Difference between revisions of "Gamma Spectroscopy (NORM and TENORM)"
(→Introductory tasks:) 

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# Control that the high detector tension is 500550 V.  # Control that the high detector tension is 500550 V.  
# Adjust the gain so that the detection system covers the energy region 02700 keV by using a source of 60Co (gamma energies of 1173 keV and 1332 keV) in close geometry, i.e. at the detector surface. You should now be able to se the sum peak at 2505 keV to the right in the spectrum.  # Adjust the gain so that the detection system covers the energy region 02700 keV by using a source of 60Co (gamma energies of 1173 keV and 1332 keV) in close geometry, i.e. at the detector surface. You should now be able to se the sum peak at 2505 keV to the right in the spectrum.  
−  #Carry out an energy calibration of the lower half of the spectrum by using the two radionuclides  +  #Carry out an energy calibration of the lower half of the spectrum by using the two radionuclides <sup>241</sup>Am (59.6 keV) and <sup>137</sup>Cs (661.2 keV). 
−  #It is possible that we have to perform another calibration later with the two radionuclides  +  #It is possible that we have to perform another calibration later with the two radionuclides <sup>133</sup>Ba (highest peak at 356 keV) and <sup>60</sup>Co (highest peak at 1332.4 keV)<br> 
<br>  <br>  
−  <br>  +  <br> 
===== Determination of source strength of <sup>241</sup>Am in fire alarms =====  ===== Determination of source strength of <sup>241</sup>Am in fire alarms ===== 
Revision as of 14:21, 28 June 2012
Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
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We will mainly use the digiBase system from Ortec which is coupled to 2x2” NaI(Tl) detectors mounted in lead shields.
Towards the end of the exercise we will also demonstrate HpGe detectors.
Contents
Introductory tasks:
 Turn on the PC and load the Maestro program
 Control that the high detector tension is 500550 V.
 Adjust the gain so that the detection system covers the energy region 02700 keV by using a source of 60Co (gamma energies of 1173 keV and 1332 keV) in close geometry, i.e. at the detector surface. You should now be able to se the sum peak at 2505 keV to the right in the spectrum.
 Carry out an energy calibration of the lower half of the spectrum by using the two radionuclides ^{241}Am (59.6 keV) and ^{137}Cs (661.2 keV).
 It is possible that we have to perform another calibration later with the two radionuclides ^{133}Ba (highest peak at 356 keV) and ^{60}Co (highest peak at 1332.4 keV)
Determination of source strength of ^{241}Am in fire alarms
Experimental procedure:
 Mount a metal platesupported 241Am fire alarm source on an aluminium ring covered with transparent tape on both sides.
 Mount the standard source of 241Am on an aluminium ring with adhesive tape only on one side.
 Use the two sources and find a suitable counting distance between source and detector (start with a distance of about 5 cm) so that the counting rate is kept at a reasonable and not too high level,  ask advisor.
 Mount the fire alarm source at the decided distance and count the source for 300 s (livetime).
 Store the spectrum in a dedicated folder on the PC.
 Mount the standard source in the same position and count for 300 s (livetime).
 Store the spectrum in the same folder.
 Use the Maestro program and integrate the photopeaks in both spectra to derive at their respective peak areas.
Calculations:
The standard source has a known activity A_{s,i} at a certain defined date. Let us denote the time from this date until today with t_{d} (decay time, in days). One finds the standard source strength today, A_{s,t,} by the formula:
Eqn. 1 
where λ = ln2/T_{1/2} for ^{241}Am. Since both sources may be regarded as “massless” point sources and the counting geometry is the same for both sources, the total counting efficiencies are identical. Hence , since we in addition perform comparative analysis where the activity of one source is known, knowledge of this total counting efficiency is not needed. We can then put up the following simple relation:
Eqn. 2 
Solved with respect to A_{x,t} we have:
Eqn. 3 
For the calculations: Look up the halflife of ^{241}Am from the nuclide chart and obtain the certified activity of the standard source and the certification date from the lab adviser.
Source 
Recorded number of counts per 300 s, S 
Original decay rate at certification time (Bq) 
Decay rate today (Bq) 
Standard 
S_{s}= 
A_{s,i}= 
A_{s,t}= 
Fire alarm 
S_{x}= 

A_{x,t}= 
Determination of concentration of KCl in socalled “health salt” – SELTIN
SELTIN is a popular table salt in Norway. It contains substantial amounts of KCl instead of NaCl. In this section we shall determine the specific activity and the activity concentration of 40K and the fraction of KCl in weight % in SELTIN
Molweight of KCl M_{KCl} (g/mol) 
Atomic weight of K
M_{K} (g/mol) 
Avogadro’s number
N_{A} 
Natural abundance of ^{40<7sup>K YK40 (%) }  Branching ratio of 1462 keV, Iabs (%) 
Halflife of ^{40}K
T_{1/2} (years) 
74.551 
39.1 
6.023 
10^{23} 0.0117 
10.7 
1.28  10^{9}
Remember in addition that the following relation is generally valid:
Eqn. 4 
where D = D_{K40} , N = N_{K40} and λ = λ_{K40} = ln2/(T_{1/2})_{K40}.
The general relation between total counting efficiency, decay rate and counting rate is:
Eqn. 5 
ε_{T} is composed of several subefficiencies like the intrinsic detector efficiency for this gamma energy (ε_{D}), the efficiency due to the sample shape and distance to detector (ε_{G}), called geometrical factor efficiency) and the fact that the branching ratio, I_{abs}, may be less than 100%, i.e. that only a fraction of the decay events leads to emission of the detected gamma ray. For the 1462 keV gamma ray from ^{40}K, I_{abs} is found in Table 9. The total counting efficiency for the gamma energy 1462 keV, ε_{D'G},is independent of the nuclide. Here, we shall use ^{40}K to determine this efficiency and has to take into account the value of I_{abs}. Then:
Eqn. 6 
When calculating the total counting efficiency for the 1462 keV gamma energy on the basis of recorded counting rate, one has to consider the fact that only 10.7% of the decays result in emission of a gamma ray for the ^{40}K 1462 keV gamma energy (I_{abs} = 10.7%). The counting efficiency is found from Eqn.6.
Since the sample weight and geometry are nearly identical for the two samples, we can suppose that the total counting efficiency, ε_{T}, is also the same for both samples. Therefore, the decay rate of ^{40}K in SELTIN is calculated by the formula:
Eqn. 7 
Solving this equation for D_{S} gives:
Eqn. 8 
Practical Procedure:
 Start the spectrometer with a preset livetime of 30 min for recording of background.
 Weigh in a counting sample of KCl (w_{KCl}) in a suitable lidcovered plastic box.
 Weigh a similar sample of SELTIN (w_{S}) in an identical counting box.
 If the background counting has not finished already, stop the spectrometer, note the counting time (livetime) t_{B} and store the spectrum in a dedicated file.
 Mount the KClsample, count for a preset time t_{KCl} and store the spectrum in a dedicated file after the spectrometer has stopped.
 Mount the SELTIN sample, count for a preset time t_{S} and store the spectrum in a dedicated file after end of counting.
Since the sample weight and geometry are nearly identical for the two samples, we can suppose that the counting efficiency is also the same for both samples.
Spectra handling and calculations:
Determine the net area (S±σ_{S}) for the 1462 keV gamma peak in the three recorded spectra. Then carry out the calculations to fill into the table below:
Operations 
Result 
Net area of 1462 keV in background spectrum, S_{B}±σ_{SB}  
 Counting time t_{B}  
 Background counting rate, R_{B}±σ_{SB}  
Net area of 1462 keV in KCl spectrum, S_{KCl}±σ_{SKCl}  
 Counting time t_{KCl}  
 Backgroundcorr. KCl sample counting rate, R_{KCl}±σ_{RKCl}  
Net area of 1462 keV in SELTIN spectrum, S_{S}±σ_{SS}  
 Counting time t_{S}  
 Background corr. SELTIN sample counting rate, R_{S}±σ_{RS}  


Net weight of the KClsample, w_{KCl}  
 Weight of K in the KCl sample, w_{K,KCl}  
 Number of Katoms in the KClsample, N_{K,KCl}  
 Number of ^{40}K atoms in the KCl sample, N_{K40,KCl}  
 Calculated decay rate of ^{40}K in the KCl sample, D_{K40,KCl}  
 Specific activity of ^{40}K in the KCl sample, A_{s,KCl}  
 Activity concentration of ^{40}K in the KCl sample, A_{c,KCl}  
 Total counting efficiency of ^{40}K in KCl sample,ε_{T,KCl} </sup></sup> 

 Total counting efficiency of 1462 keV in KCl sample, ε_{DG,KCl}  


Net weight of the SELTIN sample, w_{S}  
 Decay rate of ^{40}K in the SELTIN sample, D_{K40,S}  
 Activity concentration of ^{40}K in the SELTIN sample, A_{c,S}  
 Number of ^{40}Katoms in the SELTIN sample, N_{K40,S}  
 Number of Katoms in the SELTIN sample, N_{K,S}  
 Weight% of KCl in SELTIN  
Gamma spectroscopy on thorium and uranium minerals and progeny
Procedure:
 Record gamma spectra with the NaI(Tl) detector for the following samples and store the spectra in dedicated files:
a. Uranium oxide of natural uranium isotope composition.
b. Uranium with about 1.5% enrichment in ^{235}U.
c. Uranium with about 20% enrichment in ^{235}U.
d. Th(NO_{3})_{2}.  Determine the energies of the strongest peaks and find the corresponding radionuclei. Use the attached spectra and tables.
 Plot the spectra and attach the plots to this report with energies and radionuclides indicated on the plot.
 Determine the energy counting efficiency, ε_{DG}, for the energies in the range around 600 keV for a certain counting geometry (ask lab supervisor) by using a standard source of ^{137}Cs (661.6 keV).
 Prepare a small piece of native radioactive rock, record its weight and accumulate a gamma spectrum in this counting geometry.
 Plot the spectrum and find the corresponding radionuclei. Use the attached spectra and tables. Attach the plot to this report.
 If the activity in the rock is predominantly from ^{238}U and progeny: Integrate the peak at 609 keV (belonging to ^{214}Bi) and determine the activity concentration of ^{214}Ra in the rock. Find necessary data for Iabs in the attached tables.
 If the activity in the rock is predominantly from ^{232}Th and progeny: Integrate the peak at 583 keV (belonging to ^{208}Tl) and determine the activity concentration of ^{208}Bi in the rock. Find necessary data for Iabs in the attached tables.
 Make a table of data used and the results achieved for the rock sample.
 Prepare a counting sample of scale from the North Sea petroleum operations in a small lidcovered plastic container, and determine the net weight.
 Accumulate a gamma spectrum of the scale sample and determine the energies of the strongest peaks. Attach spectrum.
 Integrate the 609 keV peak of ^{214}Bi and determine the activity concentration of ^{214}Bi in the sample.
 Make a table of data used and the results achieved for the scale sample.
 Perform an energy calibration of a HpGesemiconductor detector. Let the lab supervisor demonstrate the superior energy resolution of this detector for some of the samples analysed with the NaI(Tl)detector. Determine the gamma energies and the corresponding nuclei. Attach the plots.
 Compare the HpGeand NaI(Tl)spectra: Describe the main differences and indicate advantages and disadvantages with the two detectors for TENORMcontaining scale analysis for the petroleum industry.
Questions:
1. In mineral samples: Is it more or less likely that ^{208}Tl is in equilibrium with ^{228}Ra than that ^{214}Bi is in equilibrium with ^{226}Ra? Argue for your answer
2. For scale samples, the same question.
3. For the scale sample: Supposing equilibrium between ^{214}Bi and ^{226}Ra,  what is the activity concentration of ^{226}Ra in the sample? Is the activity concentration above or below the exemption level?