Difference between revisions of "Gamma Spectroscopy (NORM and TENORM)"

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(Gamma spectroscopy)
(Gamma spectroscopy)
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| <math>R=\epsilon_{T}\cdotD</math>  
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| <math>R=\epsilon_{T}\cdot D</math>  
 
| Eqn. 5
 
| Eqn. 5
 
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| <math>\frac{R_{KCl}}{R_{s}}=`frac{D_{KCl}}{D_{s}}</math>  
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| <math>\frac{R_{KCl}}{R_{s}}=\frac{D_{KCl}}{D_{s}}</math>  
 
| Eqn. 7
 
| Eqn. 7
 
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| <math>D_{s}=`frac{D_{KCl}\cdotR_{s}{R_{KCl}</math>  
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| <math>D_{s}=\frac{D_{KCl}\cdot R_{s}}{R_{KCl}}</math>  
 
| Eqn. 8
 
| Eqn. 8
 
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''Spectra handling and calculations:''
 
''Spectra handling and calculations:''
  
Determine the net area (S <math>\pm\sigma_{s}</math>S) for the 1462 keV gamma peak in the three recorded spectra. Then carry out the calculations to fill into the table below:
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Determine the net area (S<big>±σ</big><sub>S</sub>) for the 1462 keV gamma peak in the three recorded spectra. Then carry out the calculations to fill into the table below:

Revision as of 09:24, 22 June 2012

Gamma spectroscopy

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.

Introductory tasks:

  1.  Turn on the PC and load the Maestro program
  2.  Control that the high detector tension is 500-550 V.
  3.  Adjust the gain so that the detection system covers the energy region 0-2700 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.
  4. Carry out an energy calibration of the lower half of the spectrum by using the two radionuclides 241Am (59.6 keV) and 137Cs (661.2 keV).
  5. It is possible that we have to perform another calibration later with the two radionuclides 133Ba (highest peak at 356 keV) and 60Co (highest peak at 1332.4 keV)


Determination of source strength of 241Am in fire alarms
Experimental procedure:

  1. Mount a metal plate-supported 241Am fire alarm source on an aluminium ring covered with transparent tape on both sides.
  2. Mount the standard source of 241Am on an aluminium ring with adhesive tape only on one side.
  3. 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.
  4. Mount the fire alarm source at the decided distance and count the source for 300 s (live-time).
  5. Store the spectrum in a dedicated folder on the PC.
  6. Mount the standard source in the same position and count for 300 s (live-time).
  7. Store the spectrum in the same folder.
  8. 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 As,i at a certain defined date. Let us denote the time from this date until today with td (decay time, in days). One finds the standard source strength today, As,t, by the formula:

[math]A_{s,t}=A_{s,i}\cdot e^{-\lambda_{t}}[/math] Eqn. 1

where λ = ln2/T1/2 for 241Am. Since both sources may be regarded as “mass-less” 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:

[math]\frac{S_{s}}{S_{x}}=\frac{A_{s,i}\cdot e^{-\lambda}}{A_{x,t}}[/math] Eqn. 2

Solved with respect to Ax,t we have:

[math]A_{x,t}=\frac{S_{x}\cdot A_{s,i}\cdot e^{-\lambda t_{d}}}{S_{s}}[/math] Eqn. 3

For the calculations: Look up the half-life of 241Am from the nuclide chart and obtain the certified activity of the standard source and the certification date from the lab adviser.


Table 1. Data handling and calculations for 241Am
Source
Recorded number of counts per 300 s, S

Original decay rate at certification time (Bq)
Decay rate today (Bq)
Standard
Ss=
As,i=
As,t=
Fire alarm
Sx=

Ax,t=


Determination of concentration of KCl in so-called “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
MKCl (g/mol)
Atomic weight of K

MK (g/mol)

Avogadro’s number

NA

Natural abundance of 40<7sup>K

YK-40 (%)

Branching ratio of 1462 keV, Iabs (%)
Half-life of 40K

T1/2 (years)

74.551
39.1
6.023[math]\cdot[/math]1023
0.0117
10.7
1.28[math]\cdot[/math]109

Remember in addition that the following relation is generally valid:

[math]D=\lambda\cdot N[/math] Eqn. 4

where D = DK-40 , N = NK-40 and [math]\lambda[/math] = [math]\lambda[/math]K-40 = ln2/(T1/2)K-40.

The general relation between total counting efficiency, decay rate and counting rate is:

[math]R=\epsilon_{T}\cdot D[/math] Eqn. 5

[math]\epsilon_{T}[/math] is composed of several sub-efficiencies like the intrinsic detector efficiency for this gamma energy ([math]\epsilon_{D}[/math]), the efficiency due to the sample shape and distance to detector ([math]\epsilon_{G}[/math]), called geometrical factor efficiency) and the fact that the branching ratio, Iabs, 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 40K, Iabs is found in Table 9. The total counting efficiency for the gamma energy 1462 keV, [math]\epsilon_{DG}[/math], is independent of the nuclide. Here, we shall use 40K to determine this efficiency and has to take into account the value of Iabs. Then:

[math]\epsilon_{T}=I_{abs}\cdot\epsilon_{DG}[/math] 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 40K 1462 keV gamma energy (Iabs = 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, [math]\epsilon_{T}[/math], is also the same for both samples. Therefore, the decay rate of 40K in SELTIN is calculated by the formula:

[math]\frac{R_{KCl}}{R_{s}}=\frac{D_{KCl}}{D_{s}}[/math] Eqn. 7

Solving this equation for DS gives:

[math]D_{s}=\frac{D_{KCl}\cdot R_{s}}{R_{KCl}}[/math] Eqn. 8

Practical procedure:

  1. Start the spectrometer with a preset live-time of 30 min for recording of background.
  2. Weigh in a counting sample of KCl (wKCl) in a suitable lid-covered plastic box.
  3. Weigh a similar sample of SELTIN (wS) in an identical counting box.
  4. If the background counting has not finished already, stop the spectrometer, note the counting time (live-time) tB and store the spectrum in a dedicated file.
  5. Mount the KCl-sample, count for a preset time tKCl and store the spectrum in a dedicated file after the spectrometer has stopped.
  6. Mount the SELTIN sample, count for a preset time tS 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±σS) for the 1462 keV gamma peak in the three recorded spectra. Then carry out the calculations to fill into the table below: