Detection of atomic nuclear reaction products via optical imaging

In this paper we propose a new method for measuring the cross section of low-yield nuclear reactions by capturing the products in a cryogenically frozen noble gas solid. Once embedded in the noble gas solid, which is optically transparent, the product atoms can be selectively identified by laser-induced fluorescence and individually counted via optical imaging to determine the cross section. Single-atom sensitivity by optical imaging is feasible because the surrounding lattice of noble gas atoms facilitates a large wavelength shift between the excitation and the emission spectrum of the product atoms. The tools and techniques from the fields of single-molecule spectroscopy and superresolution imaging in combination with an electromagnetic recoil separator, for beam and isotopic differentiation, allow for a detection scheme with near-unity efficiency, a high degree of selectivity, and single-atom sensitivity. This technique could be used to determine a number of astrophysically important nuclear reaction rates.

Figure 1

Graphical representation of the SAM concept (not to scale; noble gas film thickness exaggerated for clarity). Left: Basic capturing scheme without a recoil separator. The nuclear reaction takes place in inverse kinematics, where the recoiling products and low-intensity unreacted beam are captured in a noble gas film. Right: Schematic of optical excitation and fluorescence imaging of the captured recoil atoms onto a CCD camera. The excitation light is separated from the emitted fluorescence light using optical bandpass filters.

Figure 2

Generic energy level diagram for a three-level system with ground state $a$, excited state $b$, and metastable state $m$. Excitation is labeled by the double arrow; emission, by single arrows; and nonradiative transfer, by the dashed arrow.

Figure 3

Off-resonance suppression factor ${\sigma }^i\left({\nu }_{\gamma }\right)/{\sigma }_0^i$ for Gaussian absorption lineshapes. Far from resonance, the probability of excitation decreases exponentially, suppressing the probability of impurity fluorescence.