Cerenkov Counting Technique for Beta Particles: Advantages and Limitations K. Rengan Eastern Michigan University, Ypsilanti, MI 48197 Madame Curie ( I ) first observed in 1910 that a hluish-white light is associated with strongly radioactive solutions. This light emission was investigated in detail experimentally by Mallet (2) and Cerenkov (3).Frank and Tamm (4) explained the results theoretically; they attrihuted the light emission to particles traveling with velocities greater than that of light in a given medium. The light emitted is termed Cerenkov radiation. This phenomenon is responsihle for the familiar blue glow seen around a nuclear reactor core. Flashes of light observed by Apollo astronauts in deep space, with their eyes shut, were, in major part, attrihuted to Cerenkov radiation from vrimarv cosmic rav warticles enterine astronauts' eves (5,6):~eteciorsbased on 'Cerenkov radiatGn have been uied for several decades (7-9) for measurement of hieh " enerev "charged particles. In 1961De Volwi and Horrocks (10) used Cerenkov radiation for the measurement of radioactivity of 56Mn in an aqueous solution, utilizing a liquid scintillation counter. This is the first application of Cerenkov radiation for counting heta particles. The advances and improvements made in photomultipliers and electronics associated with liquid scintillation counters in the last two decades have made possible efficient detection of Cerenkov radiation with commercial liquid scintillation counters. The fact that large volumes of aqueous solution containing ionic materials could be used directly for counting has made this technique a popular one in radiochemical experiments. A number of reviews of this technique have appeared in the last decade ( I , 10-16). In spite of such developments in Cerenkov counting, the technique is not used frequently in biochemical and biological tracer work involving high energy heta emitters. A survey of articles published in 1979 in a biochemistry journal indicated that only one in twenty researchers utilized the Cerenkov counting technique in 32Ptracer studies. Part of the reason for this status could he that many experimenters are still not very familiar with this technique since the traditional courses and textbooks in radiotracer methodoloav do not aive sufficient imvortance to of this article is to review Cerenko; counting and point out its advantages and disadvantages. Hopefully, this article will serve as a source material for students and instructors for a discussion of Cerenkov counting.
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Journal of Chemical Education
Table 1.
Medium
H20 HCI HCI NaCl NaCl
Toluene Benzene
Threshold for Cerenkov Radiation Emission In Typlcal Countino Media M
Refractive indexa
Threshold (kW
1.3325 1.3372 1.3790 1.3380 1.3769 1.494 1.496
262 259 23 1 258 232 178 175
0.50 6.00 0.50 5.00
~~
-
-~~
'Values are horn 6WI edition of "CRC Hendbmk of Chemistry and Physics." lntemolaied value5 aregiven lor HCI and NaCl solutions.
Theory When a beta particle (or any charged particle) travels with a velocity greater than light in a specified medium, Cerenkov radiation is emitted. The radiation can he compared to shock wave generated by a plane traveling through the atmosphere with speed greater than that of sound. The condition for Cerenkov radiation can he expressed as
where u is the velocity of the particle, c is the velocity of light in vacuum, and n is the refractive index of the medium. The radiation is emitted at an angle 6 from the direction of the particle where 0 is defined by the relation (7) cod
=C
un
(2)
The radiation emitted is in the violet-ultraviolet region of the electromagnetic spectrum ( 1 0 , I I ). There is a threshold for Cerenkov radiation emission and the value depends on n, the refractive index of the medium. The threshold value for electrons can he calculated as follows. Kinetic energy of electron = T = me2 - moc2where m is the mass of the electron traveling with velocity u and moc2 is the rest mass of the electron, which is 511 keV. From relativistic relation,
From the threshold condition expressed in eqn. (I),
Using eqn. (3) the threshold for Cerenkov radiation in different media can be calculated. Table 1shows the Cerenkov threshold calculated for typical reagents and solvents used in tracer work. In water, electrons with kinetic energy greater than 262 keV will emit Cerenkov radiation. Since the particles emitted in a heta decay have continuous energy spectrum, the Em,, (i.e., the maximum energy of the heta particles) should he much higher than the threshold for usable Cerenkov radiation. Of the three most commonly used tracers in biological systems, namely, 3H(Em,, 18.6 keV), I4C (Em, 156 keV) and 32P(E,,, 1710 keV), the latter is ideally suited for Cerenkov connting. Nuclides like S6Cl with an Em,, of 709 keV can he counted (1). Organic solvents like toluene and benzene are not commonly used for Cerenkov counting (since they offer no advantage over liquid scintillation counting). Additional theoretical information can be obtained from Parker and Elrich ( I ) , Marshall (7), Ross (111, and Jelley (17). Sample Preparation Sample preparation for Cerenkov connting is simple. In much biological or biochemical tracer work the material to he assayed is often present in aqueous solution or is present in a typical separation medium (ion-exchange resin, thin-layer chromatograph, or paper chromatograph) from which it can he leached out with appropriate aqueous solvent (dilute acid, alkali, or complexing agent). The aqueous solutions ohtained can be used directly for Cerenkov counting. If the material to be assayed is not directly soluble in aqueous media, it could be decomposed with concentrated acid or alkali forming an aqueous solution for counting. It is essential to ascertain that m&rials present in the sol;tiun do n t d ithsorh violet u 1 ~ a viulet radiatiun. Ilse of polvcthylent~vials is advuntnge~,u,u\w glass because of the decreased absorption of ultriviolet radiation. Color Quenchlng Presence of materials which absorb in the violet-ultraviolet region reduce the pulse height attainable from Cerenkov radiation (relative to a solution containing no absorbing material). This phenomenon is similar to that which occurs in liquid scintillation counting and is referred to as color quenching. Corrections can be applied for color quenching using the common technianes used in liauid scintillation countine. The correction techniq~lrsincludc internal st~nd.lrdi7atio11,external standardiznti~~n. and the channels ratio nwl h d Various quench correctibn methods applied for Cerenkov conntine have been discussed in ionrnal articles. Stuhhs and ~:icksol;(la1 nnd hloir I IYI describe channels ratiu rnrthud qurnch currertim fur Crrenkur rndi,,rh,n. Elrivh and Parker 120)and Parker (12, revie&tht. \wious uptims arailal~lrior c ~ m v t i n gcvlvr quenrhinx. Knrnp and Rl;mchard 121I discuss(.~the channels r ~ r i umet hvd imd extrrnitl s~rurcr-vh;tnnels ratiu merhod. Fric and Palwcikova ,221 iiund vhitnnrL ratio method to be satisfactory for the Cerenkov counting of a2P in plant extracts. In general, color quenching correction is comparable to that of low energy beta emitters in liquid scintillation counting. Chemical Quenchlng The mechanism of Cerenkov radiation emission is very different from that of liquid scintillation phenomenon. The presence of various chemicals do not interfere with the gen-
eration of Cerenkov radiation. However, there are two aspects that should he considered regarding the presence of different solutes in counting media: (1) The ability of solute to absorb generated Cerenkov radiation and thereby reducing the pulse height and the counting efficiency; (2) Large changes in solute concentration will lead to changes in refractive index and density of the counting medium. As seen from Table 1an increase in refractive index lowers the Cerenkov threshold. This will increase the fraction of particles that can generate Cerenkov radiation and thus lead to increased counting efficiency for a given beta emitter. On the other hand, an increase in density of the medium will reduce the path length for the electrons, decreasing the number of Cerenkov photons emitted. Hence, large changes in solute concentrations bring changes in counting efficiency. The effect, as can he expected, will depend sensitively on the shape of the heta spectra. If a large fraction of beta particles fall below 262 keV, the threshold in water, large changes in efficiency can be expected with large changes in reagent concentration. The changes in efficiency found by Rigot and Rengan (23)in HCl medium for three different tracers are shown in Table 2. The largest change was observed for 20T1, with the lowest average heta energy ... value. The data emphasizes the fact that even thoueh Cervnkuv counting is nor sulrject tochemiial quenching, i t is still visential to nit~inr.~in rhc chemical cmicrntratims in the counting media constant in order to reproduce counting efficiency. Counting Efficiency The counting efficiency depends on several factors, the important among them being the shape of beta spectrum, the refractive index of the counting medium, the type of counting vials, and the counting system. For a given counting sygtern and the type of vial the counting efficiency will increase with increasing Em., or increasing average beta energy value. With Searle Analytic Delta 300 system the author was able to obtain an efficiencyof 51%for 32P(Em, = 1710 keV) and 35%for 40K (Em,, = 1325 keV). Ross and Rasmussen (14) have summarized in a table the counting efficiencies for a number of nuclides in aqueous systems ohtained experimentally by different workers. Gnzzi et 81. (26) have determined Cerenkov counting efficiencies for a number of nuclides produced by (n, y) reaction. In general. the Cerenkov countine" efficiencv is lower than that i18r the corrt.rpondmg liquid itintillatiun nnmting. For exam~le,the rfiicirnw ior ''P ia lower hv a iwtor oitwu: rhr facto; will be larger f i r lower heta energies. The lower kfficiency is partly offset by the fact that larger volumes of aqueous solutions (20 ml) can be taken for Cerenkov counting; in the case of liquid scintillation counting only 1to 2 ml of aqueous solution can be incorporated in the liquid scintillation medium. Table 2. Change In Relatlve Counting Efficiency in HCI for Three Beta Emitters Counting Medium Hz0 1.0 MHCl 2.0 MHCl 4.0 MHCl
Refractive
Relative Counting Efficiencyfor
Index
204Ti
32P
1.3325 1.3411 1.3492 1.3649
1000 106.9 111.8 123.8 240
100.0 101.6 103.0 104.0 690
Average beta energye Eg(keV)
42K 100.0 100.4 101.1 101.9 1450
Maximum beta energy 763
EdkeV)
1710 -
3560 (0.81) 1970 (0.18)
--
'me
average beta energies are from reference (M. 0 ~ and ~ 1 32Pthere is onhl one beta group: 42K has two prominent beta groups. The franionel value for the two groups are given in pa-
bmevalues are from reference (29. Fw 2
renlherer.
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Number 8 August 1983
683
The counting efficiencv can he increased hv use of wavelength s h i f t i n g ~ o m ~ o u n in d sthe counting medium. A majority of Cerenkov emission occurs in the ultraviolet repion; alsothe emission is not isotropic. The wavelength shifting compounds function similarly to the way the secondary solute functions in liquid scintillation system. They absorb the Cerenkov radiation in ultraviolet region and emit in longer wavelength region where the photocathodes work more efficiently. In addition, the re-emitted radiation will he isotropic. These two factors increase the overall counting efficiencv. Ross (27) has investigated the use of a number ofkavelengths shifting compounds like &naphthol and 4-methylumhelliferone for Cerenkov counting. He found that for low energy heta emitters like36C1 the counting efficiency can be increased by nearly a factor of five whereas for high energy heta emitter like 3 2 P the increase in efficiencv could he close to a factor of two. I t should he noted that, with the addition of waveleigth shifters the system is not pure Cerenkov system and chemical quenching is quite likely. Ross (28) has overcome the urohlem of chemical auenchine by isolating the wavelength shifter from the ~ e r e n k o ; counting medium. Ross uses a specially designed counting vial containing two concentric chambers. The inner chamber contained the counting sample while the outer chamber contained the wavelength shifting solution. A counting efficiency increase of 88%was observed by Ross for a moderately energetic heta emitter like s9Sr ( E _ ~ ,= 1488 keV) using dimethyl POPOP in toluene as wavelennth shiftine solution. I.arger I n c r t i w i in ei~icienc?can he c x p t . ( . t r d fur lhw energy I ~ r t acmittrri. An advantare t,f rhis ~ C I ~ I I I I QisI ItIh~a t it i-.true Cerenkov counting and, assuch, chemical quenching will not
that the average heta energy should he at least as high as the Cerenkov threshold in the counting medium or the Em,, should he approximately three times the threshold, to obtain reasonable counting efficiency. A variety of experiments can he performed in the laboratory using Cerenkov counting technique.' In the radioisotope course being taught at Eastern Michigan University the count rate due to 40K in natural potassium is measured by Cerenkov countine. If Cerenkov countine efficiencv for "OK is measured or a s s u h , the half-lift,uf j"i( can he'calcul.~tcdusing the measured l i count rat? ior a knuwn weight potnssiun.
'
Summary
Cerenkov counting is a useful technique for assaying medium- and high-energy heta emitters in aqueous solutions. This technique has at least three distinct advantages, if not more, over liquid scintillation counting. The advantages are (1) simple sample preparation, (2) being able to handle large volume of aqueous solution for counting, and (3) absence of chemical quenching. Cerenkov counting is also less expensive. Acknowledgment
The author gratefully acknowledges the efforts of Carolyn Hayes and P a t Parks for typing the manuscript. Literature Cited (1) Parker, R. P., and Elrich, R. H. L" "The current status of Liquid Scintillation Counting,.) Bransome. E. D., (Editor). Grune and Strattun. New York, 1970. p. 110. (2) Ma1let.L.. C R. Aead. S o . (Paris), 188.451 (1934). (3) Cerenkov, P., Akod. Nnuk SSSR, Dokl., 2,451 (19341. (4) Frank, l.,sndTsmm.I.,Akod. Nouk SSSR,Dakl.. 14,107 (1937). (5) Fszio, G. G., Jei1ey.J. V.,and Charman, W. N..Noturo,228,260(19701. (6) McNulty, P. J.,Pesse,V. P.. and Bond. V. P..LiieSn. Spncr Res, 14,205 (1976). (7) Marshall, J.,Phys.Roo., 86,685 (1952). (8) Hutchinsun. G. W.,PioC Nucl. Phys.. 8,197 (1960). (9) Lilt, J., Mcunier, R., Ann. Re", NucI. Phys., 23, l(19731. (101 Horroeks, D. L.. "Liquid Scintillation Counting,.) Academic Press. New York, 1974. P. 263. (11) Ross, H. H.,Anai. Chem.dl.1260 (19691. (121 Parker, R. P., in "Liquid scintiitation Cnunting,'~Proceedings of a symposium held in Brighton, England. Crook. M. A,, and Johnson, P., IEdims). Heyden, London. 1973, Voi. 3 , p 231. (13) Fmmms. B..Imtwn. J. Nuel Mod. Biol., 1.1(1973l. (14) Ross, H. H., and Rasmussen. G. T.. in "Liquid Scintillation Counting: Recent Develo p m e n w Stanley,P. E., and Smggins, B. A , (Editors),Acadmicprosa. New Ynrk, ,a"" " """. 2O"">
' Cerenkov counting has a wioe variew of app ications. The article
Parker (reference1 12) isted in Literature Cited I summar zes a number of applications. Some selected applications are listed in references (29-36). by
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Journal of Chemical Education
Berry, D., and Volz, P. A., App!. Enuiion. Miciobialogy, 38,751 (1979). Bais,R.,Anol. Riorhom.,63,271 (1975). Johnson,J. E., and Hsrtsuek, J. M., Hra!lhPhys.. 16,755 11969). Haviland, R.T.. and Bieber, L. L., A n d Riorhem ,33,323 (1970). Pietra, R., Sabbioni, E.. and Girsrdi. F.. Rodiochrm. Rodiaona!. Lett., 22, 243 (1875). (36) Kararnanw, R. E., Bettsny, J.R.,Rennie, D. A.,Can. J. SoilSci.55.407 11975).
131) 132) (33) (34) (36)
