Gamma-Screening with HPGe Well and Analysis of: SNOLAB, CarboTech Activated Carbon

Low Background Group

Date: 2022/04/01


This page provides the results of gamma counting with the Well HPGe detector at SNOLAB for the sample of interest. Collected spectra are analyzed with a custom C++-based set of scripts. It is a root-based implementation. Peak detection efficiencies are estimated from analyzing the Geant4 MC simulations of the sample. Radioactive contamination levels of the sample from U/Th/K/Co/Cs isotopes are listed per line and per decay chain below.

Sample description:

Sample of CarboTech - Activated Carbon - CMS-H2.
Sample was filled loosely in the test tube vial and placed inside the well of the detector.

Sample mass is 4.2g.

1. Event Rate Monitoring

An automatic cut removes runs which show an excess in the event rate in the overall energy region 100keV-3MeV w.r.t. the run average rate. Fig 1.1 shows the event rate.

Fig. 1.1: Trending plot of the overall energy region.

Plot range (keV): 0 - 200 200-1000 1000-2000 2000-3000

Fig. 2: Energy Spectrum for different ROI
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2. Rate Calculation

"Standard" gamma-lines of interest include lines from isotopes in the 238U, 235U, 232Th, 40K, 60Co, and 137Cs.

For each line of interest, we set a window in energy whose lower and upper limits are given by the integer channels nearest to C(E- fW(E)) and C(E+ fW(E)). This window is highlighted in yellow on the plots. The number of counts N(E) in this window is determined by counting.

Per line, we consider two background contributions:

  1. The peak background from the counting system that was measured by long-duration counting with no sample. The number of counts of this type in the peak under consideration will be called Bsystem(E).
  2. The second type of background that may appear in the peak region is associated with the sample, such as the Compton background produced when gamma rays at higher energy deposit a fraction of their energy in the window at the peak. The number of background events of this type is Bcontinuum(E), it is measured by calculating the average number of counts in the windows above and below the peak window of equal width. Those windows are highlighted in green in the plots.
The number of events ascribed to the signal is given by Nsignal(E) = N(E) - Bsystem(E) - Bcontinuum(E). As a special case there will sometimes be a peak from some other isotope that occurs in either the window above or the window below the peak. In such cases the window with this extraneous peak is neglected and only one window contributes to the determination of the continuum background. That region will be highlighted in red on the plots.
Line of interest (keV): 295 351 609 1120 1764 46 143 163 185 205 911 969 238 727 583 860 2614 1461 1173 1332 661
Isotope: Pb-214 Bi-214 Pb-210 U-235 Ac-228 Pb-212 Bi-212 Tl-208 K-40 Co-60 Cs-137
Chain: U-238 chain U-235 chain Th-232 chain Individual Isotopes

Fig. 3: Energy spectra per line. Yellow region highlights the counting region for N(E). The green side-bands represent the windows in which the energies are above and below the signal region.
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