Light Filter for Reduction of Background in Liquid Scintillation Counting HOWARD A. SWARTZ Department o f Pharmaceutical Chemisfry, Butler University, Indianapolis, lnd. 46208
b The sample vial and solvent system phosphorescence contribution to the background count was reduced over 50% by means of an OX 7 glass light filter. The filter was evaluated in toluene systems with 2,5-diphenyloxazole, p-terphenyl, 2,5-diphenyloxazole, and naphthalene as the scintillators, at room temperature and -20" C.
T
and composition of background counts in liquid scintillation counting are complex and varied in nature. An accurate measurement of background depends upon an awareness of the various contributing factors and the manipulations necessary to ensure stability, or a minimum of fluctuation. A low background is preferred for lowlevel activities, but equally important to counting statistics is a constant background count. External gamma radiation originating from natural radioactive emitters in the air and the surrounding structures, fission products in the environment and various radioisotopes in neighboring laboratories can contribute to observed background activity. These factors can be controlled or reduced by detector shielding and isolation. Cosmic radiation effects are similar, and like external gamma, the observed counts are low and not subject to rapid fluctuations. Internal contamination can occur due to alpha, beta, and gamma emitters by incorporation into the shield (b), sample vial, photomultiplier, and possibly the scintillator solution itself. The contribution of lead in shields can be reduced by coating with mercury or iron (2, 13). lluminum and its alloys, which are widely used in photomultiplier attachments, show higher activity than iron ( 2 ) and have been reported to contain about 3.0 X curie per gram of Ra ( 1 9 ) . Glass content of K40 in the sample vial and photomultiplier envelope can add to background by the production of fluorescence or Cerenkov radiation (4). One gram of potassium emits 28 beta particles and 3.6 gamma per second with energies of 1.3 and 1.5 1n.e.v. ( 2 5 ) . Uranium ( 7 ) and radium (19) are additional known constituents of photomultiplier components. The photomultiplier tube is a major source of HE. ORIGIN
2080
ANALYTICAL CHEMISTRY
background and the contributions have several origins besides naturally occurring nuclides. Thermionic emission (tube noise or dark noise) is the main origin of background counts encountered in scintillation counting (20). I t originates from the cathode and occasionally from the dynode, and may be partially eliminated or circumvented by refrigeration, discrimination, and the use of coincidence circuits (26, 27). Spurious light flashes may occur in many parts of the photomultiplier assembly due to fluorescence of the glass envelope (26), Cerenkov effects in the vials or photomultiplier tube ( 3 ) ,and optical feedback (light dark current) (11, 15, 25) associated with the electron avalanche near the anode. This factor is most troublesome in coincidence work due to the interaction between photomultipliers (6). A third origin of background counts from photomultiplier tubes is caused by ionic feedback (satellite or after-pulses) (10, 1 6 ) . Positive ions created near the first dynode may produce photoelectrons upon impact with the photocathode, contributing to the photomultiplier output. The K4O content of glass counting vials may contribute appreciably to the background, but the contribution can be reduced by employing vials with a low potassium content (S), or by using quartz vials ( I ) , or polyethylene containers (22). Glass vials are also
subject to photoactivation with a subsequent phosphorescence ( 8 , 1 7 ) . Coincidence circuits reduce this contribution (20) as well as dark adaptation, but with low-level beta activity it is often difficult to differentiate from the photoactivation phosphorescence and the sample activity ( 8 ) . The phenomenon may arise from excitation of either or both the vial and solvent system by light irradiation during sample preparation or prior to insertion in the detector, and also from the sample itself through chemiluminescence (9, 12, 17, 1 8 ) . Phosphorescence consists of the emission of single photons, not large pulses, and is emitted at random. Hence, spurious and random counts would be observed, and background contribution would be random and have wide fluctuations. This could result in serious counting error, particularly with 101% energy and low activity beta samples. Phosphorescence wavelength has been shown to be greater than that of fluorescence (14), and it has been suggested that a filter opaque to phosphorescent wavelengths but one which would transmit fluorescent wavelengths (27) could be employed to reduce the phosphorescence contribution to background without seriously reducing the sample count. The purpose of this study was td, evaluate the application of a glass filter to reduce the phosphorescence contribution to background in liquid scintillation counting.
EXPERIMENTAL
D
Figure 1 ,
Counting iig
A. V i a l and sample B. Outer /ig C. OX 7 glass fllter or 2-mm. circular ring D. Inner lig
Apparatus. The glass filter employed was a Chance OX 7, 2 mm. thick, which transmits in the 2500 to 4000 A. range. The filter was placed into the vial jig as illustrated in Figure 1. A circular ring, 2 m m . thick, was used to maintain sample geometry for determination of counts without the filter. T h e counting equipment employed was a n Ekco Liquid Scintillation Detector (Model X 6648) and associate scaler (Model N 610A). The photomultiplier was an EM1 9514S, a 13-stage multiplier with a cathode diameter of 44 mm. Photosensitivity is in the averaqe order of 25 pa. per lumen with a low dark current of about 1 pa. maximum. The spectral range of 3100 t o 6000 A. has a peak of 3800 to 4200 -4.
Y
I
N
7
1
ice a 0:
C D E
IO
20
30
40
IO
50
UPPER D I SCRIM I NATO R
Figure 2. Background count with p-terphenyl in toluene and carbon-1 4 sample
30
50
40
Figure 3. Background count with PPO in toluene and tritium sample lower discriminator a t 5, temperature - 2 0 ' A. Sample B. Sample with glass filter C. Blank D. Blank with glass filter E. Photomultiplier tube
tower discriminator a t 10, room temperature A. Sample B. Sample with glass filter C. Blank D. Blank with glass filter E. Photomultiplier tube
Sample Preparation. Three solvent systems were employed, as follows: 2,5-diphenyl'oxazole (PPO), O.4yG in toluene; p-terphenyl, 0.3% in toluene; and PPO, 0.4%, naphthalene, 8.0% in toluene. Ten milliliters of t h e solvent was added to low potassium glass counting vials and 1 ml. of toluene containing a known d.p.m. of hexadecane-l-C14 or hexadecane-6-H3 was added. Blank samples were prepared of all the eyst'emsfor background measurement'. .1solvent system containing 0.4ye of PPlO and 0.01% of [ 1,4-di-(2-5-~phenyloxazoyl)POPOP benzene] was prepared and C14 of known d.p.m. was added. This solvent system was compared on the basis of efficiency t o a solvent system containing only PPO. KOdifferensee was observed, and POPOP was not added to the systems employed. Counting Procedures;. The balance pcint [maximal value of (Eff. in yo)*/Bg] was determined for each solvent system to establish t>he operational voltages a t room temperature (22" C.), and at -20" C. for systems containing PPO. The contribution of the photomultiplier tube to the background count was determined at each voltage setting for room temperature and
20
UPPER DlSCRl MI N ATOR
-20" C. with constant lower discriminator settings but with decreasing settings of the upper discriminator (see Figures 2, 3, and 4). All solvents were counted with C14 and blanks a t similar settings as those for the photomultiplier tube, with and without the glass filter. At -20" C. both the CI4 and H3, plus blanks, were counted in t h e solvent systems contsining PPO, but not p-terphenyl. Tenminute counts were used in all instances. The observed activities are illustrated in Figures 2 , 3 , and 4. Carbon-14 samples in a solvent system containing PPO and naphthalene in toluene were counted at room temperature without dark adaptation after a 5-minute exposure t o fluorescent lighting with and without the OX 7 glass filter. The same samples were then counted with a 2-minute dark adaptation after a similar light exposure.
RESULTS A N D DISCUSSION
The counting efficiencies and balance points for each solvent system are tabulated in Tables I and 11. The activities are compared in Table 111.
C.
If the total blank counts over the observed photomultiplier counts are considered separately, the OX 7 glass filter with a few exceptions, afforded over a 5001, reduction in the count at both the C14 and H3 settings. As is readily seen from the figures, the photomultiplier tube is the major contributor to the background count. This contribution, as reported previously (26, 27) and illustrated in the figures and tables of this paper, can be reduced by refrigeration and discriminators. Refrigeration was observed also to reduce the blank phosphorescence contribution, which is in agreement with previous reports (21). Thus with a quiet tube or where photomultiplier contribution has been reduced by the above methods, the OX 7 glass filter would allow counting with extremely !ow background. T h e value of the filter is associated not only with the reduction of the phosphorescence counts, but also in that it will eliminate phosphorescence fluctuations in the background count that may occur in different samples. The reduction of sample net counts by the filter was not serious, and in fact VOL. 36, NO. 1 1 , OCTOBER 1964
2081
A
N
0
i /==
Figure 4. Background count with PPO and naphthalene in toluene with carbon- 14 sample Lower discriminator a t 5, temperature A. Sample B. Sample with glass filter C. Blank D. Blank with glass fllter E. Photomultiplier tube
I
- 20'
C.
Table II. Counting Efficiencies and Balance Points at -20" C. Cl4, PPO in toluene
Ld.D 5
10
50 40 30 20 10 50 40 30 20
With filter
X o filter
__- Rd.
Bal.
78 77 71 63 33 58 57 53 51
42.6 43 9 40.1 35 4 11 2
~7~ p t , ~ E V ~ 7 2 3 2
5
42 42 38 32 11
2 9 6 2
84 98 106 124
1 82 6
3 81 4 2 74 9 5 A7 3 4 35 8 7 6 4 8
62.6 61 0 58 8 56 7
Pt.~
8.5.2 95.4 111 5 128 6
H3, PPO in toluene 5
L IO
20 UPPER
30
40
50
DISCRIMINATOR
10
Table I. Counting Efficiencies and Balance Points at Room Temperature
With filter N o filter Cl4, PPO in toluene __Bal. Bal. Ld.a U d b E% pointc E% pt.C 10
20
70 1 11 1 72 9 638 9 7 6 5 3 7 7 5 6 6 562 33.9 3 . 2 35.3
50 40 30
5 1 . 9 26.4 6 6 . 1 3 4 . 7 4 4 . 2 21.5 52.9 27.4 3 9 . 8 19.8 47.2 2 7 . 1
C14,
10
20
20
a c
PPO, naph., in toluene
40 30 20
62.8 609 544 363
50 40 30
44.8 36.2 30.6
50
C14,
10
10 9 8 9 6 9 2.8
50 40 30 20
8.7 68.0 8 8 6 5 0 7 1 5 8 1 3 6 4 0 7
8.9 8 3 5 0 3 6
1 8 . 5 64.3 31.8 14.2 5 6 . 2 2 9 . 2 1 2 . 3 5 0 . 2 28.0
5
10
457 442 401 271 17 9 339 32 3 31 5 77 4 75 5 73 2 61 5 45 4 47 8 44 4 42 7 39 5
2 0 1 8 1 5 0 8 0 6 1 4 15 16 41 0 40 1 39 9 31 8 18 7 55 7 54 8 60 8 65 0
509 2 3 4 9 0 2 1 4 7 2 2 1 3 0 2 0 9 18 9 0 ,5 389 1 7 3.5 8 1 6 34 3 17 82 2 41 7 42 1 80 c5 77 5 41 7 63 8 31 3 46 4 17 8 63 1 84 6 61 3 85 4 58 3 99 9 57 5 118 1
H3 PPO, naph. in toluene 5
10
10
b
50 40 30 20 10 50 40 30 20
441 436 418 332 17.6 36.5
1 9 1 8 1 7 1 2 0.5 1.6
513 4 9 4 473 3 4 2 19.9 39.3
2 2 2 2 2 1 1 2 0.5 1.7
356 332 296
1 6 386 1 6 3 6 4 1 4 3 4 4
1 7 1 6 1 6
Ld. Lower discriminator setting. lid. Upper discriminator setting.
Balance point, maximal value of l(E%)2/Bg1.
p-terphenyl in toluene 70 9 68 0 601 498
50 40 30 20 50 40 30
52 6 48 5 38 6
11 7 77 11 5 75 9 1 6 3 7 2 4 8
2 5 5 9
25 1 57 5 26 3 55 4 17 6 54 2
12 13 9 6
Balance point:
9 7 7 3
27 1 29 1 31 9
Ld. Lower discriminator setting. Ud. Upper discriminator setting.
maximal value of
[(E%)'/Bg.I
2082
on the basis of the balance point evaluation, the filter values were equal and in some instances higher. The marked drop in efficiencies observed with solvents containing naphthalene where the filter was employed and the lower discriminator raised to 20 could be caused by the wavelength emission of the naphthalene mixture. Naphthalene has an emission of 3480 A . (94) as compared to 4160 A . for PPO and 3910 A. for p-terphenyl. The naphthalene and PPO mixture could be emitting a low wavelength which results in a excessive discrimination with the lower discriminator raised to higher settings. The efficiencies for the different scintillators were not lowered to any degree, and with the observed reduction in the blank count as mentioned previously, gave similar balance points with and without the filter. p-Terphenyl loss was least, which coincides with the re-
50 40 30 20 10 50 30 20 50 40 30 20 10 50 40 30 20
ANALYTICAL CHEMISTRY
Table 111.
Phosphorescence Counts Caused b y Vial and Solvent Light Irradiation"
Filter No dark adaptation
S o filter
Blank c.p.m.
Sampleb c.p.m.
Blank c.p.m.
482 438
1984 1910
724 485
Samplec c.p.m. 2.502 2268
Dark adaptationc a &minute exposure. C14sample. Discriminator range 10 to 50 at room temperature. Values are corrected for background, 2-minute dark adaptation.
ported e n i i t t d wavrlength, but the P P O solvents did not have noticeable lobses, and at room temperature the balance Iioinlzi for the C14 were higher when the filter was employed. The filter was observed to markedly rtducr the count of blank and C1* s a n q i l ( ~upon esl)osure to light. I’pon 2-mini~tedark adaptiorl the blank count had returned to the previous value n-hich was obtained u1)on 24-hour dark adaptation. I t is reasonable to speculate that the use of the filter would afford lower background with less fluctuation in counting samples where chemiluminescence is a problem. This applica,tion would be of great w l u e in liquid scintillation counting. LITERATURE CITED
W.,AucZeon?cs 15 (10) 106 (19.57). ( 2 ) Anderson, B. C., Arnold, J. R., Libby, \!-. F., Rev. Scz. fnstr. 22, 225 (1951). 1 3 ) Arnold, J . R . , “Licluid Scintillation Countincr.” D. ‘129. Peraamon Press, k e w YoFk, lb58. (1) iigranoff, B.
(4) Audric, B. N., Long, J. V., J . Sei. f n s t r . 30, 467 (1953). (5) Barendsen, G. W., A‘ucleonics 16,
K. H., Symposium on the Detection and Uses of Tritium in the Physical and Biological Sciences, International (11) 197 (1958). Atomic Energy Agency, Vienna, p. (6) Bibron, R., Delibrias, G., Leger, C., 263, 1962. Proc. Intern. SvmDosium on Nuclear (18) Mayneord, W., Anderson, H. D., Electronics, Yo!. iI, p. 157, InterRosen, D., Radiation Research 3, 379 national Atomic Energy Agency, (1955). (19) Miller, C. E., Marinelli, L. D., Vienna, 1959. ( 7 ) Boyce, I. S., Cameron, J. F., “SymRowland, R. E., Jose, J. E., Il’ucleonics posium on the Detection and Uses of 14 (4)40 (1956). Tritium in the Physical and Biological (20) Packard, L. E.. “Liquid Scintillation Sciences.” p. 231, International -%tomic Counting,” p. 50, Pergamon Press, Energy Agency, !.ienna, 1962. New York, 1958. (8) Davidson, J. D., Fiegelson, P., Intern. (21) Perrin, F., Ann. Phys. 12, 169 (1929). (22) Rapkin, E., Gibbs, J. A., Intern. J . J . Appl. Radiation Isotopes 2, 1 (1957). Appl. Radiation Isotopes 14, 71 (1963). (9) Drew, H. D., Trans. Faraday SOC. 35, 207 (1939). (23) Roucayrol, J. C., Oberhaussen, E., (10) Godfrey, T. N., Harrison, F. B., Science 122, 201 (1955). Keuffel, J. W., Phys. Rev. 84, 1248 (24) Sangster, R. C., Irvine, J. W., J . (1951). Chem. Phys. 24, 670 (1956). i l l ) Haves. F. S . . Hiebert. R. D.. (25) Sawyer, G. A., Wiedenbeck, M. L., SchucL, R. L., Scihnce 116, 140 (1952): Phys. Rev. 79, 490 (1959). (26) Sharp, J., Thomson, E. E., “Proc. (12) Herberg, R. J., Ibid., 128, 199 (1958). (13) Hodgson, T. S., Gorden, B. E., 2nd Intern. Conf. on the Peaceful Ackermin, 52. E., L\rucleonics 16 (. 7,) Uses of Atomic Energy,” Geneva, 89 (1958). 1958. 114) Kasha. M.,Chem. Rev.41.401 11947). (27) Swank, R. K., “Liquid Scintillation ( i 5 j Firsten, F. A., Proc. Second ~ y i Counting,” p. 23, Pergamon Press, posium on Advances in Fast-Pulse Kew York, 1958. Techniques for Nuclear Counting, BerkRECEIVEDfor review April 27, 1964. ley, UCRL-8706, 1959. (16) Lanter, R. J., Corwin, R. W., Rev. Accepted July 10, 1964. Second Annual Sci.Instr. 23, 507 (1952). Oak Ridge Radioisotope Conference, April (17) Lloyd, R. A., Ellis, S. C., Hallowes, 20, 1964, Gatlinburg, Tenn. ~
Emission Spectrographic Determination of Boron in Plutonium and Uranium Nitrate Solutions Following Cation Exchange Separation ALBERT W. WENZEL aiid CHARLES E. PlETRl
U.
S. Afomic
Energy Commission, New Brunswick, N. 1.
b Trace amounts of boron are determined b y emission spectrography following separation from plutonium or uranium in dilute nitric acid by cation exchange. Mannitol complexing is used to prevent boron losses upon concentration of the boron-rich effluent b y evaporation to dryness prior to the spectrographic determination. Boron is subsequently determined using a zinc internal standard and an indium oxide matrix b y excitation in a d.c. arc. The lower limit of detection varies from 0.007 to 0.001 kg. of boron depending upon the instrumental setup. The rellative standard deviation for 0.1 to ;!-gram uranium samples in the 0.1 to 30-p.p.m. range and 0.1- to 0.4-gram plutonium samples in the 3- to 30-p.13.m. range was within 9% while a t the 0.05-p.p.m. level in 2 grams of uranium it was 1 2y0. Within experimental limits complete recovery of boron in all synthetic samples was obtained a t all levels. The influence of the boron blank values a t the 1 to 0.1 -pg. level is discussed.
-
-
T
of impurities have been separated from interfering plutonium and uranium for spectrographic analysis by anion exchange methods. These procedures employ 6 to 12N hydrochloric acid ( 2 ) or 8N nitric acid (6, I I ) , and the subsequent concentration of the impurity-rich effluent by evaporation to dryness causes partial or even complete loss of the more volatile elements such as boron. In many instances this loss may not be detected when both samples and standards are processed similarly. The total amount of boron available for determination, however, may be considerably reduced thereby substantially decreasing the spectrographic sensitivity for this element. An earlier attempt was made to prevent this loss of boron by using a mannitol complexing technique ( 5 ) but the amount of mannitol required for 8 S nitric acid solutions was excessive and made the spectrographic determination unreliable a t times (12). A method was required which would quantitatively separate microgram RACE A h f o c x T s
amounts, or less, of boron from plutonium and uranium without loss prior to emission spectrographic determination. Previous work with silicon indicated that boron could be separated from plutonium by cation exchange in 0.2.V nitric acid ( I O ) . Eberle, Lerner, and Kramer ( 4 ) , and Barnett and Milner ( I ) used a similar separation for the removal of uranium prior to boron determination by colorimetry or titration. ;\ccordingly, a cation exchange separation of plutonium and uranium from boron was investigated using mannitol complexing in the evaporation of the effluent from the separation step since much less mannitol was reqnired for 0.2N nitric acid than for the 8N acid solutions previously used. EXPERIMENTAL
Apparatus and Reagents. The glove boxes used for handling plutonium, the ion exchange column used for the separations, and tmheemission spectrographic equipment used for the boron determination have been previously described in detail ( I O ) . VOL. 36, NO. 1 1 , OCTOBER 1964
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