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

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Written and developed by [http://www.mn.uio.no/kjemi/personer/vit/torbjor/index.html Prof. Tor Bjørnstad] (IFE/UiO) 
 
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<br> back to [[Naturally Occuring Radioactivity - NORM and TENORM]]
 
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N = &nbsp; number of nuclides<br> D = &nbsp; disintegration rate (Bq, Ci, dimension s-1)<br> A = &nbsp; activity is disintegration rate D per unit of volume or unit of mass (dimension s-1)<br> &nbsp; &nbsp; We distinguish between two notions:<br> &nbsp; &nbsp; As(i) = specific activity is activity per unit of mass of that particular element the actual<br>&nbsp; &nbsp; &nbsp; radionuclide i belongs to. This notion is also used to express the activity of a<br>&nbsp; &nbsp; &nbsp; particular radiolabelled molecule in relation to the total mass of the same molecule<br> &nbsp; &nbsp; &nbsp; (labelled and unlabelled) in the sample (for instance when the inactive molecule is present<br> &nbsp; &nbsp; &nbsp; as a carrier).<br> &nbsp; &nbsp; Ac(i) = activity concentration is the activity of a special radionuclide ''i'' or all present<br> &nbsp; &nbsp; &nbsp; radionuclides (AcT = A<sub>c</sub>(∑i) = total activity) per unit of volume or unit of weight of a radioactive<br> &nbsp; &nbsp; &nbsp; sample.<br>
 
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N,D and A may, in addition, have an index t (i.e. N<sub>t</sub>, D<sub>t</sub> and A<sub>t</sub>). This denotes the parameter values at time t relative to a starting time t = 0.<br> R = &nbsp; counting rate (cps, cpm or generally cpt where t should be defined, dimention s<sup>-1</sup>)<br> S = &nbsp; number of counts
 
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N, D, A, R and S may, in addition, have indexes B = background, T = total or N = net. The relations between them are: R<sub>N</sub> = R<sub>T</sub> - R<sub>B</sub> and S<sub>N</sub> = S<sub>T</sub> - S<sub>B</sub>. Other indexes may also be used, for instance S<sub>Cr-51</sub> in order to separate from S<sub>Fe-59</sub>.
 
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===== Examples:  =====
 
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We consider the following three samples:<br>P1: 1 kg pure iron (Fe). This sample contains10kBq of the radionuclide <sup>59</sup>Fe.
 
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P2: 1 kg steel consisting of 70 weight% iron and 10 weight% of each of the elements Cr, Ni and Mo. The sample contains 10 kBq of <sup>59</sup>Fe and 10 kBq of <sup>51</sup>Cr.
 
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P3: A 1L aqueous solution with 100 g dissolved FeCl<sub>3</sub> and 100 g dissolved CrCl<sub>3</sub>. This corresponds to 35 g dissolved Fe and 31.6 g dissolved Cr. The solution contains 10kBq <sup>51</sup>Cr and 10 kBq <sup>59</sup>Fe.
 
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For sample P1 we then have:<br> &nbsp; &nbsp; &nbsp; As(<sup>59</sup>Fe) = 10 kBq/kg Fe<br> &nbsp; &nbsp; &nbsp; Ac(<sup>59</sup>Fe) = 10 kBq/kg sample
 
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For sample P2:<br> &nbsp; &nbsp; &nbsp; A<sub>s</sub>(<sup>59</sup>Fe) = 10 kBq/0.7kg = 14.3 kBq/kg Fe<br> &nbsp; &nbsp; &nbsp; A<sub>s</sub>(<sup>51</sup>Cr) = 10 kBq/0.1kg = 100 kBq/kg Cr<br> &nbsp; &nbsp; &nbsp; A<sub>c</sub>(<sup>59</sup>Fe) = 10 kBq/kg sample<br> &nbsp; &nbsp; &nbsp; A<sub>c</sub>(<sup>51</sup>Cr) = 10 kBq/kg sample<br> &nbsp; &nbsp; &nbsp; A<sub>cT</sub> &nbsp; &nbsp; &nbsp; &nbsp; = 20 kBq/kg sample<br>
 
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For sample P3:<br> &nbsp; &nbsp; &nbsp; As(59Fe) = 10 kBq/0.035 kg Fe = 285.7 kBq/kg Fe<br> &nbsp; &nbsp; &nbsp; As(51Cr) = 10 kBq/0.0316 kg Cr = 216.5 kBq/kg Cr<br> &nbsp; &nbsp; &nbsp; Ac(59Fe) = 10 kBq/1.2 kg sample = 8.33 kBq/kg sample and/or 10kBq/l sample.<br> &nbsp; &nbsp; &nbsp; Ac(51Cr) = 10 kBq/1.2 kg sample = 8.33 kBq/kg sample and/or 10kBq/l sample<br> &nbsp; &nbsp; &nbsp; AcT &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;= 20 kBq/1.2 kg sample = 16.66 kBq/kg sample and/or 20 kBq/l sample.
 
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Indexes may be omitted when the meaning of the parameters cannot be misunderstood.<br><br>
 
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===== Gamma spectroscopy  =====
 
===== Gamma spectroscopy  =====
  
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''Calculations:'' The standard source has a known activity '''A'''<sub>s,i</sub> at a certain defined date. Let us denote the time from this date until today with '''t'''<sub>d</sub> (decay time, in days). One finds the standard source strength today, '''A'''<sub>s,t,</sub> by the formula:  
 
''Calculations:'' The standard source has a known activity '''A'''<sub>s,i</sub> at a certain defined date. Let us denote the time from this date until today with '''t'''<sub>d</sub> (decay time, in days). One finds the standard source strength today, '''A'''<sub>s,t,</sub> by the formula:  
  
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{| width="200" cellspacing="1" cellpadding="1" border="1"
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{| width="200" cellspacing="0" cellpadding="0" border="0"
 
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| <math>A_{s,t}=A_{s,i}\cdot e^{-\lambda_{t}}</math>
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| <math>A_{s,t}=A_{s,i}\cdot e^{-\lambda_{t}}</math>  
 
| Eqn. 1
 
| Eqn. 1
 
|}
 
|}
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where <span class="texhtml">λ</span> = ln2/T<sub>1/2</sub> for <sup>241</sup>Am. 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:
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{| width="200" cellspacing="0" cellpadding="0" border="0"
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|-
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| <math>\frac{S_{s}}{S_{x}}=\frac{A_{s,i}\cdot e^{-\lambda}}{A_{x,t}}</math>
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| Eqn. 2
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Solved with respect to '''A<sub>x,t</sub>''' we have:
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{| width="200" cellspacing="0" cellpadding="0" border="0"
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|-
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| <math>A_{x,t}=\frac{S_{x}\cdot A_{s,i}\cdot e^{-\lambda t_{d}}}{S_{s}}</math>
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| Eqn. 3
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|}
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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.
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<br>
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{| cellspacing="1" cellpadding="1" border="1" style="width: 536px; height: 67px;"
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|+ Table 1. Data handling and calculations for <sup>241</sup>Am
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| Source<br>
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| Recorded number of counts per 300 s, S<br><br>
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| Original decay rate at certification time (Bq)<br>
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| Decay rate today (Bq)<br>
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| Standard<br>
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| S<sub>s</sub>=<br>
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| A<sub>s,i</sub>=<br>
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| A<sub>s,t</sub>=<br>
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|-
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| Fire alarm<br>
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| S<sub>x</sub>=<br>
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| <br>
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| A<sub>x,t</sub>=<br>
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<br>
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<u>Determination of concentration of KCl in so-called “health salt” – SELTIN</u>
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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

Revision as of 09:53, 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

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