ML19224A966
| ML19224A966 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/20/1974 |
| From: | Metropolitan Edison Co |
| To: | Mullinix W NRC/IE |
| References | |
| 1958, TM-0198, TM-198, NUDOCS 7906130329 | |
| Download: ML19224A966 (8) | |
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UNIT #1 PLMT CHEMISTRY PROCECURE 1958 GAMMA SPECTRCMETRY
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12/14/73 Revision 0 Plant Chemistry Procedure No. 1958 Gamma Spectrometry 1.0 Summary The use of gamma spectrometry as a qualitative tool is based upon the principle that, in the process of radioactive decay, gamma and x-rays are emitted at discrete energies which result in the production of such characteristic spectra. The production of such spectra, each unique to a specific radianuclide, in conjunction with half-life measurements pro-vides a positive methcd of identifying specific gamma-emitting isotopes.
For those isotopes having a known decay scheme, quantitative deter-minations may also be made.
Quantitative calculations are similar to those for beta activities except that a gamma yield factor must be applied for each photopeak energy cf each isotope according to its decay scheme.
A.different efficiency of counting must also be acplied to each ga ma energy since the efficiency for detection of x and garma rays is highly enercy dependent.
All gamma and x-ray detectors depend upon conversion of x cr gamma phctons into a pulse of electrical current which is proportional to the energy of the inccming photon.
These pulses are ccmplified electrically, scrted as to pulse size by an ADC and stored in a memory unit for readcut.
Two types of detectors are ccomenly employed, a NaI (TI) scintillator in conjunction with a FM tube and a Ge(Li) solid state detecter.
There are several advantages to the NaI (TI) detector.
It has a relacively hi h detection efficiency and a vast amcung of experimental work has been 3
performed so that its response is readi.ly predictable" for standard gecmetries.
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It's significant disadvantages are its inherently poor energy resolu-tion and its calibration shift with relatively minor fluctuations in the power supply.
It is best employed for counting samples containing low levels of only a few nuclides.
The Ge(Li) detector's principal advantage is its excellant energy resolution.
It is also much less sensitive to fluctuations in its power supply than the NaI(TI) detector and has a much higher peak-to-compton ra tio.
Its chief disadvantage is that it has a much lower detection efficiency with efficiencies on the order of 5% to 20% of that of a 3" X 3" NaI(TI). Another disadvantage is that the detector 'must be maintained at liquid nitrogen temperatures and will require extensive repairs if this temperature is not maintained. This detector maj be used effectively for counting samples containing moderate to bigh levels __ _ _
of many nuclides.
Quantitative determination of simple gamma spectra from either type of detector may be made using the techniques of compton continuum subtraction, spectrum stripping or simultaneous equations.
More complex spectra, particularly those resulting frca the use of a Ge(Li) detector require ccmputer capability for data handling.
2.0 Accaratus 2.1 Hewlett - Packard 5403 gamma spectrometer systcm.
2.2 3" X 3" NaI(TI) Detector and Shield.
2.3 Ge(Li) detector 3.0 Reagents 3.1 No reagents necessary.
4.0 Procedure 4.1 Ccmpton continuum subtraction - NaI(TI) detector.
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Revision 0 4.1.1 Ascertain that a calibration check has been run on the system within the past 24 hrs.
4.1.2 Prepare the sample to be counted so that it conforms to a geometry for which an efficiency vs. energy curve exists.
5 4.1.3 Place the sample in the shield and allign to conform to the selected counting gecmetry.
4.1.4 Select a 512 channel portion of the spectrcmeter memory, erase to be sure that there is no residual in the memory, and accum-ulate for a time sufficient to show well-defined photopeaks.
A background, previously accumulated in a negative counting mode, may be transferred :nto the memory segment giving auto-matic background subtractica before counting the cample if desired.
4.1.5 Print cut the data. Also ake a semi-log plot of energy cn the X axis vs. log of counts on the Y axis.
4.1.6 From the plot or frcm tre peak channel numbers and kev /ch deter-mine the energy of each photopeak.
4.1.7 Frcm sources such as " Heath" or " Table of the Isotopes" list possible isotcpes for each photopeak energy.
4.1.8 Eliminate any of the peaks frcm the possible list by using half-life, standard spectra, etc.
Particular attention should be paid to isctcpes having ultiple peaks. A second count.
c.t a later time will be necessary to determine half-life.
4.1.9 When the photopeak isotpoes have been identified, draw a line representing the best smooth line curve through the _ises of the photopeaks. All counts belcw this line are assumed to be Compton contribution.
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- 1958 12/14/73 Revision 0 4.1.10 Sum the 9 highest channels in each phottpeak.
4.1.11 Take 9 times the counts at the intersection of a line drawn from the top of each photopeak perpendicular to the baseline.
4.1.12 For
-h photopeak subtract the counts from Step 4.1.11 from the counts from Step 4.1.10.
4.1.13 Subtract background from the ccunts in each photcpeak.
If automatic background subtraction was employed in Step 4.1.4, this step should be disregarded.
4.1.14 Calculate the activity of each isotope according to the following formula:
uCi/ml Counts
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5 vol(ml) X time ((min) X yyield X eff X 2.22X10 4.i.15 The same method may be used to interpet simple Ge(Li) spectra provided that fewer photopeak channels are used to determine counts.
Refer to the Ge(Li) calibration data for the specific number of channels to be used.
4.2 Spectrum stricping - NaI(TI) detector.
4.2.1 Prepare and count the sample as in section 4.1.
Use automatic background subtraction if desired.
4.2.2 Identify the highest energy photopeak and determine to which isotope it is attributable.
a.2.3 Calculate its activity by summing the 9 phctopeak channels, sub-tracting backgrcund if not already accounted fce, and using the formula from Step 4.1.14 calculate its activity.
4.2.4 Enter a standard spectrum of is isotope into the spectrcme'.
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'1958 12/14/73 Revision 0 4.2.5 Determine a multiplicatior, factor necessary to adjust the photopeak height of the standard spectrum to that of the sample.
4.2.6 Multiply the standard spectrum by this factor and subtract it from the sample spectrum.
4.2.7 Select the next highest energy photopeak and perform steps 4.2.2 - 4.2.6 inclusive.
Repeat until no spectrum remains.
Note:
If a photopeak is encountered for which there is no standard spectrum an error will be introducod into the determination of all lower energy photo-peaks. The magnitude of the error is determined by the activity of the unknown isotope in relation to other activity present.
4.2.8 The same methed maybe used to interpet simple Ge(Li) spectra provided that fewer photopeak channels are used to determine the counts.
Refer to the Ge(Li) calibration data for the specific number of channels to be used.
4.3 Simultaneous Equation method - NaI(Ti) detector.
4.3.1 Prepare and count the samples as in secticn 4.1.
Use automatic background subtraction.
4.3.2 Identify by isotope all photopeaks.
4.3.3 Determine, using standard spectra, the contribution of each isotope present to the photopeak areas of the other isotopes.
Express this " interference factor" as a percentage of the counts in the photopeak.
For example, in a spectrum containing i
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- 1958 8/20/74 Revision 1 3 isotopes (A,B,&C), observed counts in A photopeak area is the sum of the contribution of all three isotopes or; Counts in A = contrib A + contrib B + contrib C.
Expressed as interference factors this becomes; Counts in A = 1.0A + dB + eC where d and e represent the #ractional interference factors of isotopes B and C to isotope A photopeak energy area es deter-mined from the standard spectra of isotopes B and C respectively.
4.3.5 The simultaneous equations from Step 4.3.4 may be solved by determinants.
The solution for each unknown represents the number of counts in a photopeak area which are actually due to one specific isotope.
4.3.6 Substitute the values from Step 4.3.5 into the equation from Step 4.1.14 to obtain activities.
4.3.7 The same method may be used to interpet simple Ge(Li) spectra provided that fewer photopeak channels are used to determine the counts.
Refer to the Ge(Li) calibration data fof the specific number of channels to be used.
4.4 Analysis of complex spectra - Ge(Li) detector.
4.4.1 Analysis of complex Ge(Li) spectra requires the use of a large computer. Refer to specific program instructions.
4.5 Readicactive Decay correction for Radio-chemistry analysis.
4.5.1 All samples analyzed must be decay corrected back to the original time of sampling.
4.5.2 Calculate the sample decay by using the following ' formula:
A= Ace -At/Th wnere: A = Sample activity after a time interval (pCi/cc)
Ao = Sample activity at scme original time (pCi/cc) e = Base of natural logarithms; 2.7133...
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elapsed time (mins. hrs.) days, etc.)
Th = Radioactive element half life.
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