COMPARISON OF THE ACTIVITY RESULTS
MEASURED IN THE WELL-TYPE HPGe
DETECTOR WITH AND WITHOUT TRUE
COINCIDENCE SUMMING CORRECTIONS
A. N. Solmaz1, H. Yücel2, E. Köse1, D. Bor1
1Institute of Nuclear Sciences of Ankara University 2Turkish Atomic Energy Authority
Scope
In cases of close counting geometry
conditions and cascading transitions in decay
schemes, determining the true coincidence
summing corrections for the measured
activities are essential for gamma-ray
spectrometry with HPGe well detector.
Objectives
The objective of this study is to determine true
coincidence summing correction factors for
238
U and
232Th
210Pb
212Pb
214Pb
228Ra
226Ra
228Th
TRUE COINCIDENCE SUMMING (TCS)
METHODOLOGY
TCS occurs for radionuclides emitting two or more photons in
sequence within the resolving time of the spectrometer.If this
happens a pulse will be recorded which represents the sum of the
energies of the individual photons.
True coincidence summing results in losses/increases in peak areas
As it is a function of a solid angle subtended at the detector by the
source, TCS effect increases with decreasing source-detector
distance and with increasing detector effciency.
(However, beyond a certain distance, which depends upon the detector size, TCS losses will be negligible in practice. dsource todetector =10-12 cm, Rel. Eff. =15-30%)Materials and Methods
HPGe well-type detector
p-type closed-ended coaxial Relative efficiency: % 44.8
Energy resolution: 2 keV at 1332.5 keV of 60Co
Multinuclide Standard Source
(Isotope products Lab,traceable to PTB)
Energy range: 47 keV to 1836 keV
210Pb, 109Cd, 57Co, 123mTe, 51Cr , 113Sn, 85Sr, 137Cs , 88Y, 60Co
12 CRMs (purchased from CANMET,Canada) were used
Certified Reference Materials Used
Material
description Code Concentration (%) Certified activity values for radionuclides (Bq kg
-1 )
Thorium
amount Uranium amount
210Pb 226Ra 228Ra 228Th
Rare
Earth-Thorium ore OKA-2 2.893±0.058 0.02186±0.00082 - - - -Uranium-Thorium ore DL-1a 0.0076±0.0004 0.0116±0.0003 1400±20 1400±40 - -Uranium-Thorium ore DH-1a 0.091±0.003 0.2629±0.0003 30800±900 31500±1100 - -Uranium tailing sample UTS-1 - - 3250±35 3670±38 680±16 710±17 Uranium tailing sample UTS-2 - - 4550±42 5600±46 1000±20 920±19 Uranium tailing sample UTS-3 - - 12600±70 13300±71 - -Uranium ore BL-2 - 0.453±0.005 - - - -Uranium ore BL-3 - 1.02±0.01 - - - -Uranium ore BL-2a - 0.426±0.002 - - - -Uranium ore BL-4a - 0.1248±0.0007 15600±500 15500±500 - -Uranium ore CUP-1 - 0.128±0.002 - - - -Uranium ore RL-1 - 0.201±0.006 - - -
-Measurement Setup
Two “close” counting geometries
(a) Cylinder beaker (CB), 3.6 cm filling height, 4.3 cm internal diameter
(b) Tube (TW), 3.5 cm filling height, 1.4 cm internal diameter
The Method for TCS Determination
Peak to Total Efficiency Calibration Curve
The coincidence-free point sources were measured on the top of the detector
end-cap.
241Am (59 keV), 109Cd (88 keV), 57Co (122 keV), 137Cs (662 keV)
P/T values were determined by the logarithmic polynomial function using Genie2000
The Method for TCS Determination
After determining the P/T ratios for counting geometries the
fitted parameters were introduced into “TrueCoinc” calculation
program together with peak efficiency values for specific
geometries.
Then this “TrueCoinc” program was employed to calculate TCS
factors using the existing ENDSF database (a library of
National Nuclear Data Center, BNL) for the input nuclide, which
can be read from a CD-ROM version of the Table of Isotopes.
TrueCoinc program
a ln (E ) a ) E ( ln n 3 i 1 i i 0 p
a a ln E a ln E
T P ln 0 1 2 2 TCS Correction Factors
TCS correction factors were calculated for all analytical
peaks from
238U and
232Th decay series.
Nuclide Energy(keV) probability (%)γ-ray emission True coincidence factors Cylinder on the end-cap
(CB) Tube in the well (TW)
210Pb 46.54 4.25 1.000±0.015 1.000±0.010 212Pb 238.63 43.3 0.998±0.035 0.999±0.035 214Pb 295.21 18.5 1.029±0.003 0.999±0.008 228Ac 338.32 11.2518 0.971±0.003 0.910±0.007 214Pb 351.92 35.8 0.997±0.005 0.996±0.009 208Tl 583.19 30.58 0.792±0.020 0.806 ±0.021 214Bi 609.31 44.7905 0.830±0.004 0.829±0.007 228Ac 911.20 26.6 0.915±0.006 0.935±0.010 228Ac 968.97 16.1728 0.927±0.003 0.935±0.008 214Bi 1120.29 14.7968 0.741±0.015 0.722±0.015
Activity Measurements
Sample Code TCS Corrected/Uncorrected Pb-210 Pb-212 Pb-214 Ra-226 Ra-228 Th-228 OKA-2 1.00 1.00 0.99 1.15 1.06 1.23 DL-1a 1.00 1.00 1.00 1.13 1.07 0.94 DH-1a 1.00 0.96 0.98 0.86 1.03 0.87 UTS-1 1.00 1.00 0.99 1.07 1.07 0.85 UTS-2 1.00 1.00 0.99 1.01 1.07 0.85 UTS-3 1.00 1.00 1.00 0.83 - -BL-2 1.00 - 0.99 1.13 - -BL-3 1.00 1.00 0.99 1.12 - -BL-2a 1.00 - 0.99 1.12 - -BL-4a 1.00 1.00 0.99 1.11 1.06 1.07 CUP-1 1.00 1.00 0.99 1.05 1.05 1.14 RL-1 1.00 1.00 1.00 1.17 - - Pb-210,Pb-212,Pb-214,Ra-226,Ra-228,Th-228 acitivities were calculated with TCS and
without TCS correction for CB and TW counting geometries using also self absorption corrections.
Mean values were obtained from measurements using CB and TW counting geometries. The ratios of corrected/uncorrected acitivities for TCS effects were calculated.
Activity Measurements
The ratios of certified/measured activities were calculated for 210Pb, 212Pb, 214Pb, 226Ra, 228Ra, 228Th.
Sample
Code Activity (Bq kg
-1)
Pb-210 Cer./Mea. Pb-212 Cer./Mea. Pb-214 Cer./Mea. Ra-226 Cer./Mea. Ra-228 Cer./Mea. Th-228 Cer./Mea . OKA-2 118287±4195 0.99 2777±71 0.97 3574±74 0.76 74044±412 1.96 117785±3774 1.00 DL-1a 1426±38 0.98 322±11 0.96 1441±37 0.97 1467±32 0.95 354±4 0.87 247±7 1.25 DH-1a 29342±231 1.05 3471±122 1.06 33044±835 0.95 30150±646 1.04 3699±27 1.00 2928±86 1.26 UTS-1 5217±45 0.62 3872±99 0.95 3589±76 1.02 595±5 1.14 406±12 1.75 UTS-2 8887±97 0.51 5461±140 1.03 5191±111 1.08 762±6 1.31 537±16 1.71 UTS-3 38236±421 0.33 12853±465 1.03 12146±383 1.09 BL-2 51302±1857 1.09 54738±1663 1.02 BL-3 129092±4672 0.98 130634±3912 0.96 BL-2a 55278±2001 0.95 54095±1643 0.97 BL-4a 17870±460 0.87 17907±383 0.87 CUP-1 15975±578 0.99 15773±473 1.00 RL-1 23394±858 1.06 23161±676 1.07
Conclusion
For the “close counting” geometry conditions:
(1) on the end-cap (2) in the well of the detector;
No TCS correction is necessary for 210Pb at 46.5 keV because of its decay
scheme properties.
In the detector well, the TCS factors for the CRMs measured are
substantially higher such as for 208Tl at 583.2 keV (583.2-2614.5 keV
summing) and 214Bi at 1120.3 keV (609.3-1120.3 keV summing) due to their
decay properties.
However the peaks at 295.2 keV (214Pb) and 351.9 keV (214Pb) for 226Ra and
238.6 keV (212Pb) for 232Th seem to be suitable for the analytical purposes in
