Bragg scattering technique [22][23][24][25][26]). …”