Analysis of Radionuclide Mixtures Using a Gamma-Beta Scintillation Spectrometer R. E. CONNALLY AND M. B. LEBOEUF Hanford Works, General Electric Co., Richland, Wash. Beta and gamma scintillation detectors have been designed for use with an automatic recording, energy spectrometer. The application of this instrument to the analysis of mixtures of radionuclides which decay by gamma emission has been investigated. Integration of the area under the photoelectric peak of a differential pulse height scan obtained with the gamma energy spectrometer permits quantitative determination of each gamma emitter present. Using this instrument, the gamma activities present in solutions containing three gamma emitters have been identified, and the individual gamma ac-
C
HEMICAL processes for the separation of fission products from the fissionable material produced by neutron irradiation of uranium require supporting analytical methods to determine the composition of many radionuclide mixtures. The analysis of mixed radionuclides is also of fundamental interest in other chemical, biological, and metallurgical studies. In general, the determination of an individual species, present in a mixture of other radionuclides, has involved chemical separations and measurement of radioactivities (S). This classical approach is undoubtedly sound. I t does, however, suffer several disadvantages: I t is time-consuming; a separation which is suitable for one component may cause losses of another; and the separation may be incomplete. Since the advent of electronic scintillation counting techniques, beta and gamma scintillation spectrometers have found application in the determination of decay schemes ( 2 , 7 ) , the study of bulk shielding (IO),and the study of high energy particles and photons (6). An instrument similar to that used for decay GAMMA SOURCE
ALUMINUM SAMPLE CARD
SHIELD AND COLLIMATOR AIR SPACE No1 ( T I ) CRYSTAL ALUMINUM REFLECTOR
GLASS WINDOW SELECTRON NO 5026 LIGHT PIPE FIRST
PHOTOTUBE TYPE 5819
wr
LAST DYNODE
OUTPUT PULSE
Figure 1. Pictorial Diagram of Gamma Sensing Element
tivities have been determined with an average precision of *7% (95% confidence limits) for triplicate determinations over concentrations of 10 to 70q' of total gamma activity. Analyses of solutions containing mixed gamma emitters were performed directly, without preliminary separations, in less than 40 minutes. This instrumental method for the analyses of radionuclide mixtures should be of particular value to radiochemical process control because of its speed and reliability; in addition, it is expected to find application in any studies where mixed gamma emitters are used.
studies has been modified to make it suitable for the analysis of mixtures of radionuclides. THEORY OF GAMMA SPECTROMETER
This spectrometer consists of a scintillation crystal, phototube, linear amplifier, differential pulse height analyzer, counting rate meter, and automatic recording, strip chart potentiometer. It automatically plots the rate of occurrence of gamma interactions against the pulse analyzer bias which is proportional to the quantity of energy lost to the scintillation crystal by the incident photon. A maximum is found on this scan at a bias proportional to the energy of the incident photon, which may serve to identify the gamma emitter (9). The gamma-sensing element is shown in Figure 1. The gamma photon produces a highly ionizing electron upon undergoing either a Compton or photoelectric interaction with the sodium iodide (thallium) crvstal. This recoil electron loses its energy in ionizing and exciting the crystal. -4 fraction of the excitation energy is released in the form of visible light ( I 6 ) ,which reaches the photocathode directly or by reflection. At this point the over-all conversion of incident gamma photon energy to light transmitted to the photocathode of the photomultiplier tube is in the order of 3% ( 7 ) . The light photons reaching the photocathode result in ejection of photoelectrons into the first dynode of the electron niultiplier structure. After amplification, the current in the last dynode is fed through a load resistor to develop a voltage pulse, the amplitude of which is proportional to the number of electrons incident on the first stage of the photomultiplier, and hence, to the energy of the initial recoil electron (16). The voltage pulse from the photomultiplier is further amplified and then fed to the input of a differential pulse height analyzer (IT), which classifies pulses according to their amplitudes. A scan of differential counting rate us. pulse height, or energy, is thus produced by the rate meter-recorder system. Gamma photons interact with the crystal according to severaJ processes: photoelectric effect, Compton effect, and pair production. In the photoelectric effect, recoil electrons of energy equal to that of the original gamma photon are produced and result in a peaked response in the pulse height man, known as the photoelectric peak. Each photopeak occurs a t a pulse height prcportional to the incident gamma energy and approaches a normal distribution. It can be shown that the width at half-maximum (rho) will vary with the square root of the gamma energy; thus, rho = k ( E ) ' / *(8). k varies inversely with the square root of the number of electrons collected on the first multiplier dynode. Therefore, an increase in efficiency of the photoelectron produc1095
ANALYTICAL CHEMISTRY
1096
tion and collection process will decrease k and improve the photopeak resolution. In the Compton effect, recoil electrons are produced whose energies range from zero to values approaching that of the gamma-ray. The pulse distribution of the Compton electrons obtained from the interaction of cesium-137 gamma photons xith the crystal appears in Figure 2 as the Compton continuum. .4t high gamma energies, a pair production peak may appear 1.02 m.e.v. below the photopeak energy. At 3 or 4 m.e.v., the pair production cross section becomes large enough to produce a pair peak detectable above the Compton continuum. 1.4
I
l
l
ENERGY I N MEV
Figure 2.
Gamma-Ray Spectrometer Scan of Cesium-I37
If a gamma photon has initially undergone a Compton scattering, there is a possibility that it may lose the remainder of its energy to the crystal in secondary interactions and thereby produce a pulse equal to a photoelectric pulse. This more complex type of interaction has the desirable result of increasing the nurnber of pulses having photopeak amplitude.
+I
1
E
The gamma disintegration rate of a source may then be obtained from the photopeak area and the counting yield. Counting yield is defined by:
Y =
counts per minute under photopeak gamma photons per minute from source
and must be determined as a function of photon energy, as in Figure 4. DESIGN OF SPECTROMETER
The gamma and beta scintillation crystals are composed of SaI(T1) and anthracene, respectively. Each orystal is optically coupled to a Type 5819 photomultiplier tube. A cathode follower couples the output signal into the linear amplifier. A switch is provided for selecting the beta detector, gamma detector, or test pulse generator as input to the spectrometer (Figure 5 ) . The linear amplifier (Atomic Instrument Co.) operates into a eingle-channel differential pulse height analyzer (Atomic Instrument &.), which functions by accepting only those pulses within a narrow range of amplitudes called the slit width. This acceptance slit is made to scan the entire region of pulse amplitude from 0 to 100 volts by rotating the pulse analyzer bias potentiometer continuously over the range 0 to 100 volts. The output pulses from the pulse height analyzer operate a counting rate meter (General Radio Co.) whose output is plotted against the analyzer bias voltage by the strip chart recording potentiometer. The pulse height analyzer can also be operated into a scaler when higher precision measurement is required (14). The recorder produces a scan of differential counting rate us. gamma energy by gearing the recorder chart drive to a 360" potentiometer (11). This potentiometer is switched into the analyzer circuit to replace the customary manually operated bias potentiometer. The speed of the recorder drive mechanism is adjusted 90 that an automatic scan is made from maximum to zero gamma energy every 30 minutes. Scanning ranges of 4,2, 1, and 0.5 m.e.v. are provided by an appropriately labeled stepwise gain adjustment in the linear amplifier.
___c
( € 1 ENERGY
Figure 3. Typical Gamma-Ray Spectrometer Scan
Let us now consider the basis for quantitative interpretation of a gamma energy scan. Quantitative determination of gamma emitters is possible because the area under the photopeak (Figure 3) is directly related to the rate a t which photons are undergoing interaction with the crystal. This area, expressed in counts per minute, can be obtained exactly from A = f r d E , or approximated from A = Z r A E , where r = counts per minute per slit width and AE = increment of energy equal to the slit width. A simplification is possible because the shape of the photopeak closely approaches a normal curve of error. Thus, the area is given by A = Sr(max) rho, where rmax = maximum photopeak height in counts per minute rho = width a t half-maximum in units of slit width S = 1.07 and is derived from a table of the normal curve of error, as follows: 1.00 2 X.(ordinate a t t = 0.00) ( t a t 1/z max. ordinate) t = abscissa of error curve
s=
0.20
0.1
02
oa
0.4
os
0.6
0.r
0.0
0.9
1.0
GAMMA ENERGY IN M E V
Figure 4. Yield vs. Energy Calibration
A canned h'aI(T1) crystal (Harshaw Chemical Co.) 4.4 cm. in diameter by 5.1 cm. deep is used. When coupled to a photomultiplier tube of average characteristics, one obtains a photopeak width a t half-maximum of 14% for the 0.663 m.e.v. photon of cesium-137. This corresponds to rho = 0.114 ( E ) ' / * . Tests with smaller crystals yielded photopeak widths as low as 9%. The poorer resolution of the large crystal is thought to result from a lower output of scintillation photons because of losses in the crystal. This disadvantage is outweighed in the present use of the spectrometer by the following desirable characteristics: The large crystal minimizes the ratio of Compton continuum to
1097
V O L U M E 2 5 , NO. 7, J U L Y 1 9 5 3 Table I. Effect of Lead Collimator with 2.54-Cm. Diameter Orifice on Ratio of Compton Continuum to Photopeak Height (Cs-137, 5 X 10s disintegrations per minute) Rfaximum Height of Compton Thickness of Continuum Region 70of 0.663 b1.e.v. Cs-lJ7 Photobeak Height Collimator, Cm. S o collimator 70 1.27 38 2 54 36
Table 11.
Effect of Lead Shielding on Background and Compton Continuum Counting Rate
(Cs-187, 5 X 106 disintegrations per minute) Thickness of Lead (Differential BackgroundCounting Scan5 Maximum Height of Shielding Rate), Counts/Min. ‘Ontinuurn Surrounding % of 0.663 Region’ h1.e.r. XaI(T1) Crystal, Cni. At 0.1 m.e.r. A t 1.0 m.e.v. Cs-187 Photopeak 36 S o shielding 1000 20 38 1.27 140 10 56 5.0 30 4 a Range 0 t o 1 m.e.v., slit width 0.02 m.e.v.
DETECTOR
I
c INPUT
GENERATOR
LINEAR AM PLI FIE R
DIFFERENTIAL PULSE HEIGHT
DETECTOR
COUNTING
wi
METER
In
1
Figure 5. Block Diagram of Scintillation Spectrometer
photopeak height; and it reduces the loss in photopeak yield with increasing energy, The amplifier pulse shaping controls are adjusted for minimum pulse width while maintaining the following characteristics: not more than 1% nonlinearity of the amplifier-pulse height analyzer response over the pulse amplitude range 0 to 100 volts: and less than 0.1 volt change in slit width over a bias range of 0 to 100 volts. The ensuing pulse has a rise time (10 to 90% of maximum value) of 0.5 peec. and a decay time constant of 2.5 p e e . The 0.663 m.e.v. gamma photon of cesium-137 is used to calibrate the spectrometer. The linearity of the amplifier-pulse height selector response is checked with a test pulse generator, with a pulse shape closely matching the scintillation pulses. A lead collimator, which confines the gamma-rays to the center region of the SaI(T1) crystal, is placed b e k e e n the gamma source and the spectrometer crystal to increase the probability of multiple gamma interactions and thus minimize the ratio of Compton continuum to photoelectric peak height, The effect of a collimator of diameter 2.54 cm. used with a cesium-13i source of equal diameter is shown in Table I. The collimator 1.27 cm. thick was chosen because its use results in an increased geometry over
the 2.54-em. thick collimator, with only slightly increased ratio of Compton continuum to photopeak height. The maximum usable gamma interaction rate in the crystal waa found to occur with a source activity of 3 X lo7 gamma photons per minute for cesium-137. As the source activity exceeds this level, the photopeak begins to broaden noticeably and the region of Compton continuum increases in amplitude. The loss in resolution a t the higher counting rates is probably due to pile up and overshoots effects in the linear amplifier. A gamma source activity of 3 X 106 photons per minute is normally the highest level of activity chosen. If the activity of the gamma source exceeds 3 X lo7gammas per minute, the source is placed on one of five sample shelves of decreasing geometry. The low geometry shelves not only reduce the counting rate, but they also enhance the effect of the lead collimator in minimizing the Compton to photopeak ratio. This effect must be considered when graphically solving a composite scan for the individual photopeak areas. The thickness of lead shielding to be placed around the gammadetecting crystal for optimum spectrometer operation was found to be a compromise between reduction in background with increasing thickness of lead and reduction in Compton continuum region with decreasing thickness, as shown in Table 11. Since the differential background counting rate peaks sharply a t lower energies, a shield 1.27 em. thick is sufficient to reduce the total back-ground counting rate 807,. The observed increase in Compton continuum with increasing thirkness of lead shield is due to backscattered radiation from the lead (4). The total background count with 1.27 cm. of lead shield is 2800 counts per minute, and its differential scan has a continuous distribution rising a t the low energies. Therefore, a single photopeak with 3000 integral counts per minute (equivalent to 3.0 X lo6 gamma photons per minute for cesium-137) can be readily detected as a peaked response rising well above the continuous background spectrum. The performance characteristics of the gamma-beta scintillation spectrometer are shown in Table 111.
Table 111. Performance Characteristics of Gamma Spectrometer hlinimum usable sample decay rate Maximum usable sample decay rate Error in energy assignment Error in quantitative detn. of mixtures Lower detectable limit of minor constituent Maximum practical sample volume Time required for gamma spectrometer scan
3 0 X 10‘ photons per minute 3 0 X 107 photons per minute =t1.8% (std. der.) Average absolute 3’% error, 7.4% (95% lim.) for concentrations of 10 t o 705% of total gamma activity 570 of total activity 100 ml.
30 min.
IDENTIFICATION O F GAMMA EMITTERS
The usefulness of this instrument in identifying radionuclides in a mixture is based on the observation that the photopeak energy of a gamma-emitting radionuclide may uniquely identify it or limit its identity to a few possibilities. A practical criterion for the resolution and identification of gamma emitters is that the energies of the principal gamma photons of the possible components differ by more than the half-maximum width (rho) of the instrument. The effectiveness of the method may be judged by considering a system in which only the radionuclides listed in Table IV may be present. I t is useful to choose one of the gamma radiations from each emitter to characterize it and confine attention to the corresponding photopeak in interpreting gamma energy mans. For this reason, the minor gamma radiations of these emitters have been omitted in Table IV, and only the principal gamma and beta radiations are shown (18). Applying the criterion for resolution mentioned above, it is seen in Table V that seven pairs of
ANALYTICAL CHEMISTRY
1098
beta energy of other beta emitters can then be estimated similarly by extrapolating the straight-line portions of the scan to zero counting rate for each component. This is illustrated for the Principal beta energy scan of a mixture of iodine-131, ruthenium-rhodiumGamma Principal Beta Half-Life Kuclide Energy, X1.e.v. Energy Emax. we.>.. 106, and niobium-95, designated as mixt,ure 1 in Figure 7. The 1 2 . 8 d. (Ba)-La-140 1 . 6 5 (75) 1 . 4 0 (70) beta emitter having greatest energy is established with cert'ainty, 290 d. Ce-Pr-144 1 . 2 5 .. 2.87 (90) 33 d. Te*-Te-129 0.80 . . 1 , 8 0 (100) and in the favorable case shown, other maximum beta energies 35 d. Nh-95 0.77 (100) 0 .. 4 10 5 (100) are determined. The present development of this technique 65 d. Zr-95 0 . 7 1 (98) 0 (98) 33 y. Cs-Ba*-137 0 . 6 6 (95) 0 . 5 2 (95) permits estimates of maximum beta energies to &lo%. The 11.3d. Nd-147 0 . 5 3 (40) 0 , 8 4 (60) 0 . 4 0 (40) 1 2 , 8 d. Ba-140 0 . 5 3 (40) 1.oo (eo) 0 . 4 7 (40) use of the beta spectrometer to decide among unresolved gamma 1 y. Ru-Rh-106 0 , 5 l (23) 3 . 5 5 (68) emitters is indicated in Table VI. 40 d. Ru-103 0 . 4 9 (94) 0 . 2 2 (94) 8 d. 1-131 0.364 (75) 0.60 ( 8 5 ) The correct component can be established in six out of seven unresolved gamma pairs. In practice, the origin of the sample Asterisk indicates metastable state. Figure in parentheses, % of total disintegrations. and its chemical and physical history will serve to eliminate some of the ambiguous cases mentioned above. Table V. Resolution of Gamma Scintillation Spectrometer for Radionuclides Having Most Similar Gamma Energies -
Table IV.
Principal Gamma and Beta Radiations of Radionuclides Formed during Fission
Pair of Gamma-Emitting Kuclides Considered 1 2 &Ba)a-La-140 Ce-Pr-144 e-Pr-144 Te*-Te-129 Te*-Te-129 Nb-95 Zr-95 Te*-Te-129 Te*-Te-129 Cs-Ba*-137 Nh-95 Zr-95 Nb-95 Cs-Brt*-137 Cs-Ba*-137 Zr-95 Nd-147 Zr-95 Kd-147 Cs-Ba*-137 Nd-147 Ba-La-140 Nd-147 Ru-Rh-106 Kd-147 Ru-103 Nd-147 1-131 Ru-R h- 106 Ru- 103 Ru-Rh-106 1-131 a
El
h1.e.v. 1.65 1.25 0 80 0.80 0.80 0.77 0.77 0.71 0.71 0 66 0.53 0.53 0.53 0.53 0.51 0.51
E>, h1.e.v. 1.25 0 80 0.77 0.71 0.66 0.71 0.66 0.66 0.53 0.53 1.650
0.51 0.49 0.364 0.49 0.364
El - Ez, M.e.v. 0.40 0.45 0.03 0.09 0.14
0.06 0.11 0.05 0.18 0.13 1.12 0.02 0.04 0.17 0.02 0.15
Resolution Rho a t El,' hl e.v. 0 . 146 0.127 o 102 O. 0.102 0.100 0.100 o 096 0.096 0.093 0,083 0,083 0,083 0.083 o, 081 0.081
.4ssuming t h a t La-"Q is present by decay of Ba-l47.
/BETA
SOURCE
ANTHRACEN CRYSTAL
ALUMINUM LIGHT SHIELO
SELECTRON NO. 5026 L I G H T PfpE
PHOTOTUBE
Figure 6. Pictorial Diagram of Beta-Sensing Element
nuclides having closely similar gamma energies are not resolved by the gamma spectrometer alone. It is difficult or impossible to assign the photopeaks correctly in a gamma energy scan involving these pairs unless additional information is available. All other combinations, for which the principal photopeaks are separated by considerably more than rho, will be easily identified. Fortunately, in many cases it is possible to confirm the presence or absence of a doubtful beta-gamma emitter by means of a beta spectrometer scan. Beta energy spectra may be measured using the same instrument by substituting an anthracene crystal for the sodium iodide (thallium) ( 5 ) , with the physical arrangement shown in Figure 6. The beta spectrum so obtained approaches a straight-line slope toward increasing energy for a simple beta decay, By extrapolating the straight-line portion of the spectrum to zero counting rate, an intercept is obtained which is directly proportional to the maximum beta energy of the emitter. T h e spectrometer is calibrated using a beta emitter of known energy, such as thallium-204, E,,, = 0.78 m.e.v. (18). The
QUANTITATIVE MEASUREMENTS
To make oossible the determination of gamma emitters with this instrument, it was necessary first to establish the photopeak yields a t various energies. Having established the relation between photopeak area and gamma decay rate for an individual nuclide, it then becomes feasible to apply the same relation for the determination of this nuclide in a mixture of gamma emitters. To determine photopeak yield factors, beta-gamma n-ot sources were prepared as standards using solutions Resolved of the following individual nuclear species (16) for which the decay schemes (18) are known: 0.364 m.e.v. iodine-131, 0.400 m.e.v. gold-198, 0.513 m.e.v. ruthenium-rhodium-106, 0.771 m.e.v. niobium-95. The beta counting rates of the iodine-131, gold-198, and ruthenium-rhodium-106 sources were measured with a mica window Geiger-Muller counter. Corrections were applied for air and window absorption, backscatter, sample spreading, and geometry to obtain beta disintegration rates (13). The niobium-95 source was measured using a windowless flow counter. Its beta counting rate was corrected for backscatter ( 1 ) to obtain the beta disintegration rate. The best available decay scheme data were applied to the beta disintegration rate to obtain the gamma disintegration rate for each solution (18). Kext, aliquots of each solution were deposited on watch glasses, taken to dryness, and measured on the gamma ray spectrometer. The resulting integrated photopeak counting rates were divided by the known sample gamma disintegration rate to establish the photopeak yields of Figure 4. The extrapolated curve beyond 0.77 m.e.v. is calculated from the theoretical total gamma cross section for sodium iodide ( 7 ) . The extrapolated region below 0.36 m.e.v. shows the expected combined effect of the inch thick aluminum beta absorber and the photoelectric absorption in the crystal. In the process of evaluating a gamma energy scan, the Compton continuum, pair peak, minor photopeaks, and background must
Result Resolved Resolved ixotreaoll.ed Not Resolved Kot resolved Resolved Xot resolved Resolved Resolved Resolved Not resolved Not resolved Resolved
Y
Table VI. Maximum Beta Energy of Pairs of Nuclides Not Resolved by Gamma Energy Analysis Pair of Gamma Emitting Xuclides Considered 1 2 Te*-Te-129 T e *-Te-129 Nh-95 2r-95Nd-147 Nd-147 Ru-Rh-106
Nb-95 Zr-95 Zr-95 Cs-Ba-137 Ru-Rh-106 Ru-103 Ru-103
Emsx of Most Energetic Major Beta, M.e.\-. 1 2 0.15 1.8 0.40 1.8 0.40 0.15 0.52 0.40 3.55 0.84 0.22 0.84 0.22 3.55
Results Easily resolved Easily resolved Easily resolved Not resolved Easily resolved Easily resolved Easily resolved
V O L U M E 25, NO. 7, J U L Y 1 9 5 3 Table VII.
Isotope 8d 1-18' 42d Ru-loa 290 Ru-Rh-108 37y cs-iar 65d Nb-96
1099
Standard Gamma Spectrometer Scans Normalized to Peak Height of Principal Gamma Energy
Energy of Principal Gamma Photon, h1.e.v. 0.05 0.364 22 0.494 45 0.513 107 0.663 49 0.771 61
0.10 0.15 23 21 50 35 91 80 43 44 58 48
Per Cent of Principal Photopeak Height at J1.e.v. Energies 0.20 0 . 2 5 0 . 3 0 0.35 0 . 4 0 0 . 4 5 0.50 0.55 0 . 6 0 0 . 6 5 19 13 24 100 46 2 2 2 3.8 4.4 35 35 30 18 16 37 100 37 6.5 1.7 33 70 100 55 39 24 67 56 45 36 42 41 40 40 40 30 19 15 35 100 52 50 48 47 45 43 42 35 19 16
of: 0.70 0.75 0 . 8 0 0.85 2.5 0 0 6 51 47
be subtracted from each major photopeak in order to establish an accurate integrated photopeak area. The curve of Figure 2 shows the Compton region and minor photopeak normalized to a principal gamma photopeak height of unity. Normalized curve; Pr-ere prepared for a group of gamma emitters. Data from these curves are shown in Table VII. Multicomponent scans are analyzed as folloas: The height of the most energetic principal photopeak is measured. The heights of the Compton continuum, pair peak, and minor photopeaks associated with the most energetic principal photopeak are evaluated using normalized curves. These values are drawn in below each lesser gamma energy photopeak of interest, as shown in a scan of a three-component solution (Figure 8 ) , designated as solution 1. By subtracting, the net height of the next most energetic gamma photopeak can he established. ( I n a few cases there may be minor gamma photopeaks of a particular isotope which are more energetic than the principal photopeak. This will require correcting the height measured in step 1.) When all the net peak heights have been determined, i ho is measured as discussed previously and the integrated photopeak areas are calculated. The photopeak area in counts per minute is divided by the photopeak yield to obtain the gamma disintegration rate oi each isotope. Finally, considerations, such as decay scheme data, half-life, and atomic weight. may be applied to carry the results to a ueight basis.
!I!//! 1
0 10
100
0 55
8
0.90
0
i
O L
ENERGY INMEV
Figure 8. Gamma-Ray Spectrometer Scan of Solution 1
width occurred with the 0.364 m.e.v. photopeak of iodine-181 and \vas in the order of 4%. K i t h the increase in rho there is also a decrease in peak height such that the effect of a 20 K.e.v. slit width on photopeak area is negligible. Table VIII.
Analysis of Standard Solutions Average Spectrometer Anal sis,
Precision of Av. 93% Conf. RadioKnown % Level Average nuclide yc Gamma 'Ilin. Triplicate Detn. i:% % ' Errol Standard I 1-131 28.5 32 2 7 5 +i3 n Ru-Rh-106 30 6 26 6 9 9 -13 2 Sb-95 40 9 41 1 4 0 +06 Standard I1 1-131 8.5 10 s 5.4 +27 n Ru-Rh-106 18.2 17 1 7.9 -6 2 Fib-95 73.3 72 1 2 6 -1 6 Standard I11 1-131 17.3 17.8 10.6 f ? !I Ru-Rh-106 49.5 48.8 3.3 -1 .5 Fib-95 33.2 33.4 11.5 +0.6 A v . precision (of triplicate determinations over range of concentrations 8.5 to 73.3% gamma/min.) = 7.0 Av. absolute % error in determined value = 7.4
-Y7k
~
5
1''
O60MEV
I
I
2 0 ENERGY IN MEV
30
Ru-Rh Os 355M V
0 0
Figure 7 .
10
40
Beta-Ray Spectrometer Scan of Mixture 1
The photopeak n%lth a t half-maximum is increased slightly in escess of the value given by rho = k(E)' l 2 due to scanning with a finite slit width. T h e effect of slit width on rho can be calculated from Sheppard'e correction (1%')as follows: rho* = rho2 -
L-
--
2.05, rho = corrected width a t half-maximum rho = observed width a t half-maximum c = slit m-idth N
The gamma scans described in this work were taken Kith a slit width of 20 K.e.v., and the greatest increase in rho due to this slit
QUANTIT4TIVE DETERMIN4TION OF K N O W 3 M I X T U R E OF RADIONUCLIDES
I n order to obtain correct results by the procedure given for analyzing gamma scans of radionuclide mixtures, it is necessary that the follo~vingcondition be met. The photopeak areas computed from a complex scan must be the same as those ohserved when each gamma emitter is measured individually The accuracy of the method was tested with mixtures of known composition, designated as solutions I, 11, and 111. The per cent gamma activity of each component is given in Table VIII. The solutions were prepared to assure compositions known accurately to 1% of the total activity. The total gamma disintegration per minute of the solution was in the order of 4 X lo6 gamma photons per minute. Three gamma spectrometer scans were made of each solution. The solutions were measured direct]>-. as the water and 5-ml. volumetric flask cause negligible absorption of these gamma photons. The known gamma energies of the radionuclides used and the results of the gamma spectrometer scans are given in Table I S .
1100 Table IX.
A N A L Y T I C A L CHEMISTRY Comparison of Gamma Spectrometer Energy Measurements to Standard Values
0 364 hZ.e.v., 1 - 1 3 1 0 513 M.e.v. Ru-Rh-146 0 771 M.e.v., Nb-95 (hleasured Measured Measured ,energy, Error, energy, Error, energy, Error, E h m ,e.Y. % m e . Y. % m.e.v. 70 -0.8 0.520 0.361 +1.3 0.760 -1.5 $1.1 0.755 +1.3 0.520 0.368 -2.1 -0.6 0.766 0.516 +0.6 -0.7 0,362 +4.0 +3.9 0,777 0.533 +0.8 0.380 +1.7 0.762 -1.2 $1.3 0.520 0.370 fl.1 -0.4 0.768 +1.3 0.520 0.368 +1.7 0.752 -2.5 -1.0 0,508 0.370 +1.7 0.510 -0.6 0.740 -4.1 0,370 -0.6 0.518 $1.0 0.758 -1.7 0.362 Std. dev. of 27 measurements 2~ 1.8%.
The experimental energy assignments have a standard deviation of =!=1.8%,and the emitter responsible for each photopeak is easily determined. The calculated results are compared to known compositions in Table VIII. The nine triplicate measurements have an average precision within 3 ~ 7 %(%yo confidence limits) over a range of concentrations 73.3 to 13.3% of the total gamma activity. .4s could be expected, minor constituents of loner energy, such as iodine-131 in solution 11, are determined with the least accuracy, and major constituents with the highest energy, such as niobium-95 in solution 11, are determined with the highest accuracy. It is estimated that in a single energy scan for the more unfavorable cases a minor constituent present to the extent of only 5y0 of the total gamma activity can be determined within 3 ~ 5 0 %of the true value, and for the more favorable case mentioned above, a major constituent can be determined to within =!=2%of the true value. DISCUSSION
The most significant advantages of the gamma scintillation spectrometer are its speed and reliability. Studies involving mixtures of radionuclides, which v-ould normally be unreasonably tedious, are thereby facilitated. This is well illustrated by the
fact that 27 measurements were obtained on the standard solutions in a total instrument time of 4.5 hours. Other workers will undoubtedly find applications of the method -for example, in the determination of trace impurities by neutron activation, wear and corrosion studies of alloys, the investigation of chromatographic separations using radioactive tracers, and the study of any system involving several radionuclides. LITERATURE CITED
(1) Christian, D., et al., Sucleonics, 10, S o . 5 , 41 (1952). (2) Cook, T. B., and Haynes, S. K., Phys. Rev., 86, 190-5 (1952).
(3) Coryell, C. D., and Sugarman, N., “Radiochemical Studies. The Fission Products,” Xational Nuclear Energy Series, 1‘01. 9, Div. IV, Xew York, McGraw-Hill Book Co., 1951. (4) Hofstadter, R., and McIntyre. J. A . , .Vucleonics, 7, S o . 3, 32 (1950). (5) Hopkins, J. I., Rec. Sci. Instr., 22, 29 (1951). (6) Johansson, S. A., Yature. 166, 794-5 (1950). (7) Jordan, W. H., “ilnnual Review of Nuclear Science,” Vol. I, Stanford, Calif., Annual Reviews. Inc., 1952. (8) Kelley, G. G., Nucleonics, 10, No. 4, 35 (1952). (9) Maeder, D., and Wintersteiger, V., Phys. Rev., 87, 537-8 (1952). (10) Afaienshein, F. C., Oak Ridge Xational Laboratory, Oak Ridge, Tenn., Document ORNL-1142 (1952). (11) Parsons,, J. H., Atomic Energy Commission, Docuneiit AECD1827 (1948). (12) Rider, P. R., “Statistical Methods,” London, John Wiley & Sons, 1939. (13) Schwendiman, L., Hanford Works, Richland, Wash., Documenf HW-18258(1950). (14) Strickler, T. D., and Wadey, I$‘. G., Rev. Sci. Inst., 24, S o . 1 , 13 (1953). (15) Taylor, C. J., et al., P h y s . Rev., 84, 1034 (1951). (16) U. S. Atomic Energy Commission, Isotopes Division, Oak Ridge, Tenn., “Isotopes Catalog and Price List No. 4.” (17) Van Rennes 4 B., Nucleonics, 10, No. 8, 22 (1952). (18) Way, K., et hi.,’“Nuclear Data,” National Bureau of Standards, C~TC 499 . including Supp. 1, 2, and 3 (1950). BEC documents may be obtained from U. S. Atomic Energy Cornmission, Reference Branch, Technical Information Service, P. 0. Box 62, Oak Ridge, Tenn. RECEIVEDfor review January 13, 1953. Accepted April 6, 1953. Presented before t h e Division of Analytical Chemistry at t h e 123rd Meeting of t h e AMERICAN CHmiIcAL SOCIETY, Los Angeles, Calif.
Metal Column for Distillation of Corrosive Gas Mixtures at low Temperatures N. C. ORRICK AND J. D. GIBSOY’ Carbide and Carbon Chemicals Corp., K-25 Plant, Oak Ridge, Tenn.
A
METHOD of analysis was needed for complex multicompo-
nent gaseous mixtures containing corrosive materials such as fluorine, chlorine, and hydrogen fluoride. The identity of some of these components was not known and others were desired in the pure form for further study. Fractional distillation could, in many instances, give a complete analysis as well as pure components and was consequently chosen as a suitable method for the analysis of these mixtures. Even in cases in which distillation has a limited application, because components have boiling points a t about the same temperature or azeotropes are formed, this column is useful in producing distillate samples with a smaller number of components than the original sample. These less complex mixtures are then more readily analyzed by other methods of gas analysis, such as infrared absorption, mass spectrum, and gas density measurements (8). h number of low temperature distillation columns are described 1
Present address, Engineering Staff. Ford Motor Co., Dearborn Mich.
in the literature (1, 3-6); houever, none is suited for corrosive mixtures. The glass in the column, the mercury in the manometer, or the metal packing will react with one or more of the gases present. The corrosion-resistant apparatus described herein is made entirely of nickel, bronze, and chlorotrifluoroethylene polymer (Kel-F, made by AI. 1%‘. Kellogg Co., Jersey City, N. J,). The design is based on that of Podbielniak ( 4 ) with certain modifications. DESCRIPTION OF APPAR4TUS
The apparatus consists of the fractionation column and pot assembly, the temperature- and pressure-measuring instruments, the distillate withdrawal unit, and the automatic coolant supply system. Fractionation Column and Pot Assembly (Figure 1). The fractionation column proper was a 30-inch section of 5 / ~ i n c h nickel tubing packed with 3,1a2-inchnickel helices. The upper
