Home
Ionization produced in the gas converts neutral molecules to positive ions and electrons within the sensitive volume. This volume is contained between charged electrodes, one positive, the other negative. The charged species are collected at the electrodes of opposite sign.
Either a photon (X or gamma ray), producing primary electrons along its path, or a particle (alpha or beta) producing secondary electrons, will create ions that will travel to the electrodes and be collected. A sufficient potential must be applied across the electrodes to prevent ion recombination and make collection possible. As the ions are collected, a current will flow. This will be measured on a sensitive measuring circuit "C" shown in the diagram above. Alternatively, the current may be measured as a pulse by a pulse counter "P" from the collection of each primary particle.
When using portable instruments, caution should be used when extending the detector cord as this may generate electrical noise and register as "counts". Also, thin window GM tubes used to detect alpha or low energy beta particles are fragile and can easily break if dropped or punctured. In a mixed beta-gamma field, the reading due to beta radiation only will be the reading with a beta shield off the detector minus the reading with the beta shield on the detector.
Efficiencies for instruments expressing results in terms of counts rates can be calculated from the following formula:
Efficiency = Observed Standard Count Rate (cpm)
Known standard Disintegration Rate (dpm)
Divide the observed sample count rate by the detector efficiency to obtain the actual disintegration rate.
Example: A Carbon-14 standard has a disintegration rate of 85,000 dpm. Your GM counter measures a count rate of 4,500 cpm. If the background is 250 cpm, what is the efficiency of the counter?
Efficiency = 4,500 cpm - 250 cpm = 0.05 c/d x 100 = 5%
85,000 dpm
However, it is usually the counting rate which is of interest and the standard deviation becomes:
σ= N1/2
t (t=counting time)
Example: What is the standard deviation of the count rate for a sample that yielded 1,000 counts in two minutes and for a sample that yielded 10,000 counts in twenty minutes?
Count Rate = 1000 ± 10001/2 = 500 ± 15.8 cpm
2 2
Count Rate = 10000 ± 100001/2 = 5000 ± 5 cpm
2 2
One can see that in counting, greater statistical accuracy can be achieved by increasing the total counts which is usually accomplished by increasing the counting time of the sample. Generally, between 1,000 and 10,000 counts are needed for a sample to have statistical validity.
The percentage error of a counting measurement is determined entirely by the total number of counts accumulated:
ε = r ± 100%
N1/2 (r=count rate)(N= total number of counts)
To reduce the percentage error in your measurement, you must collect as many counts as possible. When expecting low counting rates, increase the counting time to lower the error to an acceptable level.
Example: What is the percentage error of the count rate for a sample that yielded 20 counts in one minute and for a sample that yielded 200 counts in ten minutes?
ε = 20 counts/ min ± 100 ÷ 201/2=20 cpm ± 22% ε = 200 counts/ 10 min ± 100 ÷ 2001/2=20 cpm ± 7%
MDA = Bkg cpm + 3x (Bkg)1/2 ÷ t
Example: What is the MDA for a counter with a background of 750 counts in ten minutes?
MDA = 75 cpm + 3x (750)1/2 ÷ 10 min = 83 gross cpm
Thus, any gross count over 83 cpm can be considered to be due to radioactivity.
However, the MDA for a counting system must be expressed in terms of a net count so that the results can be converted to dpm or µCi. Thus, the MDA becomes:
MDA = 3x (Bkg)1/2 ÷ t
To calculate the MDA (in dpm) for a known nuclide, divide by the efficiency of the nuclide. Report the MDA for any nuclide for which a net count of zero is calculated or whenever the standard deviation of the sample counting rate brings the net count at or below the MDA. Note that the MDA can be reduced by increasing the counting time and lowering the background. The lower the MDA, the more accurately the activity of samples with low counting rates can be determined.
Example: What is the MDA (in dpm) for a counter with a background of 750 counts in ten minutes and an efficiency of 50% for the nuclide of interest?
MDA = 3 x 7501/2 ÷ 10 min = 8 net cpm
= 8cpm = 16dpm or 7.2 x 10-6 µCi
0.5c/d
Not all pulses from the PMT are due to radiation from the sample. Pulses are generated by the electronics, the PMT and from environmental radiation. These "noise" pulses are identical to pulses due to scintillations from the sample. To distinguish the pulses, two PMT's are arranged in a "coincidence" mode. Because noise pulses are random events, it is unlikely that two PMT's will receive a pulse simultaneously. But most beta particles have sufficient energy to produce more than one photon in the solution. Therefore, it is probable that both PMT's will simultaneously receive photons due to a single beta decay event. A coincidence circuit is established to check if a pulse from one PMT is accompanied by a corresponding pulse from the other. The requirement that both PMT's receive a pulse within a certain time (coincidence resolving time) excludes the vast majority of noise pulses from the sample count.
Beta particles will produce PMT pulses up to a maximum amplitude. An upper level discriminator (ULD) can be introduced to the system which can exclude pulses which have a greater amplitude than the maximum amplitude for the nuclide of interest. A lower level discriminator (LLD) can be arranged to exclude all pulses smaller in amplitude than a given value. A gain control is used to determine the PMT pulse height to which a given discriminator setting corresponds. Changes in gain alter the amplitude of the pulses before analysis by the LLD and ULD. The limits of pulse height accepted by a pair of discriminators and gain setting is referred to as a "window" (see Figure 1). Correct settings of gain controls and discriminators will discriminate between pulses of given nuclide from those of another. To separate pulses from beta events in samples containing nuclides of differing energies, a number of separate channels of pulse height analysis are necessary. The instrument's operating manual should be referred to for specific procedures on how to optimize the counter for each particular nuclide to be analyzed.
Pulse height spectrum of a beta emitter showing the effect of gain. Note that the pulse spectrum is centered between the window set by LLD and ULD to give the maximum counting rate (Gain Setting Ga).
Figure of Merit = S2 where: S = net sample counts
S + 2B B = background counts
The larger the Figure of Merit, the more significant the sample measurement is.
PMT's, the scintillations appear as beta particles of lower emission energies. The effect of quenching is a shift in the pulse height spectrum (see Figure 2). Thus, some low energy events which would normally exceed the coincidence threshold in unquenched samples will produce insufficient photons for detection in quenched samples.
Because quenching occurs to some degree in all samples, a loss in counting efficiency will result. The three basic techniques used to determine sample counting efficiency in a liquid scintillation counter are Internal Standard, Sample Channels Ratio, and External Standard.
Efficiency = C2 - C1
internal standard dpm
Where: C1 is the net cpm of the sample without the internal standard.
C2 is the net cpm of the sample with the internal standard.
In order to be most accurate, the material added as the standard should be of the same material as the sample, as to not introduce quenching, and added in small volume (0.1 ml or less) so not to alter the characteristics of the original sample. The amount of activity added must be accurately determined and should be equal to or greater than the sample activity.
To use the channels ratio method to determine the efficiency of a single nuclide, one of the windows (Ch A) is set narrower than the normal window of analysis (Ch B) for that nuclide as shown in Figure 2. As the quenched standards set is counted, more and more counts will be shifted out of window B into window A. The counting efficiency in channel B and the net samples channels ratio(SCR) for each standard is calculated. A graph of the efficiency vs SCR is obtained and a curve drawn:
Sample Channels Ratio Calculations
Counting Mode: Single Nuclide (C-14)
Counting Time: 1 Minute
Background Count Rate: Channel A = 90 cpm; Channel B - 27 cpm
Quenched Standards Set:10 samples, each containing 97,600 dpm of C-14.
Sample #1 least quenched; #10 most quenched
Ch A Ch B ChB/ChA %Eff
# cpm cpm SCR
1 19787 87456 4.420 89.6
2 22541 86171 3.923 88.3
3 28738 82670 2.877 84.7
4 34977 78970 2.258 80.9
5 47505 71174 1.498 72.9
6 55311 63652 1.151 65.2
7 63448 53135 0.8375 54.4
8 65492 39859 0.6086 40.8
9 58511 24441 0.4177 25.1
10 45768 11243 0.2457 11.5
External Standard Ratio Calculations
Counting Mode: Single label, Ch A set for H-3; Ch B for C-14.
Time: 1 minute.
Background Count Rate: Ch A = 90 cpm; Ch B = 27 cpm
Quenched Standard Sets:10 samples, each containing 262166 dpm H-3;
10 samples, each containing 97600 dpm C-14.
Sample #1 least quenched; #10 most quenched.
C-14 Quenched Standard Set H-3 Quenched Standard Set
Ch A Ch B Ch A Ch B
# cpm cpm ESR %Eff # cpm ESR %Eff
1 19787 87456 1.007 89.6 1 146489 59773 1.015 55.8
2 22541 86171 0.9745 88.3 2 124867 37083 0.9586 47.6
3 28738 82670 0.8976 84.7 3 107576 23921 0.8924 41.1
4 34977 78970 0.7861 80.9 4 89517 14000 0.7816 34.1
5 47505 71174 0.6622 72.9 5 64457 5728 0.6722 24.6
6 55311 63652 0.5567 65.2 6 42870 1989 0.5184 16.4
7 63448 53135 0.3921 54.4 7 28025 759 0.3449 10.7
8 65492 39859 0.1964 40.8 8 17313 255 0.1582 6.6
9 58492 24441 0.0322 25.1 9 9348 96 0.0154 3.6
10 45768 11243 0.0011 11.5 10 4770 41 0.0000 1.8
The External Standard method can be used to determine efficiency in any sample regardless of its radioactive content and is suitable for single and dual label counting as well as for samples of low activity.
Another factor to consider when preparing samples for liquid scintillation counting is the introduction of high background count rates as a result of photoluminescence, chemiluminescence, and static electricity. In photoluminescence (also called phosphorescence), photons are generated by interactions of the ultra-violet component of light with the sample vial and contents. Therefore, samples should avoid exposure to direct sunlight and fluorescent light, and counting solutions should be stored in amber containers. Incandescent light will not cause photoluminescence. The level and duration of photoluminescence is a function of the light intensity and exposure time. When a sample has been photoactivated, it must be dark adapted until it decays to background levels.
In chemiluminescence, photons are generated during sample preparation as a result of chemical interactions of the sample components. The amount and duration is temperature dependent and the effect decays faster at higher temperatures. However, cooling the sample will slow down the effect to a point where the coincidence circuitry of the counter can discriminate between chemiluminescence and radioactive decay. Thus, storing samples in a refrigerator overnight should remove most of the background counts due to chemiluminescence and photoluminescence.
The usual method for detection of luminescence is to recount the sample after an appropriate interval. A decrease in the second count rate indicates a strong possibility of luminescence.
During dry seasons or when using an ambient counter, static electricity may be the cause of high background count rates. When a static charge deposited on a vial discharges, light photons are produced in proportion to the charge. If the vial is being counted at the time of discharge, an incorrect, high sample count rate will result. This problem can be reduced by humidifying the counter and by wiping the vials with a moist cloth.
Sample preparation for Cerenkov is simple and economical since additional scintillators are not needed and the solvent can be almost any colorless liquid. Samples analyzed by Cerenkov counting are not affected by chemical quenching, but are highly vulnerable to color quenching. Also, counting efficiencies for Cerenkov radiation are relatively low because the Cerenkov light is highly directional. As a result, light photons generated may be detected by only one PMT and thus rejected as a count by the coincidence network. Counting efficiencies can be increased by the addition of a wavelength shifter to the solution and/or by deactivating the coincidence circuitry.
Sodium iodide crystals can be made in various sizes, some small enough to use in portable survey instruments. Larger crystals (3 inches in diameter by 3 inches deep) are common for most radioisotope counting room applications such as isotope identification by characteristic photopeaks. Still others have a hole or "well" in the center, allowing the sample to be surrounded by the crystal, resulting in a very high detection efficiency. This type of detector is found in most laboratory "gamma counters" where a large number of samples can be counted automatically.
Unlike liquid scintillation counting, the sample does not need special preparation. The sample can be counted in any physical form. However, care must be taken to have the sample properly contained so as not to contaminate the counting equipment. Gamma emitting isotopes such as I-125, Cr-51, and those decaying by electron capture are best assayed using a NaI detector.
Multiple choice questions may have more than one correct response.
GM Ion Cbr NaI Ctr LSC
a) non-removable surface contamination ( ) ( ) ( ) ( )
b) X-rays from a dental machine ( ) ( ) ( ) ( )
c) H-3 labelled water ( ) ( ) ( ) ( )
d) a P-32 labelled nucleotide ( ) ( ) ( ) ( )
e) a Cr-51 labelled protein ( ) ( ) ( ) ( )
f) a Mn-54 labelled bacteria ( ) ( ) ( ) ( )
g) a 10 mr/hr radiation of beta and ( ) ( ) ( ) ( )
gamma rays
We would greatly appreciate your feedback about this site.
 RSS FeedCopyright © 2009 University of Maryland DES |
 |
UM Home | Directories | Search | Calendar Maintained by Department of Environmental Safety Direct questions and comments to safety@umd.edu |