gram is somewhat. more versatile than t,he simultaneous equation program in that it contains subroutines which are readily adapted to simpler problems. Also, because it is more closely analogous to graphical procedures for resolving spectra. t,he radiochemist can be of more help in diagnosing malfunctions in the program. The improvement in the simultaneous equations results over those previously reported can be explained by these changes: the smoothing of data to fit theoretical decay curves improves the statistical accuracy in the standard coefficients; the control of temperature in the detector and of gain in the ampli-
fier provides more reproducible data in duplicate irradiations; the preparation of samples by mixing the solutions used for standards eliniinates the difficulty of reconciling possible errors in chemical or spectrographic analyses previously used as a basis for comparison.
ACKNOWLEDGMENT
R e appreciate the valuable contributions of R. E. Greenwood, who explored the problems which might be encountered in a computer program, and of Sandra P. Guldman, who coded the IBM programs.
Determination of Radioactive Lisuid Scintillator
LITERATURE CITED
(1) dnders, 0 . U., Beamer, JV. H., .ISAL. CHEII.33, 226 (1961). ( 2 ) Breen, J$-. l l . >Fite, L. E., Gibbons, I).> Wainerdi, It. E., Trans. A m . .Vucl. SOC.4, 244 (1961). (3) Gilmore, J. T., Hull, U. E., A s . 4 ~ . CHEX 35, 1623 (1963). (4) Gilmore, J. T., Hull, 11. E., Ihzd., 34, 187 (1962). ( 5 ) Gilmore, J. T.,Hull, 1). E., Fries, B. A,, 6th J$70rld I'etroleuiii Congress, Section V, Paper 19 (1963j. (6) Ku>-kendall,JV. E., FTaiirerdi, It. El., Trans. d m . .YucZ. SOC.3 , 93 (1960). ( 7 ) Salmon, L., AVucl. Inslr. :Ilethotls 14, 193 (1961).
RECEIVEDfor review January 0 , 1964. Accepted July 15, 1964.
Noble Gases with a
DONALD L. HORROCKS and MARTIN H. STUDIER Chemistry Division, Ar!gonne National Laboratory, Argonne, 111.
b The disintegration rates of the radioactive noble gases RnZz2,Xe131m, and K P have been measured with a toluene-base liquid scintillator. This was practical because of the high solubility of the noble gases in the aromatic hydrocarbons used as solvents in liquid scintillators. The distribution coefficients, the ratios of the concentrations of the noble gases in the liquid scintillator to the space above C., it, were measured. At - 1 5 " these were 5 and 32 for xenon and radon, respectively, The differential pulse height spectra of the light output of the liquid scintillator with dissolved radioactive noble gases are shown.
L
SCIXTILLATORS have been used to obtain ihe absolute disintegration rates of beta (8, 12) and alpha ( 6 , 7 , I O ) emitting nongaseous nuclides. The fact that the heavier noble gases are appreciably soluble in the aromatic hydrocarbons commonly used as solvents in liquid scintillators ( I , $ , Q , I I ) makes practical the quantitative determination of radioactive noble gases in liquid scintillators. IQCID
EXPERIMENTAL
Apparatus. The detection and recording equipment consisted of a multiplier phototube i D u M o n t 6292), cathode follower preamplifier, linear amplifier, and multichannel analyzer. [See previous works (4, 6).] The saniple holder-light guides were Lucite
/SMALL LIGHT GUIDE for 0 2 5 mi Liquid Scintillotor (0)
Figure 1.
LARGE
LIGHT GUIDE far IOrnl,Liguid Scintillator
(b)
Light guides-sample holders
cylinders. For small samples a cylinder 1 inch in diameter and ' I 2 inch long with a 7-nim. sample well drilled parallel to the faces waq used (Figure l a ) . For large sampleq a cylinder l 7 inches in diameter and 1' inches long nith a 32-mm. sample well drilled parallel t o the faces was used (Figure l b ) . The light guides were completely painted with a TiOs reflecting paint ( 7 ) except for one face which as coupled to the face of the multiplier phototube. Optical coupling was attained n i t h a clear silicone grease betv een the light guide and multiplier phototube and a clear silicone fluid in the sample well. The samples and multiplier phototube were maintained, for most of this iTork. in a freezer a t - 1.5' C. Liquid Scintillator. The liquid scintillator used in this work consisted of 7 grams of 2,5-diphenyloxazole, PPO, and 0.5 gram of 1,4-bis-2-(4methyl - 5 - phenyloxazolyl)benzene, RI,-POPOP, in 1 liter of toluene. Sample Preparation. The samples were prepared as follows: The desired amount of liquid scintillator was placed in the sample container and flushed with argon to remove a n y dissolved oxygen. This was cooled
to liquid nitrogen temperature and evacuated. X given volume of noble gas, with radioactivc tracer, was condensed onto the liquid scintillator at, liquid nitrogrn temperature, and the sample container wa.q flame sealcd. The sample is n-armed and equilibrated a t the temperature of the experiment (room temperature or -15' c'. in the freezer). Two saniple containers, one for 0.25 ml. and the other for 10 nil. of liquid scintillator, were employed. For the 0.25-nil. size a 6-nini.>thin-n.alled, glass tube served as thc liquid containcr. On top of this n-a. a 2-mni. i.d. capillary tube for reduction of the iliace above the liquid scintillator. The 10-nil. size sample container.; were made from 30nim. o.d. g1as.W tubing. 'The small neck was for sealing the containers and reducing the silace abo\-e the liquid scintillator. The small bize containers were used for determining radon and xenon. Hon-ever. becau*e of thc high energy electrons from Kr5j, I:,,:,, 672 k.e.v., the larger size container was used to minimize wall effect?. The distribution of nohle gases between the liquid scintillator and the space above it was nieawred for 0.25 ml. of liquid scintillator in saml)le containers as >holm in Figure 2. The shorter container? w r e Iirepared from the original tallezt container by freezing the xenon and radon Ivith liquid nitrogen and successively sealing off I)art.>of the container tube. The ca1iillar>- tube made possible a minimum space above the liquid scintillator in the final samlile. I t alw reduced the geonietry for excitation of the hquid rrintillator by c'lcctrons and alpha particks producwl in the gas phase hy the decay of Xpl3lrn VOL. 36, NO. 11, OCTOBER 1964
2077
6 mm
71 Figure 2. Sample containers for solubility measurements Varying space above liquid scintillator by successively sealing off container tube
and RnZz2. Since krypton has a vapor pressure of approximately 3 mm. a t liquid nitrogen temperature, a lower temperature cooling bath would be required to use this method for solubility determination. RESULTS
The differential pulse height spectra of the light output of 0.25 ml. of the liquid scintillator with about 0.08 cc. space above it are shown for samples with RnZz2,Xe131m, and Kr85 in Figures 3, 4, and 5. The range of the alpha particles and 164-k.e.v. conversion electrons were short enough that they expended all of their energy in the liquid scintillator. However, the high energy beta particles of KrS5, Emsxof 672 k.e.v., have a range greater than the thickness of the 0.25-ml. liquid scintillator. They expended only part of their energy in the liquid scintillator and gave a distorted beta spectrum (Figure 5a). Only trace amounts of radon were added to the liquid scintillator because of the high specific activity of the 3.8-
day half-life RnZz2. The alpha peaks of RnZz2 (5.49 m.e.v.) and daughters Po218 (6.00 m.e.v.) and (7.68 m.e.v.) were superimposed upon the beta continuum of the daughters Pb2I4 and Bi214. Since alpha particles were counted with 1 0 0 ~ oefficiency in a liquid scintillator (6, 7 , 10) the disintegration rate of the RnZz2dissolved in the liquid scintillator was obtained by summing the area under the peak corresponding to the 5.49-m.e.v. alpha particles. The counts due to the part of the beta continuum under the peak have to be subtracted. The linewidths (full width a t half height divided by pulse height of the maximum) for the RnZz2 and Poz1* peaks agreed with those obtained for other dissolved alpha-emitting nuclides (6) indicating that RnZz2and Poz18were truly dissolved in the liquid scintillator. Also the disintegration rates (area under each peak) were essentially the same. However, 1 or 2 hours after the preparation of a RnZz2sample the pulse height peak for began to spread and in some cases formed a double peak which indicated an appreciable wall effect for the decays (Figure 4). Since the time between the decay of PoZ18 and was long, due to the 26.8-minute and 19.7-minute half-lives of the P02I4 precursors Pb214and Bi214, it appeared that most of the Pb2I4and/or Bi2I4 must have condensed on the walls of the container before the emitted its alpha particles. The spectrum for the Xe131mshows a double humped peak for the 97% converted 164-k.e.v. gamma rays. The two humps corresponded to 134 k.e.v. and 164 k.e.v. The lower energy was obtained when the gamma rays were K converted and the 30-k.e.v. Xe K x-rays escaped from the liquid scin-
Figure 4. Differential pulse height spectra for Xe131m sample [approximately 3 cc. (S.T.P.) of xenon] in 0.25 ml. of liquid scintillator
tillator. The higher energy was obtained when the K x-rays were absorbed in the liquid scintillator or when the gamma rays were L converted. The 4-k.e.v. X e L x-rays are totally absorbed by 0.25 ml. of the liquid scintillator ( 5 ) . The continuum part of the pulse height spectrum was due mostly to some 5-day beta emitting Xe133present in the sample. Thk disintegration rate of Xe131m dissolved in the liquid scintillator was obtained by summing the area under the peak of the spectrum remembering that the gamma rays are only 97% converted and that the part of the continuum under the peak, due to any Xe133 in the sample, had to be subtracted. As the concentration of xenon in the liquid scintillator increased there was a
30
50
0
IO0
I50
R E L A T I V E PULSE HEIGHT
Figure 3. Differential pulse height spectra for Rn222sample in 0.25 ml. of liquid scintillator 0 x
2078
- 1 hour after preparatlon - 2 4 hours after preparatlon ANALYTICAL CHEMISTRY
"
0
50
IO0
I50
200
250
R E L A T I V E PULSE H E I G H T
Figure 5. Differential pulse height spectra for KrS5sample in (a)0.25 ml. and (b) 10 ml. of liquid scintillator
+
80
c
This is of the form y = az b. The intercept of a plot V,us. 1/ A , would be equal t o -KIT8 or> =
(O'
1(intercept) -
(5)
V'
20
40
60
Such plots of the experimental data gave straight lines. The values of K obtained are shorn in Table I. The value of K for X e at room temperature (27" C.) was obtained by moving the sample and multiplier phototube out of the freezer and agreed with that obtained by others (9, 11).
80
Table I. Solubilities of Noble Gases in Toluene-Base Liquid Scintillator
a
Figure 6. Quenching effects of increasing xenon concentration in 0.25 ml. of liquid scintillator. a.
b.
1 . 3 3 cc. Xe (S.T.P.) 2.0 cc. Xe (S.T.P.)
quenching effect (Figure 6). When 7.6 cc. (S.T.P.) of xenon, tagged with Xe131m, were dissolved in 0.25 ml. of the liquid scintillator (Figure 6d) the low energy tail increased (considerably and the conversion electron. peak broadened with a loss of the fine structure. The 672-k.e.v. E,,, beta spectra for KrR5 dissolved in 0.25 ml. of liquid scintillator is greatly distorted. The high energy electrons expend only a part of their energy inL the liquid scintillator (the rest expended in the walls of the container) thus indicating a large number of low energy electrons. T o minimize this wall effect krypton (tagged with Kr55) was dissolved in 10 ml. of liquid scintillator. The pulse height spectrum obtained (Figure 5b) had essentially the shape expected for Krsj (a first forbidden unique beta transition) in 10 ml. of 'liquid scintillator ( 3 ) . The disintegration rate of the Kr85 dissolved in the liquid scintillator (both 0.25 and 10 ml. sizes) was obt,ained using the integral counting technique (8,1 2 ) . To make this assay method quantitative the distribution of the noble gases betiyeen the liquid scintillator and the space above it had to be measured.
c.
d.
2.9 cc. Xe (S.T.P.) 7.6 cc. Xe (S.T.P.)
This was accomplished using the sample containers as shown in Figure 3. The amount of radioactive noble gas in the liquid scintillator was determined by measuring its disintegration rate in the liquid scintillator, A,. Then
K __ Gas -15OC. 27" C. Kr 0.9a 5 3 Xe Rn 32 Estimate from data in (11).
From the K values it was calculated that for a sample a t -15" C. with 0.08 cc. space above 0.25 ml. of liquid scintillator 95.6 and 99.6% of the xenon and radon, respectively, ~tould be dissolved in the liquid scintillator. Also from the value of K and the slope of the plot of T', against 1 / 9 ,the total activity in the sample can be calculated ;
LITERATURE CITED
where .4, is the disintegration rate of the noble gas in the space above the liquid scintillator and A4Tis the total disintegration rate of the sample. By definition,
where I', and TYg are the volumes of the liquid scintillator and the space above it, respectively, in the sample container. The distribution coefficient, K , is defined
K
=
a,/a,
(3)
Combining the above equations gives
(1) Clever, H. L., Battino, R., Saylor, J. H., Gross, P. lI., J . Phys. Chem. 61, 1078 (1957). ( 2 ) Clever, H. L., Ibid., 61, 1082 (1957).
(3) Flynn, K. F., Glendenin, L. E., Argonne National Lab., private communication, 1962. (4) Horrocks, D. L., Sucl. Instr. Methods 27, 253 (1964). ( 5 ) Ibid., in press. (6) Horrocks, L). L., Rea. Sci. Instr. 35, 334 (1964). (7) Horrocks, D. L., Studier, 31. H., ASAL. CHEM.30, 1747 (1958). (8) Ibid., 33, 615 (1961). (9) Saylor, J. H., Battino, R., J . Phys. Chem. 6 2 , 1334 (1958). (10) Seliger, H. H., Intern. J . A p p l . Radiation Isotopes 8, 29 (1960). (11) Steinberg, hI., Nanowitz, B., Ind. Eng. Chem. 51, 47 (1939). (12) Steyn, J., Proc. Phys. SOC.(London) A69, 865 (1956).
RECEIVEDfor review June 11, 1964. Accepted July 27, 1964. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
VOL. 3 6 ,
NO. 11,
OCTOBER 1 9 6 4
2079
