gires the americium gamma activity, and the amount of americium present in the sample can then be calculated. LITERATURE CITED
( I ) Clifford, J. H., Koshland, D. E., Jr.,
U. S. Atomic Energy Commission, Rept. CN-2040 (1944) secret. (2) Freedman, M. S., Wagner, F., Jr.,
Engelkemeir, D. W., Phys. Rev. 88, 1155 (1952). (3) hfagnusson, L. B., Ibid., 107, 161 (1957). (4) MetZ, c. F.,ANAL. CHEX 29, 1748 (1957). (5) Miller, H. W., U.S. Atomic Energy Commission Rept. HIV-22267 (October 1951)unclassified. (6) Moore, F. L., Hudgens, J. E., Jr., AKAL.CHEV.29, 1767 (1957).
(7) Xewton, J. O., Rose, B., Milsted, J., Phil. Mag. 11, 981 (1956). (8) Shliagin, K. N., Zhur. Eksptl. i Teoret. Fiz. 30, 817 (1956); Soviet Phys. JETP 3, 663 (1956). RECEIVEDfor review A4pril 28, 1958. Accepted July 18, 1958. Work performed under the auspices of the U.S. Atomic Energy Commission.
Development and Preparation of a Set of Gamma Spectrometer Standards L. J. BEAUFAIT, Jr., E. E. ANDERSON1, and J. PAUL PETERSON Western Division, Tracerlab, Inc., 2030 Wright Ave., Richmond 3, Calif. b Remarkably few gamma emitters have decay schemes sufficiently well established to make good standards for accurate efficiency calibration. The absolute gamma-emission rate for any isotope is not generally probable known better than &5% error. From the extensive list of available gamma-emitters, eight isotopes were selected on the basis of their established decay scheme, availability, half life, and convenience of radiation characteristics: cadmium109, barium-1 33, tin-1 13, cesium1 37, sodium-22, manganese-54, zinc65, and cobalt-60. The method of calibrating each of these isotopes and the resulting data obtained on a standard [Nal(TI)] crystal scintillation counter setup are described.
P
uses of radioactive materials from nuclear reactors, cyclotrons, and accelerators have increased enormously in the past few years. Vitally important from the industrial standpoint are radioactive isotopes which emit gamma radiation, and their identification and quantitative measurement. Therefore, it is necessary to have accurately calibrated gamma spectrometer standard sources for calibrating both the energy of y-rays emitted and the total gamma emission rate. EACETIME
ESTABLISHING A METHOD
OF CALIBRATION
A choice had first to be made between alternatives: whether to calibrate the standard sources in terms of absolute disintegration rates of the pure radioisotopes, or to calibrate in terms of the absolute number of y-rays of one specific energy-Le., one photopeak-each prepared source would emit 1 Present address, General Atomic, LaJolla, Calif.
1762
ANALYTICAL CHEMISTRY
per unit length of time. As it would be of more practical value to eliminate the errors involved in specific decay schemes of the isotopes, it was decided to calibrate each source in terms of its y-ray emission rate. This method was also more efficient, as the worker would not have to look up each decay scheme and make corrections in his data each time he used the source. Counting Arrangement. Figure 1 illustrates the geometrical consideration of the gamma spectrometer detector head for the initial calculation of the disk-type primary standards (Figure 3).
-4, Dumont Type 6292 multiplier phototube, 2 inches in diameter, was used, standard, but especially selected from among 30 to 40 identical tubes for its high signal to noise ratio. A standard sodium iodide (thallium-activated) crystal [NaI(Tl)] 11/2 inches in diameter and 1 inch long was o p tically coupled to the phototube with high viscosity Dov-Corning 200 silicone oil. Rigid planchet mounts using spring tension were located a t distances from the crystal as shown, to ensure reproducible geometry: Samples with or without backing can be used with this support. The entire detector assembly was surrounded with 4 inches of lead shielding and connected to a conventional 50-channel pulse height analyzer. The resolution for cesium137 y-rays under these conditions is about 9%. Resolution here is defined as the width of the 661-k.e.v. cesium137 y-ray peak a t half-height in thousand electron volts, divided by the energy of the y-ray (661 k.e.v.). Basic Calibration Curve. I n Figure 2 is seen the original calibration curve obtained using the geometrical setup shown in Figure 1 for a large number of radioactive isotopes which were used to calibrate the spectrometer. The abscissa is the gamma energy in million electron volts and the ordinate
Photomultiplier Glass
-
source I I/ 1.7-2.1 c m ' \ diameter
' ;g 1 Top Shelf
j
i
'
-
7
1
2
Bottom Shelf
Figure 1. Geometry of detector head for initial calibration of standards Photopeak counting efficiency = gamma counting rate in photopeak gamma emission rate Bottom shelf efficiency = (0.207) times top shelf efficiency
is the photopeak counting efficiency. The absolute disintegration rates of the isotopes shown were obtained by various methods. A few were primary standards obtained from the National Bureau of Standards. Some were determined by 4-pi beta counting in this laboratory. The K-electron capture isotopes of chromium-51, beryllium-7, and manganese44 were obtained from the Oak Ridge Xational Laboratory. The disintegration rates used were also determined by that organization. The smooth curve obtained is in escellent agreement with the shape of the curves found in the general literature for isotopes in the range of 0.2 to 1.4 m.e.v. At energies less than 0.2 m.e.v. the data still give a satisfactory working curve. C H O O S I N G ISOTOPES FOR STANDARD SOURCES
The preparation of a set of r-ray
/,
.I
~
,
':AS0012' TIC
15/16'
THICK DISK DIAMETER
! , "
/ COPPER DlSK 0009' T H l C L RAD10bCTiVITY DEPOSlTED ON
314" D I A M E T E R COPPER DISU
PLASTIC DISU OOIZ', THICK is/ 6 D l A H E T E R
Figure 3. Exploded view of disk-type gamma spectrometer source Energy of Gamma for
Isotopes Cdl@(9) Ba133(6)
0 01
GAMMA ENERGY ( M p )
Figure 2. Original calibration curve obtained from geometrical setup in Figure 1 Photopeak counting efficiency VI. gamma energy for a spread source on top shelf Photopeok counting efficiency = gamma counting r a t e in photopeak gamma emission rate Bottom shelf efficiency = (0.207) times top shelf efficiency L
energy standards must start with the proper choice of the radioisotopes themselves. A number of parameters must be taken into account in order best to choose the desired standard. They are discussed in the order of their importance. Half Life. There are trro major factors to consider in the half life of the isotopes. One is that they be as long as possible, t o conserve source strength. The other is that the half-life value be known accurately, because an error in the half life would be reflected in an error in calibration a t some later date. Complexity of Decay Scheme. The ideal in a good y-ray standard would be to have a single y-ray emitted from a radioisotope per disintegration, and with no associated side effects such as internal conversion. Isotopes having K-capture producing x-rays might also interfere with the lower energy y-ray spectrum and are thus not as desirable. Radioisotopes h a h g such ideal decay schemes are, of course, extremely rare. However, one can choose the radioisotopes which have the simplest possible decay scheme. When it becomes necessary to choose an isotope with more than one y-ray per disintegration. their gamma energies should be different enough to be readily separated on a y-ray spectrometer. Further, if a radioisotope emitting more than one gamma per disintegration must of necessity be chosen as a standard, its highest energy gamma should be selected for calibration of the emission rate. For example, an isotope
having one y-ray whose energy is somewhat less than another of Cigher energy should not be chosen as the standard, because the photopeak of the gamma is superimposed on the Compton smear of the higher energy gamma. This can produce rather large errors in determining the gamma emission rate. Energy of y-Ray. The accuracy of the y-ray energy used in calibrating the standards is just as important as knowing half-lives and decay schemes accurately. However, the error involved is small, in that most gamma energies for the more stable radioisotopes are known rather accurately. Selection of the isotopes whose y-ray energies cover a practical range and which are spaced a t nearly equal intervals over this range in such a manner that no holes or gaps exist in the range from 0.050 to 1.5 m.e.v. are theoretically desired. However, the energies obtainable from the desired gamma-emitting isotopes are not as conveniently placed as this. Factors Influencing Choice of Isotope Sources. Price and availability are important, in that it would be practical t o make sources of isotopes n-hich are either exceedingly difficult t o produce or prohibitively expensive. After the foregoing had been taken into consideration, eight isotopes from the extensive and readily available list of artificial radioisotopes were chosen for preparation as standard sources. These isotopes, listed in order of their increasing energy of the gamma used as standards, are as follows:
Sn"3 (I, 4 ) CS'37 (8, 3) Mn6' (6)
Zn"
(10)
Na22 (8)
COB ( 2 , '7)
Calibration Half Purposes, M.E.V. Lives 0.087 470 days 10 years 0.356 119 days 0.393 30 years 0.662 300 days 0 84 245 days 1.12 2 6 years 1.28
5 27 years
1.33
Types of Source Mount. There are in general two types of source mounts in use today for routine gamma spectrometry measurements: a flat planchet mount (disk-type) for counting solid materials, and a small vial (well-type) for counting liquid samples. As it was too difficult to fabricate a single source t o cover both types adequately, the decision was made to pattern the sources after the two general types. The dimensions of the flat disk-type source mount were set a t essentially 1 inch on the over-all diameter, and as thin as practical. If further backing is desired, this disk could be simply mounted on any commercially available mount. The well-type source mount was designed to simulate an inexpensive comniercially available ll/z-dram flint glass vial, 4/s inch in diameter and 21j4 inches long. The prime consideration in the preparation of the disk- and n ell-type sources n-ere that they should produce the least possible back-scattering, that they have a low 2 material cover, that the activity be permanently fixed, and that they be homogeneously distributed for reproducibility in counting and resistant t o shock and handling. Source Mount Material. Plastic materials, in general have displayed all of these qualifications as materials of construction. They are easily machined, can be obtained cheaply, and are available in various stages of rigidity. Plastics also fulfill the requirements of a lo^ Z material for keeping the low-energy x-ray and y-ray absorption to a minimum. AEC Requirements. A very important consideration in the ultimate VOL. 30, NO. 1 1 , NOVEMBER 1 9 5 8
1763
Figure 4. Exploded view of well-type gamma spectrometer source manufacture of source sets mas the insurance that they fulfill both the AEC requirements on the sale of unlicensed quantities of radioisotopes and be properly sealed sources. Disk-Type Source Mount. An aliquot of the isotope to be used is spread uniformly over the surface of the copper disk as shown in Figure 3. I t is then dried under a heat lamp. The sources are next coated with a plastic spray to hold the evaporated radioactive material rigidly on the mount through the ensuing operations. The copper disk is then laminated between two pieces of plastic on a conventional commercially available laminator, along with a label identifying the isotope and serial number. A close tolerance machine punch is used to punch out each source after proper centering. Well-Type Source Mount. I n the well-type mount, as shown in Figure 4, an absorbent plug is saturated with the isotope, dried, and spraycoated. The plug is pushed to the bottom of the Lucite cylinder, and the Lucite rod inserted and glued in place. The source is then painted and an engraved aluminum label placed on the one end, giving the isotope and serial number. A special Alnico permanent magnet probe has been fabricated to promote easy withdrawal of the standard sources from the well counter assembly. The quantity of activity put in each source was in the amount necessary to produce about 3000 counts per minute in the photopeak region under the geometry conditions shown previously in the calibration curve. CALIBRATING G A M M A SPECTROMETER STANDARDS
Reference Standards Calibration Curve. The first step in calibrating
Figure 5. Calibration curve for a gamma spectrometer Photopeak counting efficiency = gamma counting rote in photopeak gamma emission rate
a set of gamma spectrometer standards is to prepare one source of each isotope. This set is designated as "tlie reference standards ." Each reference stand,ard has had at least three complete gamma spectra determined on it. Then the gamma emission rate was determined for the source on the basis of an average of the three spectra, using the primary (Figure 2) calibration curve. This curve (Figure 5 ) is considered to be accurate to 5 5 % of the original calibration curve based on the best available decay scheme and half life. These sources thus become the reference standards for future comparison with manufactured sources. From this point on, future manufactured sources need only be counted on a conventional gross gamma scintillation counter with fixed geometry. A simple ratio of the counting rate between the reference standard and the new source was sufficient to establish the true gamma emiwion rate of the new source. Any number of new standards could thus be manufactured as desired. To eliminate half-life errors further, the reference standards are restandardized periodically in the calibrated spectrometer before future standards are made. Thus each new production set is free from possible half-life errors in the reference- standards. Uses. Any laboratory carrying out END OF SYMPOSIUM
1764
ANALYTICAL CHEMISTRY
research and development work in the field of gamma spectrometry, or in gross gamma measurement, whether it be concerned with radiochemical analysis or pure investigative programs, will find it necessary to have a set of standard sources for a constant check on instrumental setups for energy, efficiency, and consistency of counting. ACKNOWLEDGMENT
The authors express their appreciation for technical assistance provided by J. D. Buchanan and E. R. McLaughlin of this laboratory. LITERATURE CITED
(1) Avignon, P., Ann. Phys. 1, 10 (1956). (2) Bay, Z., Henri, V. P., McKernon, F., Phys. Rev. 94, 7808 (1954). (3) Bhattacherhee, S. K., Waldman, B., Miller, W. C., Ibid., 95, 404 (1954). (4) Gardner, G., Hopkins, J. I., Ibid., 101, 999 (1956). (5) Grace, M. A., Johnson, C. E., Kurti, N., Lemmer, H. R., Robinson, F. N. H., Phil. Mag. 45, 1192 (1954). (6) Katcoff. S.. Abrash., H.., Phus. Rev. " 103, 966 '( 1956). (7) Keister, G. L., Ibid., 96, 8858 (1954). (8) Learner, R. D., Hinman, G. H., Ibid., 96, 1607 (1954). (9) Mateosian, E. D., Ibid., 92, 938 (1953); 87, 193A (1952). (10) Perkins, J. F., Haynes, S. K., Ibid., 92, 687, 1096A (1953). \
I
RECEIVEDfor review June 9, 1958. Accepted September 2, 1958.
