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Register a new userOn the Analytic Estimation of Radioactive Contamination from Degraded Alphas
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Richard W. Kadel
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The high energy spectrum of alpha particles emitted from a single isotope uniformly contaminating a bulk solid has a flat energy spectrum with a high end cutoff energy equal to the maximal alpha kinetic energy ($T_{alpha}$) of the decay. In this flat region of the spectrum, we show the surface rate $r_btext{,(Bq/keV-cm}^{2})$ arising from a bulk alpha contamination $rho_b$ (Bq/cm$^3$) from a single isotope is given by $r_b =rho_b Delta R/ 4 Delta E $, where $Delta E = E_1-E_2>0 $ is the energy interval considered (keV) in the flat region of the spectrum and $Delta R = R_2-R_1$, where $R_2$ ($R_1$) is the amount of the bulk material (cm) necessary to degrade the energy of the alpha from $T_{alpha}$ to $E_2$ ($E_1$). We compare our calculation to a rate measurement of alphas from $^{147}$Sm, ($15.32%,pm,0.03%$ of Sm($nat$) and half life of $(1.06,pm,0.01)times,10^{11} text{yr}$, and find good agreement, with the ratio between prediction to measurement of $100.2%pm 1.6%,text{(stat)}pm 2.1%,text{(sys)}$. We derive the condition for the flat spectrum, and also calculate the relationship between the decay rate measured at the surface for a [near] surface contamination with an exponential dependence on depth and an a second case of an alpha source with a thin overcoat. While there is excellent agreement between our implementation of the sophisticated Monte Carlo program SRIM and our intuitive model in all cases, both fail to describe the measured energy distribution of a $^{148}$Gd alpha source with a thin ($sim200mu$g/cm$^2$) Au overcoat. We discuss possible origins of the disagreement and suggest avenues for future study.
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A strontium iodide crystal doped by europium (SrI2(Eu)) was produced by using the Stockbarger growth technique. The crystal was subjected to a characterization that includes relative photoelectron output and energy resolution for gamma quanta. The intrinsic radioactivity of the SrI2(Eu) crystal scintillator was tested both by using it as scintillator at sea level and by ultra-low background HPGe gamma spectrometry deep underground. The response of the SrI2(Eu) detector to alpha particles (alpha/beta ratio and pulse shape) was estimated by analysing the 226Ra internal trace contamination of the crystal. We have measured: alpha/beta=0.55 at E_alpha=7.7 MeV, and no difference in the time decay of the scintillation pulses induced by alpha particles and gamma quanta. The application of the obtained results in the search for the double electron capture and electron capture with positron emission in 84Sr has been investigated at a level of sensitivity: T_1/2 sim 10^{15}-10^{16} yr. The results of these studies demonstrate the potentiality of this material for a variety of scintillation applications, including low-level counting experiments.
We present measurements of bulk radiocontaminants in the high-resistivity silicon CCDs from the DAMIC at SNOLAB experiment. We utilize the exquisite spatial resolution of CCDs to discriminate between $alpha$ and $beta$ decays, and to search with high efficiency for the spatially-correlated decays of various radioisotope sequences. Using spatially-correlated $beta$ decays, we measure a bulk radioactive contamination of $^{32}$Si in the CCDs of $140 pm 30$ $mu$Bq/kg, and place an upper limit on bulk $^{210}$Pb of $< 160~mu$Bq/kg. Using similar analyses of spatially-correlated bulk $alpha$ decays, we set limits of $< 11$ $mu$Bq/kg (0.9 ppt) on $^{238}$U and of $< 7.3$ $mu$Bq/kg (1.8 ppt) on $^{232}$Th. The ability of DAMIC CCDs to identify and reject spatially-coincident backgrounds, particularly from $^{32}$Si, has significant implications for the next generation of silicon-based dark matter experiments, where $beta$s from $^{32}$Si decay will likely be a dominant background. This capability demonstrates the readiness of the CCD technology to achieve kg-scale dark matter sensitivity.
A simple model for the estimation of the light yield of a scintillation detector is developed under general assumptions and relying exclusively on the knowledge of its optical properties. The model allows to easily incorporate effects related to Rayleigh scattering and absorption of the photons.The predictions of the model are benchmarked with the outcomes of Monte Carlo simulations of specific scintillation detectors. An accuracy at the level of few percent is achieved. The case of a real liquid argon based detector is explicitly treated and the predicted light yield is compared with the measured value.
To improve the ability of particle identification of the RIBLL2 separator at the HIRFL-CSR complex, a new high-performance detector for measuring fragment starting time and position at the F1 dispersive plane has been constructed and installed, and a method for achieving precise Br{ho} determination has been developed using the experimentally derived ion-optical transfer matrix elements from the measured position and ToF information. Using the high-performance detectors and the precise Br{ho} determination method, the fragments produced by the fragmentation of 78Kr at 300 MeV/nucleon were identified clearly at the RIBLL2-ETF under full momentum acceptance. The atomic number Z resolution of {sigma}Z~0.19 and the mass-to-charge ratio A/Q resolution of {sigma}A/Q~5.8e-3 were obtained for the 75As33+ fragment. This great improvement will increase the collection efficiency of exotic nuclei, extend the range of nuclei of interest from the A<40 mass region up to the A~80 mass region, and promote the development of radioactive nuclear beam experiments at the RIBLL2 separator.
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