Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Zone refining of ultra-high purity sodium iodide for low-background
  detectors   There has been a growing interest in ultra-high purity, low-background
NaI(Tl) crystals for dark matter direct searches. Past research indicates that
zone refining is an efficient and scalable way to purify NaI. In particular, K
and Rb -- two elements with radioisotopes that can cause scintillation
backgrounds -- can be efficiently removed by zone refining. However, zone
refining has never been demonstrated for ultra-high purity NaI which became
commercially available recently. In this article, we show that many common
metallic impurities can be efficiently removed via zone refining. A numerical
model for predicting the final impurity distribution was developed and used to
fit the ICP-MS measurement data to determine the segregation coefficient and
the initial concentration. Under this scheme, the segregation coefficient for K
is estimated to be 0.57, indicating that zone refining is still effective in
removing K from ultra-high purity NaI. As zone refining tends to move the
impurities to one end, elements with concentrations too low to be measured
directly in the unprocessed powder can potentially be detected in the end due
to the enrichment. We also present an analysis technique to estimate the
initial concentrations of impurities with partial data, which effectively
enhances the sensitivity of the spectrometer. Using this technique, the initial
concentration of $^{85}$Rb is estimated to be between 5 ppt and 14 ppt at 90%
CL, at least 14 times lower than the detection limit of ICP-MS and 7 times
lower than the current most stringent limit set by the DAMA collaboration by
direct counting of radioactive $^{87}$Rb. These results imply that zone
refining is a key technique in developing next-generation, NaI-based crystal
scintillators for dark matter direct detection.

13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
18
The categories of this paper are: econ
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
14
The categories of this paper are: math-ph
7
The categories of this paper are: hep-th
30
The categories of this paper are: q-alg
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex