Tests and calibrations of nuclear track detectors (CR39) for operation in high neutron flux

Abstract

The neutron beams produced by the Soreq Applied Research Accelerator Facility (SARAF) operating with the liquid lithium target (LiLiT) present scientific opportunities in nuclear astrophysics. Of interest are measurements (in our setup) of 1.5 to 3.0 MeV alpha-particles and 1.5 MeV protons, emanating from the interaction of neutrons with ^{7}Be. The deduced cross sections, are important for understanding the cosmological “Primordial ^{7}Li Problem.” However, major experimental challenges arise when measuring such low-energy charged particles due to the high flux neutron beams produced by the LiLiT (up to ∼5×10^{10} n/sec/cm^{2}). We present a study of the operation of CR39 nuclear track detectors (NTD), in such a high neutron flux. The CR39 plates were energy calibrated with 3.18 MeV alpha-particles from a ^{148}Gd standard radioactive source, and by using Rutherford backscattering of 1.5 MeV alpha-particles as well as 1.5 MeV protons from a thin (100 μg/cm^{2}) gold foil. The plates were etched in a 6.25 N NaOH solution for 30 minutes at 90^{∘}C to produce micron-sized circular pits. The alpha (3.18 MeV) and proton (1.5 MeV) etch efficiencies were measured to be 93.3±6.1% and 8.7±1.3%, respectively. After etching, the plates were scanned with a fully automated microscope. A segmentation algorithm that addressed the challenges posed by the intense neutron beam was developed. A measurement of the interaction of cold neutrons with CR39 showed that the ^{17}O(n,α)^{14}C reaction inside the CR39 was a major source of background. Since the tracks were formed inside the CR39, this background extended up to the full energy deposit of ∼2.1 MeV. This neutron-beam-induced background inside the CR39 plates, observed here for the first time, will most likely be the limiting factor for observing ∼2 MeV alpha-particles with CR39, in measurements of small cross sections (below a few mb) of (n,α) reactions. However, in measurements of larger cross sections, this ^{17}O(n,α) “background line” can be used as an internal self-calibration of alpha-particles with energies up to ∼2.1MeV, and thus allow to correct for piece-to-piece variations in the energy-to-track size calibration of CR39 plates.