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The standardization of radioactive preparations
I.NuckarEnergyI1,1956,Vol. 3.p~.347to355.PergamonPress Ltd.,London
THE STANDARDIZATION OF RADIOACTIVE PREPARATIONS* K. K. AGLINTSEV,F. M. KARAVAEV, A. A. KONSTANTINOV, G. P. OSTROMUKHOVA, and E. A. KHOLNOVA Abstract-A descriptionis givenof the methodsand apparatusused at the D. I. Mendeleev All-Union Standards Institute for the precise measurement of a number of quantitative properties of radioactive sources. The activity is measured by calorimetric, ionization, and absolute B-counting methods, the radium y equivalent by means of a 4rr ionization chamber, and the gamma dose rate with an ordinary ionization chamber. The accuracy and limitations of the measurements are discussed. To apply radioactive materials in practice it is necessary to know the principal characteristics of the isotopes used and the basic physical features of sources prepared from them. Among the most important characteristics which must be known are the end point and the shape of the /3 spectrum, the energy and number of quanta per disintegration for the y rays, and also the number and energies of the conversion electrons, For a complex ,B spectrum, this information and also the relative intensity must be known approximately for each of the components. In addition to data about the ,4 and y rays, the half-life of the radioactive isotope must also be known. Important features of radioactive sources are the activity of the preparation and the dimensions, weight, and nature of the container. For a source of relatively large volume, it is necessary to know the distribution of activity within it if this is nonuniform. The total radiation from a source containing a radioisotope of known /I and y spectrum is readily determined from the activity. However, in practice it is more important to know the external radiation; this differs from the total radiation because of self-absorption in the source and absorption in the container. The activity of a source can be measured in curies or in disintegrations per second. The y radiation from a source can be expressed in riintgens; in addition, the y radiation can be given in terms of the y equivalent of the source in grams of radium. The effect of p radiation can be measured in riintgen equivalent physical units. Uniformity of measurement is achieved by having apparatus which reproduces the unit of the physical quantity, and by a system of calibration and checking for both prototype and production-measuring equipment. The reproduction of curie and rantgen units is by means of instrumental standards, whilst the reproduction of a unit equivalent gram of radium is done using a government-owned standard consisting of two radium sources enclosed in thin-walled glass capsules. (L 2, The measurement of sources in equivalent grams of radium is the most precise method. y equivalents of sources are measured on two calibration instruments which compare the intensity of the y rays from the unknown source with that from a standard radium source. * Translated
by L. C. RONSON. 347
348
K. K. AOLINTSEVet al.
Each of the two arrangements consists essentially of a 4rr ionization chamber connected to an electrometer for measuring the small currents. Sources in the range l-1000 equivalent mg of radium are measured with an ionization chamber consisting of two concentric aluminium spheres of 5 mm wall thickness. The space between the spheres is the sensitive volume of the chamber. The ionization currents produced in the chamber by sources in the range mentioned above are from 1.5 . lo-l1 to 1.5 . 10-s amperes. Such currents are measured employing a compensating scheme’ due to TOWNSEND@) using a quadrant electrometer of maximum sensitivity 800 scale divisions per volt and a set of standard condensers. The arrangement is shown schematically in Fig. 1. The source being measured and the radium standard are placed alternately in the chamber.
FIG. 1.
Calibration apparatus for measuring y equivalents of sources in the range l-1000 mg of radium. K-ionization chamber; B,, B2, &-batteries; S-switch; C-standard condenser; V-voltmeter.
Measurements of y equivalents with this arrangement have an error 0*5-2%.‘4) Less active sources producing ionization currents comparable with the normal background current of the ionization chamber are measured with a different arrangement employing a so-called compensating chamber.(5) This consists of two similar cylindrical brass chambers separated by a lead block and connected in opposite polarity to a battery (Fig. 2); the differential current is zero in the absence of sources. The source being measured and the standard are placed alternately in one of the chambers. For sources equivalent to l-O*001 mg of radium the currents are in the range 5 . lO-‘l-5 . 10-14 amperes. For the measurement of these currents, a d.c. amplifier is used in an electrometer valve bridge circuit.15) The circuit, shown schematically in Fig. 2, measures the ionization current by a compensating method. The voltage drop produced in the output resistance by the ionization current is balanced by a potentiometer. In this case the amplifier is used only as a null indicator. In the range 1-0~001 equivalent mg of radium the error of measurement is from 1 to 5-8 %. Values of the y equivalents of sources measured by the above methods are corrected for self-absorption of the y rays, and a correction is also made for errors due to the geometrical dimensions of the sources. The value obtained for the y equivalent of a radioactive source depends on the conditions of measurement, mainly because of attenuation of the y radiation, and also on the geometry, type, and material of the walls of the ionization chamber. Figs. 3 and 4 show how the y equivalents of CoBo, (Zr + Nb)95, Agifo, and Sbrar
349
The standardization of radioactive preparations
vary with the thickness of a lead absorber. The full curve is for wide beams of y rays and the dotted curve refers to narrow beam conditions. From the measured value of the y equivalent of a source, the ionization produced at a distance of 1 metre can be estimated in riintgen per hour. Thus, since 1 gm of
FIG. 2.-Calibration ZC-compensating
apparatus
for measuring
y equivalents of sources in the range l-0901 equivalent mg of radium. chamber; P-potentiometer; &-standard cell ; E-electrometer valve; G-galvanometer; md-milliammeter; B-battery.
radium in a 0.5 mm-thick platinum envelope gives a dose rate at 1 metre of O-84 r per hour, a source equivalent to M gram of radium gives, under identical conditions, a dose-rate of 0*84M r per hour. The y equivalent of a source therefore characterizes the external y radiation under given conditions.
1.1 1.0
0.9 0.8 0.7 Q:
0.6 0.5 0.4 0.3 0.2 d, cm Pb Fig. 3
12
3’4 d,cm
5
6
7
8
Pb
Fig. 4
FIGS. 3 and 4.-Relation between y equivalent of Coao, (Zr + Nb)Os, AgIl” and Sbr”, and the filtration. Along the abscissa is plotted the thickness of absorber, and along the ordinate the ratio of the measured y equivalent to the value for 0.5 cm thickness.
The y equivalent of a source can be converted into activity (expressed in curies) only if the following quantities are known: the energy of the y quanta and their number per disintegration of the particular radioactive material, the sensitivity of the ionization chamber for various y ray energies, and also the effect of selfabsorption.
350
K. K. AGLINTSEVet al.
For absolute measurement of activity in curies an absolute /3-particle-counting method and also a calorimetric method are used. Although simple in principle, the absolute p-particle-counting method is in fact exceedingly difficult if an accuracy of the order of 3 % is required. Fundamental difficulties arise chiefly because lowenergy /I particles are always present in the continuous spectrum, and these are easily absorbed. The absolute counting method for /I particles depends on counting all the B particles from a given source over the whole solid angle 4~. The “47~ counter”
4P counter
4Tcounter
type
type
I
II
FIG. 5.-The “4n counter”. l--stand; 2-tap; 3-brass base; 4-source; S-counter wire; 6-body of counter; ‘I-polystyrene cup; B-lead-through insulators; 9-aluminium foil; IO-glass bell jar.
consists of two counters between which the source is placed. For absolute measurements with this instrument corrections must be made for the following: 1. Absorption of B particles in the film supporting the source. This correction is readily determined. 2. Absorption of /I particles by the counter itself. 3. Counting error due to the “dead time” of the counter and the finite resolving time of the electronic equipment. The 47r counter is shown schematically in Fig. 5. Two designs have been used.(‘jp ‘) The first consists of two cylindrical counters, cut parallel to the axes, with the source placed between them. This instrument was filled with a mixture of argon and ethyl alcohol, and was used in the Geiger regime. With this apparatus, sources of activity 5 . 10-8-10-10 curie could be measured with an accuracy of 24% for thin sources. The second design was made up of two half-cylinders with the source placed between, and used to count @ particles under proportional-counting conditions. It was filled with methane at a pressure between 30 and 50 cm Hg. The proportional “47r counter” can measure sources of 5 . lo-‘-5 . lo-l1 curie, with an accuracy of l-3 % for thin specimens.
The standardization
of radioactive preparations
351
Although not very accurate, the defined solid-angle method for absolute counting of /I particles is widely used because of its simplicity and wide range of application. In principle the method is very simple. The radioactive source on a thin support is placed in front of the counter, and p particles within a solid angle defined by a calibrated diaphragm reach the counter and are recorded. The total number of #? particles is then given by N = (477/Q) . N,, where No is the number of B particles counted in the defined solid angle 0. For absolute @particle determinations it is necessary, however, to apply a number of corrections when using this method. c8) The need for these corrections greatly reduces the accuracy of the method and in the best case it is 4-6 ‘A. The main error is due to uncertainty in the determination of the solid angle. /3 activities of the order 1O-5-1O-s curie could be measured by this method on our apparatus. A comparison of the two methods of absolute /3 counting shows that the 4~r counter method is the more accurate and also the simpler in principle. It is also the most sensitive of all known methods for measuring p activity. If a large number of measurements of p-active materials is required, the 4n counter can readily be used with each separate radioactive isotope to calibrate an end-window counter. In order to compare the above two methods with other means for absolute p counting (such as the calorimetric or ionization methods) the quantity of radioactive material must be known exactly. Not more than some tenths or hundredths of a milligram of material can be used if the correction for self-absorption of ,!I particles is to be kept small, and exact weighing of such small quantities on a microbalance is quite difficult. To estimate radioactive material which emits y rays in addition to B particles the y activity of the source is compared with the y activity of a larger amount of radioactive material, the so-called standard or reference quantity. The reference quantity was approximately 10 mg and could be weighed with sufficient accuracy. Several reference quantities were usually prepared;t51 6, they were compared with one another and those agreeing most accurately were used. Comparison of the y activity of the reference quantity and that of the source was carried out with a scintillation y spectrometer. The use of the y ray intensity to estimate the weight of a source proved to be a very reliable method. With a pure ,8 emitter such as P32, a relatively large amount of material is weighed out, and a proportion then taken to give a quantity of a fraction of a mg. The measurement of activity with counters becomes inaccurate when the specimens are thick. For such sources the calorimetric method is more satisfactory since the thickness of the material causes no difficulty. Calorimetric measurements of activity are based on the fact that the quantity of heat produced by the absorption of radiations from a radioactive source is proportional to the number of disintegrating nuclei. Two types of calorimeters have been designed for the measurement of absolute activity: (a) a differential double calorimeter for the measurement of y radiation; (b) an isothermal calorimeter based on the principle of the evaporation of liquid nitrogen for measuring the activity of sources from their fi radiation.(g-ll) The y calorimeter consists of two similar lead spheres (Fig. 6) mounted on an ebonite support in a thermostat. The substance to be measured is placed in a cavity provided in one of the spheres, and the temperature difference between the surfaces of the spheres is measured using a number of copper-constantan thermocouples.
K. K. AGUNTSEVet al.
352
A-body
FIG. &-Diagram of the y calorimeter. of the calorimeter; B-heater coil; C-plug; D-recess for source; E-attachment point for thermocouples; 2”thermocouple.
The principal characteristics of four y calorimeters are listed in Table 1. These instruments were calibrated by using a heater-coil inside each sphere. To evaluate the activity from the quantity of heat measured, the following data must be known: (a) the energy hvi of all y lines; (b) the number CQof y quanta of TABLE
1 Calorimeter
Property No. 1
External diameter of sphere, cm Thickness of absorber, cm Weight of sphere, gm Number of thermocouples Time to establish equilibrium, hours Sensitivity, lOa mm/watt
No. 2
No. 3
j
No. 4
7
5.5 2.1 969 20
i
3.5 1.06 232 18
10 4.3 5944 24
2.8 2030 .24
8 6.44
4 I.58
3 13.3
1.5 19.6
(c) the mean energy of the /? spectrum E,. Since energy per disintegration; the dimensions of the y calorimeters do not ensure complete absorption of all y rays, it is necessary, in addition, to know sufficiently accurately the proportion of the y energy which is absorbed by the calorimeter. This value-the coefficient piwas calculated for each calorimeter for y quanta of various different energies. The results of these calculations are summarized in Fig. 7. each
FIG.
‘I.-Absorption
of y rays in the calorimeters.
1,2, 3,4-number
of calorimeter.
The standardization of radioactive preparations
353
The formula relating N, the number of disintegrations per second, and W the quantity of heat produced per second in the calorimeter, has the following form: N, = W/X hviccipi + EB = nfi(Z hvictipi + ED) , where n is the galvanometer deflection in mm; j the sensitivity of the calorimeter in mm/watt; and EB the mean energy of the j3 spectrum.
A-manometer J, H-ground
FIG. 8.-Diagram of the isothermal /? calorimeter. ; C-mercury pump ; D-reservoirs; E, F, G-Dewar joints; K, L-connections for removing nitrogen; M-vacuum N-electric heater.
; B-capillary
vessels jacket;
;
The specially designed y calorimeters have sufficient sensitivity to allow measurement of sources with activities from a few tenths of a millicurie upwards. The accuracy of absolute activity measurement using y calorimeters is between 3 and 5 %. An uncertainty of 2-3 % can be attributed to error in calculation of the y absorption in the calorimeters. The calorimetric apparatus for measuring the activity of sources from their ,6 radiation consists of three coaxial Dewar vessels filled with liquid nitrogen (Fig. 8). The heat liberated by /? particle absorption is used to evaporate liquid nitrogen. The volume of gaseous nitrogen, which is measured at 760 mm Hg and 20°C using a capillary tube, is thus a measure of the amount of heat liberated by the source during the time of measurement. Calibration of the calorimeter showed that the sensitivity was sufficient for the measurement of sources equivalent to 10-4-10-5 6
354
K. K. AGLINTSEVet al.
watts (i.e. activity of the order of 10 millicurie for a mean /l-ray energy of approximately 1 MeV). The formula relating the heat production to the number of disintegrations has the following form for a pure /? emitter: N, = W/E@. When the specimen emits y rays a correction is made for their absorption in the source and calorimeter. The accuracy of the j3 calorimeter is between 3 and 5 X. The uncertainty is made up of the calorimetric error in measuring the quantity bf heat (1:15%j and the
To the electrical measuring equipment FIG. IO.-Calibration
apparatus for the measurement of y radiation in rijntgens (schematic).
error in measurement of the mean energy of the t!?particles. For sources having a simple @ spectrum the latter is 2-3 %. For sources with a complex @ spectrum Es is known with less accuracy. In addition to separate measurements on the @ and y calorimeters, combined measurements on these instruments have also been carried out. It was shown that by combining the results from the /? and y calorimeters the activity of the source could be determined from the known relative intensities of the y lines. Simultaneously with the activity of the source the number of y quanta per disintegration is also determined. A calibration apparatus has been designed to measure radiation in rijntgen units over the energy range 250 keV to 1.5 MeVo2) (Fig. 9). This consists of an ionization chamber enclosed by a tank filled with compressed air, a collimating system, electrical measuring equipment, and a high-voltage battery for the ionization chamber. The air-pressure in the tank can be raised to 15 atmospheres at which pressure electrons with energies up to 1.5 MeV are completely absorbed. The arrangement is shown schematically in Fig. 10. The ionization chamber has a measuring electrode, two screen electrodes, and a high-potential electrode ; there is a potential divider for controlling the electric field in the measuring volume of the chamber. The distance between the electrodes is 40 cm and the applied potential may be as large as 15 kV. The y source to be measured is placed in a lead block in the collimator, and a beam of y rays is obtained using a system of calibrated diaphragms of diameter constant with an accuracy of 0.01%. The ionization current is measured on a bridge circuit using an electrometer valve: For Cldl y radiation, the apparatus was compared with the government rijntgen standard; the measurements agreed within 1.5%.
FIG. 9.-General
view of the calibration
apparatus
for measurement
of y radiation
in riintgens.
facingpage 354
The standardization of radioactive preparations
355
Using this calibration apparatus, the external y radiation of sources can be determined in riintgens to an accuracy of about 3%, and thus an accurate value obtained for the dose rate in riintgen per hour at a given distance from the source. In the calibration of radioactive sources it is important that radioactive impurities y spectrometer is used to control the radiochemical purity be absent. A scintillation and half-lives of the sources are measured. The latter examination is carried out by successive measurement of y equivalents using the compensated ionization chamber. The apparatus that has been described thus allows the determination of the activity, the total radiation, and the external radiation of a source. Table 2 summarizes the features of the equipment. TABLE2
-
-
Quantity measured
Item of equipment
Apparatus for measuring y equivalents 477counter Defined solid-angle counter y calorimeter B calorimeter Calibration apparatus for measuring y radiation in rijntgens
Activity Activity Activity Activity
l-1000 mg-equivalent Ra. O+IOlmg-equivalent Ra. 5.10-7-5.10-11 curie 10-6-10-8 curie 0.05-10 curie 0.01-10 curie
Dose rate at a given distance
10-1000 pr.sec-’
y equivalent
-
Limit of measurement
Accuracy of measurement, % 0.5-2 1-8 1-3 2-4 3-5 3-5
3
!_
By measuring radioactive sources using the methods described, their radiation characteristics (activity, total and external radiation) may be determined and checked by alternative methods. REFERENCES 1. 2. 3. 4. 5. 6. 7.
B~GOYAVLENSKY L. N. Truuy VNIIM 26 (42), 3 (1939). AGLINTS~VK. K. Trudy VNIIM 7 (52), 33 (1941). AGLINTSEVK. K. The Measurement of Zonising Radiations. AGLINI-SEV K. K. and KARAVAEVF. M. Trudy VNIIM (in KARAVAEVF. M. Trudy VNIIM (in press). ROW B. and STAUBG. Zonisution Chambers and Counters,
COHENR.
Gostekizdat, Moscow, (1950). press). McGraw-Hill, New York (1949).
Ann. Phys. 7,185 (1952). A. A. Trudy VNIIM (in press). 8. KONSTANTINOV 9. AGLINTSEVK. K. and KOLNOVAE. A. Dokludy Akudemy Nuuk XCVIII 3,357 (1954). 10. AGLINTSEVK. K. and KOLNOVAE. A. Trudy VNIIM (in press). 11. KI-IOLNOVA E. A. Trudy VNIIM (in press). 12. AGLINT~EVK. K., OSTROMUKHOVA G. P., and YUDIN M. E. Trudy VNIIM (in press).
THE STANDARDIZATION OF RADIOACTIVE PREPARATIONS* K. K. AGLINTSEV,F. M. KARAVAEV, A. A. KONSTANTINOV, G. P. OSTROMUKHOVA, and E. A. KHOLNOVA Abstract-A descriptionis givenof the methodsand apparatusused at the D. I. Mendeleev All-Union Standards Institute for the precise measurement of a number of quantitative properties of radioactive sources. The activity is measured by calorimetric, ionization, and absolute B-counting methods, the radium y equivalent by means of a 4rr ionization chamber, and the gamma dose rate with an ordinary ionization chamber. The accuracy and limitations of the measurements are discussed. To apply radioactive materials in practice it is necessary to know the principal characteristics of the isotopes used and the basic physical features of sources prepared from them. Among the most important characteristics which must be known are the end point and the shape of the /3 spectrum, the energy and number of quanta per disintegration for the y rays, and also the number and energies of the conversion electrons, For a complex ,B spectrum, this information and also the relative intensity must be known approximately for each of the components. In addition to data about the ,4 and y rays, the half-life of the radioactive isotope must also be known. Important features of radioactive sources are the activity of the preparation and the dimensions, weight, and nature of the container. For a source of relatively large volume, it is necessary to know the distribution of activity within it if this is nonuniform. The total radiation from a source containing a radioisotope of known /I and y spectrum is readily determined from the activity. However, in practice it is more important to know the external radiation; this differs from the total radiation because of self-absorption in the source and absorption in the container. The activity of a source can be measured in curies or in disintegrations per second. The y radiation from a source can be expressed in riintgens; in addition, the y radiation can be given in terms of the y equivalent of the source in grams of radium. The effect of p radiation can be measured in riintgen equivalent physical units. Uniformity of measurement is achieved by having apparatus which reproduces the unit of the physical quantity, and by a system of calibration and checking for both prototype and production-measuring equipment. The reproduction of curie and rantgen units is by means of instrumental standards, whilst the reproduction of a unit equivalent gram of radium is done using a government-owned standard consisting of two radium sources enclosed in thin-walled glass capsules. (L 2, The measurement of sources in equivalent grams of radium is the most precise method. y equivalents of sources are measured on two calibration instruments which compare the intensity of the y rays from the unknown source with that from a standard radium source. * Translated
by L. C. RONSON. 347
348
K. K. AOLINTSEVet al.
Each of the two arrangements consists essentially of a 4rr ionization chamber connected to an electrometer for measuring the small currents. Sources in the range l-1000 equivalent mg of radium are measured with an ionization chamber consisting of two concentric aluminium spheres of 5 mm wall thickness. The space between the spheres is the sensitive volume of the chamber. The ionization currents produced in the chamber by sources in the range mentioned above are from 1.5 . lo-l1 to 1.5 . 10-s amperes. Such currents are measured employing a compensating scheme’ due to TOWNSEND@) using a quadrant electrometer of maximum sensitivity 800 scale divisions per volt and a set of standard condensers. The arrangement is shown schematically in Fig. 1. The source being measured and the radium standard are placed alternately in the chamber.
FIG. 1.
Calibration apparatus for measuring y equivalents of sources in the range l-1000 mg of radium. K-ionization chamber; B,, B2, &-batteries; S-switch; C-standard condenser; V-voltmeter.
Measurements of y equivalents with this arrangement have an error 0*5-2%.‘4) Less active sources producing ionization currents comparable with the normal background current of the ionization chamber are measured with a different arrangement employing a so-called compensating chamber.(5) This consists of two similar cylindrical brass chambers separated by a lead block and connected in opposite polarity to a battery (Fig. 2); the differential current is zero in the absence of sources. The source being measured and the standard are placed alternately in one of the chambers. For sources equivalent to l-O*001 mg of radium the currents are in the range 5 . lO-‘l-5 . 10-14 amperes. For the measurement of these currents, a d.c. amplifier is used in an electrometer valve bridge circuit.15) The circuit, shown schematically in Fig. 2, measures the ionization current by a compensating method. The voltage drop produced in the output resistance by the ionization current is balanced by a potentiometer. In this case the amplifier is used only as a null indicator. In the range 1-0~001 equivalent mg of radium the error of measurement is from 1 to 5-8 %. Values of the y equivalents of sources measured by the above methods are corrected for self-absorption of the y rays, and a correction is also made for errors due to the geometrical dimensions of the sources. The value obtained for the y equivalent of a radioactive source depends on the conditions of measurement, mainly because of attenuation of the y radiation, and also on the geometry, type, and material of the walls of the ionization chamber. Figs. 3 and 4 show how the y equivalents of CoBo, (Zr + Nb)95, Agifo, and Sbrar
349
The standardization of radioactive preparations
vary with the thickness of a lead absorber. The full curve is for wide beams of y rays and the dotted curve refers to narrow beam conditions. From the measured value of the y equivalent of a source, the ionization produced at a distance of 1 metre can be estimated in riintgen per hour. Thus, since 1 gm of
FIG. 2.-Calibration ZC-compensating
apparatus
for measuring
y equivalents of sources in the range l-0901 equivalent mg of radium. chamber; P-potentiometer; &-standard cell ; E-electrometer valve; G-galvanometer; md-milliammeter; B-battery.
radium in a 0.5 mm-thick platinum envelope gives a dose rate at 1 metre of O-84 r per hour, a source equivalent to M gram of radium gives, under identical conditions, a dose-rate of 0*84M r per hour. The y equivalent of a source therefore characterizes the external y radiation under given conditions.
1.1 1.0
0.9 0.8 0.7 Q:
0.6 0.5 0.4 0.3 0.2 d, cm Pb Fig. 3
12
3’4 d,cm
5
6
7
8
Pb
Fig. 4
FIGS. 3 and 4.-Relation between y equivalent of Coao, (Zr + Nb)Os, AgIl” and Sbr”, and the filtration. Along the abscissa is plotted the thickness of absorber, and along the ordinate the ratio of the measured y equivalent to the value for 0.5 cm thickness.
The y equivalent of a source can be converted into activity (expressed in curies) only if the following quantities are known: the energy of the y quanta and their number per disintegration of the particular radioactive material, the sensitivity of the ionization chamber for various y ray energies, and also the effect of selfabsorption.
350
K. K. AGLINTSEVet al.
For absolute measurement of activity in curies an absolute /3-particle-counting method and also a calorimetric method are used. Although simple in principle, the absolute p-particle-counting method is in fact exceedingly difficult if an accuracy of the order of 3 % is required. Fundamental difficulties arise chiefly because lowenergy /I particles are always present in the continuous spectrum, and these are easily absorbed. The absolute counting method for /I particles depends on counting all the B particles from a given source over the whole solid angle 4~. The “47~ counter”
4P counter
4Tcounter
type
type
I
II
FIG. 5.-The “4n counter”. l--stand; 2-tap; 3-brass base; 4-source; S-counter wire; 6-body of counter; ‘I-polystyrene cup; B-lead-through insulators; 9-aluminium foil; IO-glass bell jar.
consists of two counters between which the source is placed. For absolute measurements with this instrument corrections must be made for the following: 1. Absorption of B particles in the film supporting the source. This correction is readily determined. 2. Absorption of /I particles by the counter itself. 3. Counting error due to the “dead time” of the counter and the finite resolving time of the electronic equipment. The 47r counter is shown schematically in Fig. 5. Two designs have been used.(‘jp ‘) The first consists of two cylindrical counters, cut parallel to the axes, with the source placed between them. This instrument was filled with a mixture of argon and ethyl alcohol, and was used in the Geiger regime. With this apparatus, sources of activity 5 . 10-8-10-10 curie could be measured with an accuracy of 24% for thin sources. The second design was made up of two half-cylinders with the source placed between, and used to count @ particles under proportional-counting conditions. It was filled with methane at a pressure between 30 and 50 cm Hg. The proportional “47r counter” can measure sources of 5 . lo-‘-5 . lo-l1 curie, with an accuracy of l-3 % for thin specimens.
The standardization
of radioactive preparations
351
Although not very accurate, the defined solid-angle method for absolute counting of /I particles is widely used because of its simplicity and wide range of application. In principle the method is very simple. The radioactive source on a thin support is placed in front of the counter, and p particles within a solid angle defined by a calibrated diaphragm reach the counter and are recorded. The total number of #? particles is then given by N = (477/Q) . N,, where No is the number of B particles counted in the defined solid angle 0. For absolute @particle determinations it is necessary, however, to apply a number of corrections when using this method. c8) The need for these corrections greatly reduces the accuracy of the method and in the best case it is 4-6 ‘A. The main error is due to uncertainty in the determination of the solid angle. /3 activities of the order 1O-5-1O-s curie could be measured by this method on our apparatus. A comparison of the two methods of absolute /3 counting shows that the 4~r counter method is the more accurate and also the simpler in principle. It is also the most sensitive of all known methods for measuring p activity. If a large number of measurements of p-active materials is required, the 4n counter can readily be used with each separate radioactive isotope to calibrate an end-window counter. In order to compare the above two methods with other means for absolute p counting (such as the calorimetric or ionization methods) the quantity of radioactive material must be known exactly. Not more than some tenths or hundredths of a milligram of material can be used if the correction for self-absorption of ,!I particles is to be kept small, and exact weighing of such small quantities on a microbalance is quite difficult. To estimate radioactive material which emits y rays in addition to B particles the y activity of the source is compared with the y activity of a larger amount of radioactive material, the so-called standard or reference quantity. The reference quantity was approximately 10 mg and could be weighed with sufficient accuracy. Several reference quantities were usually prepared;t51 6, they were compared with one another and those agreeing most accurately were used. Comparison of the y activity of the reference quantity and that of the source was carried out with a scintillation y spectrometer. The use of the y ray intensity to estimate the weight of a source proved to be a very reliable method. With a pure ,8 emitter such as P32, a relatively large amount of material is weighed out, and a proportion then taken to give a quantity of a fraction of a mg. The measurement of activity with counters becomes inaccurate when the specimens are thick. For such sources the calorimetric method is more satisfactory since the thickness of the material causes no difficulty. Calorimetric measurements of activity are based on the fact that the quantity of heat produced by the absorption of radiations from a radioactive source is proportional to the number of disintegrating nuclei. Two types of calorimeters have been designed for the measurement of absolute activity: (a) a differential double calorimeter for the measurement of y radiation; (b) an isothermal calorimeter based on the principle of the evaporation of liquid nitrogen for measuring the activity of sources from their fi radiation.(g-ll) The y calorimeter consists of two similar lead spheres (Fig. 6) mounted on an ebonite support in a thermostat. The substance to be measured is placed in a cavity provided in one of the spheres, and the temperature difference between the surfaces of the spheres is measured using a number of copper-constantan thermocouples.
K. K. AGUNTSEVet al.
352
A-body
FIG. &-Diagram of the y calorimeter. of the calorimeter; B-heater coil; C-plug; D-recess for source; E-attachment point for thermocouples; 2”thermocouple.
The principal characteristics of four y calorimeters are listed in Table 1. These instruments were calibrated by using a heater-coil inside each sphere. To evaluate the activity from the quantity of heat measured, the following data must be known: (a) the energy hvi of all y lines; (b) the number CQof y quanta of TABLE
1 Calorimeter
Property No. 1
External diameter of sphere, cm Thickness of absorber, cm Weight of sphere, gm Number of thermocouples Time to establish equilibrium, hours Sensitivity, lOa mm/watt
No. 2
No. 3
j
No. 4
7
5.5 2.1 969 20
i
3.5 1.06 232 18
10 4.3 5944 24
2.8 2030 .24
8 6.44
4 I.58
3 13.3
1.5 19.6
(c) the mean energy of the /? spectrum E,. Since energy per disintegration; the dimensions of the y calorimeters do not ensure complete absorption of all y rays, it is necessary, in addition, to know sufficiently accurately the proportion of the y energy which is absorbed by the calorimeter. This value-the coefficient piwas calculated for each calorimeter for y quanta of various different energies. The results of these calculations are summarized in Fig. 7. each
FIG.
‘I.-Absorption
of y rays in the calorimeters.
1,2, 3,4-number
of calorimeter.
The standardization of radioactive preparations
353
The formula relating N, the number of disintegrations per second, and W the quantity of heat produced per second in the calorimeter, has the following form: N, = W/X hviccipi + EB = nfi(Z hvictipi + ED) , where n is the galvanometer deflection in mm; j the sensitivity of the calorimeter in mm/watt; and EB the mean energy of the j3 spectrum.
A-manometer J, H-ground
FIG. 8.-Diagram of the isothermal /? calorimeter. ; C-mercury pump ; D-reservoirs; E, F, G-Dewar joints; K, L-connections for removing nitrogen; M-vacuum N-electric heater.
; B-capillary
vessels jacket;
;
The specially designed y calorimeters have sufficient sensitivity to allow measurement of sources with activities from a few tenths of a millicurie upwards. The accuracy of absolute activity measurement using y calorimeters is between 3 and 5 %. An uncertainty of 2-3 % can be attributed to error in calculation of the y absorption in the calorimeters. The calorimetric apparatus for measuring the activity of sources from their ,6 radiation consists of three coaxial Dewar vessels filled with liquid nitrogen (Fig. 8). The heat liberated by /? particle absorption is used to evaporate liquid nitrogen. The volume of gaseous nitrogen, which is measured at 760 mm Hg and 20°C using a capillary tube, is thus a measure of the amount of heat liberated by the source during the time of measurement. Calibration of the calorimeter showed that the sensitivity was sufficient for the measurement of sources equivalent to 10-4-10-5 6
354
K. K. AGLINTSEVet al.
watts (i.e. activity of the order of 10 millicurie for a mean /l-ray energy of approximately 1 MeV). The formula relating the heat production to the number of disintegrations has the following form for a pure /? emitter: N, = W/E@. When the specimen emits y rays a correction is made for their absorption in the source and calorimeter. The accuracy of the j3 calorimeter is between 3 and 5 X. The uncertainty is made up of the calorimetric error in measuring the quantity bf heat (1:15%j and the
To the electrical measuring equipment FIG. IO.-Calibration
apparatus for the measurement of y radiation in rijntgens (schematic).
error in measurement of the mean energy of the t!?particles. For sources having a simple @ spectrum the latter is 2-3 %. For sources with a complex @ spectrum Es is known with less accuracy. In addition to separate measurements on the @ and y calorimeters, combined measurements on these instruments have also been carried out. It was shown that by combining the results from the /? and y calorimeters the activity of the source could be determined from the known relative intensities of the y lines. Simultaneously with the activity of the source the number of y quanta per disintegration is also determined. A calibration apparatus has been designed to measure radiation in rijntgen units over the energy range 250 keV to 1.5 MeVo2) (Fig. 9). This consists of an ionization chamber enclosed by a tank filled with compressed air, a collimating system, electrical measuring equipment, and a high-voltage battery for the ionization chamber. The air-pressure in the tank can be raised to 15 atmospheres at which pressure electrons with energies up to 1.5 MeV are completely absorbed. The arrangement is shown schematically in Fig. 10. The ionization chamber has a measuring electrode, two screen electrodes, and a high-potential electrode ; there is a potential divider for controlling the electric field in the measuring volume of the chamber. The distance between the electrodes is 40 cm and the applied potential may be as large as 15 kV. The y source to be measured is placed in a lead block in the collimator, and a beam of y rays is obtained using a system of calibrated diaphragms of diameter constant with an accuracy of 0.01%. The ionization current is measured on a bridge circuit using an electrometer valve: For Cldl y radiation, the apparatus was compared with the government rijntgen standard; the measurements agreed within 1.5%.
FIG. 9.-General
view of the calibration
apparatus
for measurement
of y radiation
in riintgens.
facingpage 354
The standardization of radioactive preparations
355
Using this calibration apparatus, the external y radiation of sources can be determined in riintgens to an accuracy of about 3%, and thus an accurate value obtained for the dose rate in riintgen per hour at a given distance from the source. In the calibration of radioactive sources it is important that radioactive impurities y spectrometer is used to control the radiochemical purity be absent. A scintillation and half-lives of the sources are measured. The latter examination is carried out by successive measurement of y equivalents using the compensated ionization chamber. The apparatus that has been described thus allows the determination of the activity, the total radiation, and the external radiation of a source. Table 2 summarizes the features of the equipment. TABLE2
-
-
Quantity measured
Item of equipment
Apparatus for measuring y equivalents 477counter Defined solid-angle counter y calorimeter B calorimeter Calibration apparatus for measuring y radiation in rijntgens
Activity Activity Activity Activity
l-1000 mg-equivalent Ra. O+IOlmg-equivalent Ra. 5.10-7-5.10-11 curie 10-6-10-8 curie 0.05-10 curie 0.01-10 curie
Dose rate at a given distance
10-1000 pr.sec-’
y equivalent
-
Limit of measurement
Accuracy of measurement, % 0.5-2 1-8 1-3 2-4 3-5 3-5
3
!_
By measuring radioactive sources using the methods described, their radiation characteristics (activity, total and external radiation) may be determined and checked by alternative methods. REFERENCES 1. 2. 3. 4. 5. 6. 7.
B~GOYAVLENSKY L. N. Truuy VNIIM 26 (42), 3 (1939). AGLINTS~VK. K. Trudy VNIIM 7 (52), 33 (1941). AGLINTSEVK. K. The Measurement of Zonising Radiations. AGLINI-SEV K. K. and KARAVAEVF. M. Trudy VNIIM (in KARAVAEVF. M. Trudy VNIIM (in press). ROW B. and STAUBG. Zonisution Chambers and Counters,
COHENR.
Gostekizdat, Moscow, (1950). press). McGraw-Hill, New York (1949).
Ann. Phys. 7,185 (1952). A. A. Trudy VNIIM (in press). 8. KONSTANTINOV 9. AGLINTSEVK. K. and KOLNOVAE. A. Dokludy Akudemy Nuuk XCVIII 3,357 (1954). 10. AGLINTSEVK. K. and KOLNOVAE. A. Trudy VNIIM (in press). 11. KI-IOLNOVA E. A. Trudy VNIIM (in press). 12. AGLINT~EVK. K., OSTROMUKHOVA G. P., and YUDIN M. E. Trudy VNIIM (in press).