Detection of hard β-emitting radionuclides in aqueous solutions using Cerenkov radiation: A review article

International

Journal

of Nuclear

Medicine

and Biology,

1973, Vol.

1, pp.

I-14.

Pergamon

Press.

Printed

in Northern

Ireland

Detection of Hard P-Emitting* Radionuclides in Aqueous Solutions Using Cerenkov Radiation: A Review Article B. FRANCOIS Hopital

de PAntiquaille,

69 Lyon 5, France

(Received 6 January 1972)

Cerenkov counting of solutions is advocated as a useful means of detecting hard ,%emitters; the present work is dedicated to the practical aspects of Cerenkov detection with commercially available liquid scintillation spectrometers. The variations of count rates with the type of vials and the volume of solutions are pointed out. W ith a spectrometer equipped with S 11 phototubes, detection efficiencies from 16 calibrated sources are found to be 23-50 per cent for emitters of Es maximum in the range 1.3-2.3 MeV and 50-70 per cent up to 3.5 MeV. The results of experiments with various optical filters suggest a direct relationship between the /? energy of the isotope and the fraction of Cerenkov light emitted in the ultra-violet region. Pulse height analysis either by calculation of the channel ratio obtained from the spectrometer or by direct oscilloscopic display of spectra enables the energy of any unknown B-emitter to be evaluated. Color quenching of biological solutions inhibits more widespread use of Cerenkov counting while solute quenching is shown not to be a problem. Changes induced by 3 1 wavelength shifters are assessed; we single out Esculin as one of the most effective. Stress is being put on the advantages of Cerenkov counting of 42K, 32P, E6Rb and 24Na and the applicability of the method to more than 40 &emitters having maximum beta energies above 1 MeV. LA

DETECTION DES RADIONUCLIDES BETA-EMETTEURS DURS EN SOLUTION AQUEUSE PAR L’EFFET CERENKOV L’effet Cerenkov peut etre mis a profit pour effectuer le comptage des radionuclides p-tmetteurs durs en solution, au moyen des spectromttres a scintillation liquides courants du commerce. Le taux de comptage d’une mCme source varie avec la nature des flacons utilisb et le volume de solution detectee. Avec un spectromttre CquipC de photomultiplicateurs de reponse S 11, les rendements de detection sont de l’ordre de 25 a 50 o/o pour les Cmetteurs d’tnergie B maxima entre 1,3 et 2,3 MeV et de 50 a 70 % jusqu’a 3,5 MeV. Les rtsultats d’essais men& avec differents filtres optiques suggtrent une relation directe entre l’tnergie /l de l’isotope et la fraction de lumitre Cerenkov qu’il tmet dans l’ultra-violet. On montre qu’il est possible d’tvaluer I’energie d’un /?-Cmetteur inconnu en solution en procedant a l’analyse d’amplitude des impulsions du spectromttre soit par le calcul des rapports des voies de comptage soit par la visualisation oscilloscopique des spectres. L’affaiblissement du signal par la couleur constitue l’obstacle essentiel a une utilisation plus repandue du comptage Cerenkov en biologie alors que l’affaiblissement chimique (du aux solutes) ne pose gutre de problemes. L’Ctude des variations apportees par 31 substances fluorescentes et d’azurage optique a permis de selectionner 1’Esculine comme un des agents les plus efficaces pour amtliorer le taux de comptage. L’accent est finalement place sur l’inttret pratique du comptage Cerenkov pour la detection de 42K, 32P, s6Rb et 24Na et sur les possibilitb d’application de la methode 8 plus de 40 /l-emetteurs d’energie p maxima suptrieure a 1 MeV. * Invited contribution. 1

1

B. Franc&

2

OBHAPYZEHBE KECTKIIX BETA-B3JIYYAIOIIJEIX PA#IOB30TOIIOB B BOAHbIX PACTBOPAX, MCIIOJIb3Yfl YEPEHKOBCKOE kl3JIYYEHME IIpennoHteno npunerreane YepermoBcKoro cqera a Kagecrae noneanoro Merona AeTeKTnPa6OTa-nOCBHIIJeHa K llpaKTu=IeCKuM BOupOCaM poBaHIlrr mecmmx rianysarenet. gepeHKOBCKOr0 aeTeKTHpOBa&yfl lIpu nOMOIJJu HCuJ(KOCTHbIXC~uHTuJIJIK~uOHHbIX CneKTpOMeTpOB,BbInyJIJeHHbIX npOMbImJIeHHOCTbIO. IIoKaaaHbI u3MeKeuun CKOPOCTUcYeTa c TunoM aMnyJlOB u 06%eMOM paCTBOpOB. flpu npMMeHeHuu CneKTpOMeTpa C IjIOT03JIeMeHTaMH Tulla s-11 nOJIyqaeTCfi 3@#eKTuBHOCTb J(eTeKTupOBaHuJJ El3 16 rpWyupOBaHHbIX UCTOgHuKOB B 23-50 %~n~~uany~aTeneimaKcuManb~oro Ep ~npe~e~~ax l&-2,3 MBBM~O-~O%JJO~,~MBB. Pe3yJIbTaTbl8KCnepuMeHTOB Cpa3JluYHbIMuOllTu¶eCKuMu~UJlbTpaMurOBOpSITOHellOCpe~CTBeHHOM CBKSU Me7KHy /%3HeprHeti u3OTOna u EOJIei YepeHKOBCKOrO CBeTa,u3JiyYeHHOrO B o6nacTu yJIbTpa#IOJleTOBOrO U3JIy~eHuJL AMnnuTy~HbIi aHaJUi3 UMlIyJlbCOB, UJIU npu nOMO~uBbI=IUCJleHuROTHOmeHUflKaHaJIOB,IlOJly~eHHOrO OTCneKTpOMeTpa,UJlllnpUnOMOIJJU HeIIOCpeACTBeKHOZt OCIjUJIJIOCKOnWIeCKOZtMHJ(UKaquU CneKTpOB,nO3BOJIuT OIJeHUTb 3HeplWO mooboro Heu3BeCTHOrO /%u3JIy~aTeJIEI. IJseTKoe rameHue Guonoruqecrurx pacTr3opoa aa~ep~usaer 6onee pacnpocrpaaenrroe npuMeHeHue =repeHKoBCKOrO CqeTa, a nOKaaaHo,qTO pacTBopuMoe rameHue-HeT npo6neMa. O~eH~TCRU3MeHeHUH,uH~yKTupOBaHHbIenpUnOMO~U 31 IJBeTOC~BUraIO~UX~o6aBoK. npu 8TOMMbIBbI~eJiKeM3CKy~UHKaKO~UHu3CaMbIX3~~eKTuBHbIX. nOfl9epKllBaeMAOCTOUHCTBa ¶epeHKoBcKoro weTa B cnyqae arK, srP, 8’LRb, =Na, a TaKme npUMeHHeMOCTb MeTOAa K bonee,seM 40 p-u3JIy'KaTeJ'IRM,uMeIoIQUM MaKCuMaJiHyIO /?-3Hepl'UPJ CBbIme 1 M3B.

NACHWEIS HARTER BETASTRAHLER-RADIO NUKLIDE IN WASSERIGEN L&XJNGEN UNTER VERWENDUNG VON CERENKOV-STRAHLUNG Die Cerenkov-Zlhlung von Liisungen wird empfohlen als brauchbares Mittel zum Nachweis harter Betastrahler; die vorliegende Arbeit befasst sich mit den praktischen Gesichtspunkten des Cerenkov-Nachweises mit im Handel erhaltlichen Fltissigkeits-Szintillations-Spektrometern. Die Veranderungen der Zahlgeschwindigkeiten mit dem Typ der Phiolen und dem Volumen der Liisungen werden aufgezeigt. Mit einem S-l 1 Photozellen enthaltenden Spekfindet man Nachweisausbeuten aus 16 geeichten Quellen von 23 bis 50% fur Strahler mit einem max. Ep im Bereich von 1,3-2,3, MeV und 50 bis 70% biz zu 3,5MeV. Die Ergebnisse von Experimenten mit verschiedenen optischen Filtern deuten aufeine direkte Beziehung zwischen der Betaenergie des Isotops und dem Bruchteil des im Ultraviolettbereich emittierten Cerenkov Lichtes. Die Energie eines unbekannten Betastrahlers kann zahlenm%s.sig bewertet werden durch Impulshbhenanalyse, entweder durch Berechnung des vom Spektrometer erhaltenen Kanalverhlltnisses, oder durch direkte Oszilloskopanzeige der Spektren. Farbenloschung biologischer Lijsungen verhindert die weitergehende Verwendung der Cerenkov-Z&lung, wlhrend gezeigt wird, dass Liischung der gel&ten Substanz kein Problem ist. Durch 31 Wellenllngenschieber induzierte Anderungen werden ausgewertet; Eskulin von 4aK, wird als am meisten wirksam herausgestellt. Die Vorteile der Cerenkov-Zahlung s2P, s6Rb und 24Na werden hervorgehoben tmd die Anwendbarkeit des Verfahrens auf iiber 40 Betastrahler mit Ho&t-Betaenergien von tiber 1 MeV wird betont.

INTRODUCTION and usefulness of detecting hard p-emitters in water through Cerenkov radiation have been well documented.(l-lo) The use of this type of detection has been made possible by an increase in number and a constant technical improvement of liquid scintillation spectrometers. These have turned out to be nondirectional, reliable and performing Cerenkov counters. As an illustration, all the data reported in this paper have been obtained with an unmodified, commercially available spectrometer initially set for 3H and 1% detection by the liquid scintillation method.

THE

FEASIBILITY

Cerenkov light is produced in aqueous media when the theoretical level of 260 keV is reached; with current equipment, the actual working level may be set at 500keV. Since it constitutes less than 1 per cent of the energy lost by moving charged particles in the medium, the intensity of Cerenkov radiation is very weak: as this paper will show, detecting Cerenkov light from common hard p-emitters is similar to counting tritium with liquid scintillation techniques. Reporting work done between 1966 and 1970, this paper is a plea for more Cerenkov counting of hard p-emitters in solution.

Detection of hard B-emitting radionuclides in aqueoussolutions using Cerenkovradiation

IOIYrnI 01 rollllDn 100,

4

6

,O

12

14

16

la11

20

'

FIG. 1. Relative detection efficiency with glass

and polyethylene vials. Packard Tricarb 3314, EM1 6255 B phototubes; three samples per volume, per type of vial and for each isotope.

DETECTOR RESPONSE AND DETECTION EFFICIENCY Since it is knownol) that Cerenkov radiation may occur in polyethylene and in glass, the influence of the vial material and geometry on detection efficiency has been investigated with three p-emitters of decreasing energy (Fig. 1). The better results obtained with plastic vials can be accounted for by a better transparency for U.V. photons, the diffusive property of polyethylene as well as geometry conditions: for a given volume, the height of solution in the vial as seen by the photomultipliers is smaller in most plastic vials than in glass vials. Since they provide a higher counting rate and a lower background, plastic vials are to be preferred more especially as the beta energy is lower and the volume of solution to be assessed is larger (Fig. 1). Seven series of experiments have been performed to demonstrate the linearity of detectors using 9-12 different activity levels, 3-4 samples per concentration and a constant volume of 20 ml of solution in polyethylene vials. The following isotopes have been assessed = 42K and snP (l80 nCi), O”Y and sgSr (7-70 nCi), 40K (O-7862.622 g KCl) . Making allowance for minute experimental errors, a perfect linearity has been observed for all isotopes tested with the Packard Tricarb 3314 Spectrometer (H.V. = 182Ov., gains 50 and 100 per cent, thresholds 50-co) and with the Intertechnique Abac SL 40. Cerenkov detection eficiency has been deter-

3

mined with 16 calibrated sources* of 11 exclusively or predominantly #?-emitters and five chiefly y-emitters (Tables 1 and 2). The absolute activity was 78-480 ,uCi for ,&emitters and l7 ,uCi for y-emitters, & 1.5 per cent. After appropriate dilution in a carrier solution of the same chemical form or, if not available, a NaCl 1 % solution, each source was measured in plastic vials with 13 different volumes from 2 up to 26 ml (four samples per volume of solution). As a permanent check of the detection set-up, any counting of a source was followed by the counting of s04T1 and 40K standards; the sample changer was operated at a constant temperature of +4”c. In the counting conditions of the Tricarb 3314 Spectrometer equipped with EM1 6255 B phototubes, the maximum efficiency was observed for volumes in the range 8-14 ml and in most cases for 10 or 12 ml. The shape of the response curve is of the type y = a sin nx + b cos nx, where y is the relative eficiency (percentage of the maximum observed efficiency) and x is the volume of solution in the vial. As the sample volume deviates from its optimum value, the decrease in relative efficiency becomes more pronounced with low beta energies (Fig. 2). In Table 1 are reported the optimum values of the apparent absolute e$iciency, namely the ratio cpm/dpm. The counting rate (cpm) was obtained with a fixed high voltage (the value used in routine practice ofliquid scintillation detection of sH and 14C) while the amplifier settings were those resulting in the best figures of merit. The theoretical dpm values were calculated according to the measured absolute activity of calibrated sources after subsequent dilutions and the decay scheme of the radioisotope. Beta emitters have been characterized by their average energy though it is acknowledged that neither the average energy nor the end point of the spectrum take into account the shape of the spectrum and thus they fail to be fully descriptive. The relation between detection efficiency and beta energy is not reducible to a simple function as a power function or a third degree polynomial; however in the interval Ep 0.330-l-450 or E, max 0.932-3.550 MeV, the efficiency may be described as a logarithmic function of the energy * Laboratory of Metrology, clkaires de Saclay, France.

Centre d’Etudes

Nu-

B. Francois TABLE 1. Detection efficiency (cpm/dpm) of 11 calibrated sources of pure or predominant p-emitters. Packard Tri-Carb spectrometer with EM1 6255 B phototubes; H.V. set at 1820 V; Thresholds: 50-w. Maxim u m gain. Solutions counted in plastic Packard vials, 8-14 ml in volume. Average j3 energy values ( 1) after HINE and BROMTNELL(~) ; (2) calculated after HINE and BROWNELL and WIDMAN et aI.( (3) after DILLMAN

Ep max

-4 WW 204T1 143Pr lgsAu

24Na 8gSr 4OK 9lY ssP WY 76As

4sK

(MN

0.240 (1) 0.330 (2) 0.330 (1) 0.555 0.560 0.570 0.610 O-695 0.931 I.140

I.450

(3) (1) (2) (1) (3) (1)

(1)

0.764 (97.7 %) 0.932 O-963 (99 %) o-290-1*370 1.390 1.460 1.360 1.545 I.710 2.270 1.76 (16%) 2.41 (31%) ’ 2.97 (50%) 1.99 (18%) 3.55 (82%)

after the equations (1) or (2) =

el = 42.37 In E, - 239*27(r = + 0.99) (1) e2 = 46.89 In E, max - 313.41 (r = + 0.99) (2) where E is the efficiency (cpm/dpm, %), E the energy (in keV) and r the coefficient of correlation. TABLE 2. Cerenkov counting efficiency of five pure For emitters with or predominantly y-emitters. more than one photopeak energy figures have been weighted (“ I+“‘)

J-$ W W 5x3 1”Ru

asZr-Nb ssFe SSY

0.320 0.510 o-750 I.180 l-840 10.900

(max) (W) ( W) (W) (max) and 1.840

Efficiency cw-ddpm (%) NO 0.18 1.50 2.90 3.90 5.90

Detection efficiency (%I 3 6.2 6.7 32.5 28.3 38.4 35.5 35.1 50.6 59 70.3

Similarly as far as we have observed and as an approximation, Cerenkov efficiency of y-emitters is a logarithmic function of the photoelectric energy of the radio element: tz3 = 2.91 In Ey -

17.87

(3)

where E is in per cent and E in keV. The data were provided by five calibrated sources (Table 2) of pure or predominant y-emitters; in the latter case, the beta maximum energy of the element was equal to or lower than 470 keV ( 5gFe) or 396 keV (g5Zr), i.e. below the Cerenkov threshold ofour detection device. Thus, equation (3) allowed us to calculate how much the gamma emission ofsome beta-emitters previously assayed contributed to the Cerenkov signal due to Compton electrons. s04T1, s4Na and 40K have been excluded from the above equations. The detection efficiency of 204T1 differed widely with the type of photomultiplier used as shown below. The efficiency of 24Na is 26 and 28.5 per cent according to equations (1) and (2) ; the actual figure is 32.5

Detection t

of

hard /kmitting

Ellicitncy

%

70 2

5

radionuclides in aqueous solutio;w using Cerenkov radiation

4

6

S

10

12

14

16

IS

Sample nlumc ,

(ml) \

20

24

22

26

FIG. 2. Relative detection efficiency ( % of maximum

in plastic vials.

observed) with different volumes of samples The radionuclides in the upper set of curves gave optimum efficiency for either 8 or 10 ml, the nuclides in the lower set for either 12 or 14 ml. 3. A comparison of Cerenkov count rate ratios (per cent) of various beta emitters using three different types of commercially available photo-multipliers

TABLE

J!?Pvolume

EM1 9635 QB EM1 6255 B

12 ml 204T1 240 keV 20 ml

188

193

198

230

12 ml 330-700 keV 20 ml

131-135

130-136

136-141

143-154

12 ml > 700 keV 20 ml

122-128 128-132

per cent and the difference is thought to represent the contribution ofthe very energetic gamma emission. A problem is unsolved with *OK (as KCI, 1 g K+ = 1619 dpm) whose expected efficiency was 24.8 per cent (equations (2) and (3)) or 29.6 per cent (equations (1) and (3)). Actually, in three series of experiments performed with high grade KC1 in 12 ml of distilled water, we found 38.4, 34.6 and 46.5 per cent. The efficiency decreased very slowly with a progressive drop in KC1 concentration and the count rate was identical

RCA 4501 E:MI 6255 B

after retaining

the sample

5

days in the refrigerated sample changer. Counting 25 g of KC1 as powder and not solution in a plastic vial (background against 25 g of pow-

dered NaCl) produced 195 cpm/g K and an efficiency of 12 per cent. We have surmised a yet undefined phenomenon (chemiluminescence ?) might be responsible for the discrepancy between the expected and the observed values. Figures from other workers with *OK in solution are much lower than ours, around 14 per cent.(s) The counting efficiencies of so*Tl, losAu, *OK, s%r, OlY, ‘J’JSrand *sK in water have been compared with two different types of phototubes EM1 6255 B and EM1 bi-alkali 9635 QB (Table 3, first column) under high voltages of respectively 1820 and 1500 V and gain settings for maximum

counting

performance.

The

ratio

9635 QB/6255 B count rates is influenced

of

both

6

B. Francois

by the volume of solution and the beta energy. Better apparent efficiencies may be accounted for by a better quantum efficiency (27 per cent vs. 23 per cent) and a spectral response of EM1 9635 QB that favors Cerenkov emission (maximum sensitivity on 380 nm vs. 410 nm). The same holds true with bialkali RCA 4501 equipping an Intertechnique SL 40 liquid scintillation spectrometer and operating under 2250 V (quantum efficiency was quoted at 28 per cent at 385 nm). In both series of experiments, the ratios EM1 9635 QB/EMI 6255 B and RCA 4501/EMI 6255 B liquid scintillation count rates for a 3H Toluene source were 135 per cent. The major contribution of the photomultipliers to the total count rate, the possibility of using different settings of the spectrometers, the influence of sample volume and of vial material make comparisons difficult with results from other workers tentatively listed below = l-3 % for ss’Tl(*) 5.4 % for ~Q’Au(~) 18 %,ts) 25 %,15) 26 %(r3) for a4Na 14 %t6) for 40K 25 %,fs) 31 %,(p3) 50 %(ll) for ssP 53 %,t3) 54 zc6) for soY 57 %,(14) 60 %,c6) 62 %,(13*1') 76 %(15) for 4aK 70 %t6) for losRuJo6Rh 54 %c6) for r44Ce-r44Pr. Incidentally, most of those values are lower than ours reported in Table 1. Moreover, it appears that efficiencies are still higher in media other than water, especially organic solvents.tr6) CERENKOV SPECTROMETRY We had intended to delineate the spectral distribution of Cerenkov light emitted in aqueous media and to characterize the amplitude distribution of pulses from the spectrometer for variThe ous /?-emitters of increasing energy. Cerenkov emission spectrum is known to be continuous, extending from near U.V. (limited at approximately 320 nm by the material of the counting vial) to near infra-red. A semi quantitative study of the optical properties of Cerenkov light has been performed with the following arrangement: the internal

walls of standard plastic vials for liquid scintillation counting have been lined with Kodak Wratten filters, cautiously handled and cut so that the edges of the filter were brought into apposition and the whole lateral surface of the vial up to the opaque cap was screened. Next, glass test-tubes were forced through the vial neck so as to hold the tubes vertically. Five ml of radioactive solution were introduced in the glass tubes, interposing the glass wall of the test tube, the optical filter and the plastic wall of the counting vial between the solution and the photocathodes. Twelve different types of filters chosen to be as selective as possible have been used, namely 2B, 2A, 2E, 34,4,9, 16,22,25,92, 89 B and 87. Filters as Kodak 2A and 2B are said anti U.V. as they let visible and infra-red light pass through; anti U.V. plus anti-visible filters as 92 and 89 B only admit the infra-red component of emitted light. All the radioactive sources have been counted in uniform conditions of high voltage (1820 V) and gain (50 per cent) on a Packard Tricarb 33 14 liquid scintillation spectrometer. Each series of experiments started with the counting of 18 different sources of the same radionuclide in test tubes set in plastic vials (as described above) but without any filter in order to determine the activity of the sources and later normalize the results. The count rates of the series of test-tubes containing the first 12 sources in plastic vials lined with the Kodak filters were compared with those of the re-assayed last six sources without filters. With filters chiefly anti U.V. as 2B and 2A (whose optical transmission is respectively 20 per cent at 400 and 415 nm, 50 per cent at 410 and 422 nm, 75 per cent at 430 nm) the ratio count rate with filter/count rate without filter seems to increase with the beta energy of the isotope (Table 4). Similar results with filter 2B have been reported by ELRICK and PARKER(*). With radionuclides of relatively low p energy as 99Mo and 40K, the filters seem to screen out about the same proportion of light regardless of the window settings. With r6As and 4sK, the filters screen out 41 and 45 per cent of the high energy counts while all or nearly all the low energy counts are recorded (Table 4). Presumably, the higher the beta energy of the isotope, the more emission by Cerenkov effect in the U.V. range.

7

Detection of hard p-emitting radionuclides in aqueoussolution using Cerenkovradiation TABLE 4. Relationship between the beta energy of the isotopes and the ratio of count rates with anti U.V. filters (2A and 2B) to count rates without filters (per cent). Different arbitrary threshold settings were used for spectrometric purposes. The interval 50-500 was chosen for the less energetic fraction of hard beta emission and the interval 50-w to encompass all signals. Measurements were made on a Packard Tri-Carb model 3314 Thresholds 4 9sMo s?Sr 40K ssP *Y ‘6As 4sK

WeV) 0.455 0.560 0.570 0.695 0.931 1.140 I.450

Beyond 700 nm, using anti U.V. plus antivisible filters Nr 92, 89 B and 87 (with optical transmission of 50 per cent at respectively 640, 716 and 796 nm), we found that the count rate was null or below 1 per cent of the standard value for a9Mo, %r, 4oK and 32P, but in the order of 4 per cent for eoY and 76As, of 8 per cent for a2K. In the visible part of the spectrum, Cerenkov light has been studied through six filters, each likely to be characterized by the wavelength at which the transmission of a monochromatic light source is half the original signal without the filter = 430 nm for filter 2E, 472 nm for filter 4, 510 nm for Nr 9, 540 nm for Nr 16,566 nm for Nr 22, and 600 nm for Nr 25. Again, the results have been expressed as the count rate ratios of each source with a given filter and standard sources without filter. The most obvious relation between the ratios and the wavelengths taken as typical of the filters is a log-log function (coefficient of correlation r = -0.98). In loglog coordinates, the slopes of regression lines are the lowest for low (“K) or high ( 4zK) /l energies and the highest for p-emitters of medium range energy as 32P,an unexpected finding. Extrapolating the regression line calculated for each isotope allowed to derive the value of the wavelength for which the ratio count rate with filter/count rate without filter would be 100 per cent. For 4aK, 76As, QOYand 32P, the derived wavelength

(divisions)

50-500

50-1000

33.3 42 34.9 44.9 66.9 94.7 >lOO

218.8 34.2 37-9 33.2 50.7 68 77.3

50-w 28.5 33.4 32.6 31.1 46.2 55.5 59.4

is 382-393 nm between thresholds 50 and co, 390-405 nm between 50 and 1000, 403-441 nm between 50 and 500. For ?+, 99Mo and 40K between 50 and co as well as 50-1000, it is found between 34.5 and 384 nm. Obviously, these figures are controversial because of the spread in optical properties of each filter as opposed to the nominal values and since the data have not been corrected for the variable spectral efficiency of the photo-cathode. With a Kodak Wratten filter such as Nr 34, transparent to U.V. and visible light down to 470 nm, the ratios of count rates come to be 41 per cent with P2K, 30 per cent with D’JY,20 per cent with 32P and *%r, a finding in keeping with prior conclusions. The background figures of 21 cpm (Tricarb) and 24 cpm (SL 40 Intertechnique) with standard empty plastic vials and an integral spectrum counting are reduced by one third with filters 2E and 4 stopping U.V. and visible photons down to 450 nm and by two thirds with filters 25 and 92 opaque down to 600 nm. In the 50-500 interval

(low pulses) of the Tricarb

spectrometer

and beyond 500 nm, the background falls below 1 cpm. We have become interested in a pulse height analysis

of counter

response

to Cerenkov

sion. Using first the three-channel the liquid

scintillation

spectrometer,

emis-

analyser of we meas-

ured channel ratios up to 60 consecutive days

a

B. Francois tion of the channel

ratios

have been:

(1) Occurrence of vapour or frost which even in minute amounts on the external wall of the vial may bring basic changes of the count rate. (2) The volume of solution or rather the height of the solution critical factor.(‘)

within

(3) The value of the amplifier stant high voltage.

the vial, a most gain for a con-

(4) Counting statistics. (5) Electronic stability.

0 I

’ 05

’ 1

15

2

2.5

3

35

'

FIG. 3. Channel ratios of count rates (C.R.) between arbitrary limits 50, co and x1 = 1000 (upper curve) or xi = 250 (below) ; 20 ml of solution in plastic vials; Packard Tricarb spectrometer set at H.V. = 1820 V, gain = 50 per cent, EM1 6255 B phototubes. By comparison, sH in Bray, 3H and l*C in toluene PPO-POPOP gave respectively 99.7, 95-l and 17.2 for channel ratio 50-1000/50-co and 39.3, 19.2 and I.6 per cent for ratio 50250/50- co.

with various B-emitters of increasing energy from aorTl to 42K (Fig. 3). The same source was used when half-life permitted, or, if not, new sources were used. The results obtained from three or four series of runs (three samples per isotope) and giving a statistical error of less than 0.6 per cent have been averaged. The stability of the detector was checked by a repetitive counting of a sH toluene standard under the conditions cited above. Tritium as tritiated water in Bray’s solution performed like a Cerenkov emitter of E, maximum 0.84 MeV and background in water (20 ml in a plastic vial) as a Cerenkov emitter of E, 3.5-l MeV. Since their beta energies were very closed, results from 4oK, s%r, erY and sap

were combined in Fig. 3. In our experience, the main causes of varia-

Counting repeatedly for 120 h the same source of a rapidly decaying element then several samples of elements of largely different absolute activities, we could observe that the channel ratios are not related to the strength of thesource and that there is no hysteresis. If the factors listed above are allowed for, any diagram like Fig. 3 may provide an easy means of determining the species of a hard and unknown p-emitter present in an aqueous solution. The relationship between the beta energy dissipated in aqueous media and the height ofpulses collected in the scaler may be exhibited through an oscilloscopic display of Cerenkov spectra. An Intertechnique SL 40 liquid scintillation spectrometer was linked to a 400~channel Intertechnique SA 40 B pulse height analyser and a photograph was taken of each Cerenkov spectrum as displayed on the SA 40 B oscilloscope. The spectrum of a sH toluene liquid scintillation standard was stored and used as a reference for the position and the shape of Cerenkov spectra (Figs. 4-9). Those findings corroborate the previous results with channel ratios; under experimental conditions of 3H and 1% routine detection, the pulses obtained by liquid scintillation counting of sH toluene are higher than those from counting Cerenkov emitters of maximum beta energy 0.5-l -3 MeV and lower than those from emitters above I.3 MeV. The background spectrum is made of pulses scattered along the entire energy scale up to 10 MeV with a slight peak at I.5 MeV. Although there are numbers of phenomena involved(8-B) such as the energy distribution of electrons, the wavelength of light produced, the interaction with the detector materials, the spectral response of the phototubes, the losses in the coincidence circuitry, etc. . . , this simple method

d \

=

FIGS. 4-9. 4. (right) spectrum 5. 6. 7. 8.

Same Same Same (right) that 9. (left)

Cerenkov spectrum of 42K in water. For reference (on the left) a liquid scintillation of 3H toluene. Abscissa is on a logarithmic scale. Contents of peak channel = 4000 counts. conditions as Fig. 4: (right) g”Y in water; (left) 3H toluene. (left) 3H tol.uene. as Figs. 4 and 5: (right) 32P in water; 3H toluene and sgSr in water. as previous figures: 1000 counts. A spectrum identical to 3H toluene; (left) 2oiTl in water. Peak activity: 2A. of 204T1 was given by sgSr in water filtered through anti U.V. Kodak Wratten 3H toluene; (right) 13-hr background on 20 ml distilled water. Maximum activity (ordinate) = 200 counts.

Detection of hard B-emitting radionuclides in aqueoussolutions using Cewnkov radiation was shown to permit the determination energy of hard p-emitters in water.

QUENCHING AND ACTIVATION

of the

LIGHT

As previously stressed”) color quenching can be an obstacle to biologists resorting to the Cerenkov effect for the detection of hard /?-emitters in solutions, For example, the count rate given by human urine enriched in 42K is half that obtained in distilled water. Moreover, color quenching varies widely between biological samples of the same kind. A similar problem arises when colored substances are leached out of tissues by immersion in water. Sustained efforts to eliminate color due to human serum and urine have been unsuccessful. Bleaching agents as benzoyl peroxyde or ozone(r7) and protein precipitation proved in our experience poor or unsatisfactory procedures. For instance, the precipitation of human serum protein by sulfo-salicylic acid ( 1 : 3) on 12 different samples yielded average optical transmission values on the supernatant of only 25 per cent at 350 nm, 76 per cent at 400 nm and 79 per cent Mineralization techniques give at 450 nm. better results(‘) since they will regularly produce solutions highly loaded in solutes but colorless, but they are time-consuming and not widely applicable. We decided to try powerful physical agents to break down serum or urine chromogens; urine samples in plastic vials were irradiated by a 82,000 Ci 6oCo source (670 krad/hr) but the samples showed no decrease in color. High-frequency ultrasonic irradiation of other samples proved to be similarly ineffective. When the color of the sample is weak it may be accounted for by several procedures. One of those, internal standardization, has been recommended by several authors;(rO~ss) it is in our view liable to give irregular results”) and anyhow modifies the sample. Optical density determinations and channel ratio calculation@) appear either too insensitive or not accurate enough.(*) We conclude that correction by external standardization is the best method at hand. In our experience the solute quenching of Cerenkov signals in transparent aqueous solutions is far less important than color quenching, as previously shown.(7,8.10.11.16,1Q) Adding in-

9

creasing amounts of pure H,SO, (up to 18 ml for a total sample volume of 20 ml) to various /?emitters in solution demonstrates a dependency of the magnitude of quenching on the beta energy of the isotope. With 4zP and s2P in an acid solution of 2.65 Osm/kg ( measured in the Fiske osmometer), the count rate rises to 106 per cent the reference value in water; it returns to the 100 per cent initial value in solutions between 4.5 and 5 Osm/kg, then it decreases somewhat slowly for 42K and more rapidly for 32P. With a less energetic p-emitter as 14aPr, the decrease of the count rate is parabolic-97 per cent for 1.80sm., 85 per cent for 4.5 Osm., 69 per cent for 5.35 Osm. and 46 per cent for 6.2 Osm./kg. For the highest acid concentration assayed ( 18 ml H,SO,, 8 Osm/kg) the count rates are 82 per cent for 42K, 69 per cent for 32P and 47 per cent for 143Pr, compared to distilled water. In our laboratory, G. Masson extended the previous experiments to 20 different concentrations of NaCl (up to 2.5 g/l. with 42K, 32P, rb3Pr), KC1 (up to 1.77 g/l. with 42K), Na,HPO, (up to 6.12 g/l. with 32P) ; 10 samples were run for each concentration. Concentration-dependent sinusoid-like variations of the count rate seemed to be observed, yet did not exceed f2 per cent the average figure in each series. Nuclear magnetic resonance spectra and infra-red spectra on some of the solutions under study failed to show significative differences. Asdid HABERER'~) and PARKERand ELRICK(@, we conducted several series of experiments on signal enhancement produced by wavelength shifters on pulses originating from 42K and s2P in aqueous solutions, in 2 N acid solutions (the end produ.ct of wet mineralization of human urine”)), and 6 N acid solutions. Basically, any fluorescent substance may be considered a wavelength shifter when added to the solution in ppm and excited by U.V. photons it will isotropically reemit light with a larger visible fraction than the incident Cerenkov light. As shown in Tables 5 and 6,31 products have been assessed at various concentrations ranging from 0.5 to 5000 ppm; some of them were new industrial blueing agents. Uranin and Rhodamin acted like strong quenching agents owing to the color they developed in the media. With most powerful shifters enhancement is maximum at concentrations as low as 5 ppm; up to the

B. Francois

10

5. Variations of count rates of32P in distilled water with 26 products assessed as wavelength shifters. Supplied by Ciba, a molecule with a Benzo-xyazolyl-thiophen structure. Supplied by Badiche Anilin und Soda Fabrik. Products with a stillben structure, from Ciba. Supplied by Geigy. A product with a Benzy-midazol structure, from Ciba. A molecule of the Diothyl chlorhydrate type, from Ciba.

TABLE

(1) (2) (3) (4) (5) (6)

Variation of count rates %

Wavelength shifters (denomination)

Working concentrations (wm) 5-1000

+50 to +70

Esculin

+30 to +50

,&Naphthylamine @-Methylumbelliferone 2-Naphthylamine 6-8 U vitcx E.B.F. cone (1)

5-5000 1000 500-1000 1000

$10 to +30

/?-Naphtoic acid Blancophor BBU (2) U vitex NB cone (3) U vitex CF cone (3) P-Terphenyl U vitex S 35-35 (3) U vitex CF (3) U vitex CFCN (3) Tinopal RP (4)

5-1000 50-1000 50 50-1000 500-1000 50-1000 50-1000 50-1000 100-500

+5 to -5

Naphthalene Ca Tungstate U vitex S2R 300 (3) U vitex ERNC 100 (5) Tinopal 4 BM (4)

5-1000 100-1000 50-1000 50-1000 50-1000

-5 to -50

Quinin sulfate Coumarin Salicylic acid Riboflavin U vitex WGS (6) Blancophor DCB (2) U vitex SBR HC (3)

55000 5-500 5-5000 0.5-10 50-1000 50-1000 50-1000

concentration giving the maximum enhancement, the variation of the count rate (afterbefore/before) is a logarithmic function of the concentrations of the shifter. The efficiency of the wavelength shifter is unrelated to the activity of the radioactive source being detected, neither do the ways by which the sources have been handled and exposed to solar or artificial light seem critical. With seP in 2 N acid solutions, the count rates

were increased 35-50 per cent by Esculin, 2530 per cent by 6,8 2-Naphthylamine and lo-20 per cent by p-Naphtoic acid and P-Terphenyl. For 42K, the figures were 12-14 per cent with Esculin and 6 per cent with fi-Naphtoic acid and P-Terphenyl. Using the device already described based on selective Kodak Wratten filters, we tried to gain some insight into the distribution of the light emitted by a s2P source to which the two best

11

Detection of hard b-emitting radionwlides in aqueous solutions using Cerenkovradiation TABLE 6. Variations wavelength shifters.

(1) (2) (3) (4) (5)

_-

of counting rates of 42K in distilled water with 18 The results with esculin have been obtained with two distant series of experiments. Supplied by Badiche Anilin and Soda Fabrik. Ciba (see Table 5). Geigy. Ciba. Ciba. Variation of count rates (%I

Esculin

1-15 to $20

matched

weaker nounced

5-1000

+10 to +15

/?-Naphthylamine /?-Methylumbelliferone p-Naphtoic acid

5-1000 500-1000 100-1000

$5 to t-10

P-Terphenyl Blancophor BBU (1) U vitex CF cone (2)

500-1000 108-500 100

$5 to -5

2-Naphthylamine 6,8 U vitex S 35-35 (2) Tinopal (3) Naphthalene Quinin sulfate Coumarin

50-1000 50-1000 50-1000 50-1000 5-5000 0.5-500

-5 to -50

Salicylic acid Riboflavin Blancophor DCB (1) U vitex WGS cone (4) U vitex SBR HC (5)

wavelength shifters were added. Results (Table 7) make obvious the effects of converting the U.V. component of Cerenkov light into a visible one

better

Working concentrations (pm4

Wavelength shifters (denomination)

to the photocathodes.

the pulse amplitude, the more the enhancement phenomenon.

The proThe

“calculated value for relative count rates 100 per cent” is a rough approximation derived from the relation relative count rates vs. wavelengths taken as defined by 8 Kodak Wratten filters rangingfrom nm (Nr 22) to410 nm (Nr 23). In Table 8, our results obtained with Esculin are compared with data from ELRICK and PARKER(*)and LA&HLI(~~) who have resorted to

wavelength

5-5000 5-500 50-1000 50-1000 shifters

retain

some interest

for b-

emitters in the intermediate range of energy (as 32P) but show little advantage for very hard b-emitting radioisotopes. PRACTICAL IMPLICATIONS We have ascertained(‘) that it is practicable to detect 42K in biological solutions through the Cerenkov effect. The method, subsequently confirmed by other.Gal) was implementedtomeasure exchangeable potassium (the most significant

fraction

of total

body

potassium)

in 49

fonic acid, Elrick and Parker doubled the efficiency of /l-emitters with energies falling between those of 2o4Tl and asp. In our experience with

controls and 17 patients.(ss) “Spot” urine samples were collected 40-44 hr after the oral administration of 60-80 ,uCi of b2K and the urines were mineralized. As shown by the analysis of 140 spot urine samples counted 50 hr after the dose had been given to the patient, the

new,

method

amino

naphthalene

more

efficient,

disulfonic

bialkali

acid.

With

phototubes,

disul-

the

in.sures a signal-to-noise

ratio

ranging

B. Francois

12

TABLE 7. Changes induced by two wavelength shifters and Kodak Wratten filters on detection conditions and efficiency of 32P. Packard Tricarb spectrometer EM1 6255 B phototubes, HV 1820 V, gain 100 per cent for three channels. On each row of (2), (3) and (4) the figures concern respectively the three usual energy intervals 50-500, 50-1000, 50-co. “R.C.R.” or relative count rate is the ratio count rate (cpm) with a given filter count rate (cpm) without the filter

Without wavelength shifter (1) “Absolute” detection efficiency cpm/dpm ( %I

With /I-Naphthylamine

With Esculin

35

48-55

52-60

R.C.R. (%) anti-u.v. filters

45 33.2 31

85 80.8 80.3

105.7 95.7 88

(3) R.C.R. (%) anti-u.v. and anti-visible filters

0.4 0.3 0.3

0.7 0.7 0.5

7.2 4.1 3.4

(2)

(4) rl calculated value for RCR 100 % b-4

403 395 392

from 17 to 2 10, an average of 102. The data support the claim that measurements with a statistical error less than 1 per cent may be performed with oral doses most certainly smaller than 40 ,&i of baK. The reliability of the sample preparation technique has been checked by new recovery experiments = 20 nCi of a2K have been added to 30 different human urines in four series of runs; each urine was mineralized(7) and its counting rate compared to the reference value from four aqueous standard, handled and treated in the same way as the urines within each series. The average recovery was found to be 99.3 per cent, cr f 2.9 per cent, observed extreme values = 95-l-105 per cent. Those last results are identical to results published in 1966 (x = 98.8 per cent). The ratio mineralized standards/aqueous standards was 98-5 per cent and optical transmissions of the treated

urines, when checked

on,

ranged from 98.5 to 100 per cent in the interval 400-500 nm.

424 420 420

442 428 423

Several workers(8,11.12,16,1s.23)have used Cerenkov counting for detecting 32P under various conditions. In our laboratory we have tried to detect ssP in a blood red cell environment, anticipating the assessment of human red cell life span with DFP 32P; owing to an intense color quenching, that represented one of the worst possible working conditions. The combustion in a muffle furnace of a suspension of priorly dried red cells led to irregular and large (up to 40 per cent) losses of 32P due to sublimation. Later on, samples of dried red cells and of serum-free red cells stored in distilled water were mineralized; 10 samples were simultaneously treated in each case. Total processing times were respectively 1.5 and 3 hr and necessary volumes of sulfuric acid and water respectively 25 and 20 ml (dried cells), 10 and 40 ml (cells in water). After mineralization, the optical transmissions of the solutions were 86 per cent (dried cells) and 80 per cent (cells in water) at 400 nm, from 97 to 99.5 per cent at 450 nm and

Detection of hard b-emitting radionuclides in aqueous solutions using Cerenkovradiation

13

TABLE 8. Enhancement of detection efficiency (cpm/dpm%) using different types of photo-multipliers and wavelength shifters ssRb

ssP (1.71 MeV) EMI6255B (%I)* Esculin without with

35 54

EM1 9514 (S13)t Amino 2 6,8 Disulfonic acid without with

25 50

EM1 9635 QB (S13)$ Amino 7 1,3 Disulfonic acid without with EM1 9635 QB Esculin without with *This report;

(1.78 MeV)

42K (i:zz MeV)

70 82

23 46

60 85

.51 ‘70

82 93

(S13)* 49 61

79 82

t ELRICK and PARKER(*); $ LA~~cHLI(~~).

rom 98 to 100 per cent at 500 nm. Obviously, those techniques are too tedious and will be profitably replaced by automatic sample combustion devices. The Cerenkov effect has been readily used for the detection of ssRb(*,14) and 24Na.(13,24) We are now in the process of applying it on a routine basis to the determination of exchangeable sodium in man. May Cerenkov counting be extended to many other radionuclides in solutions ? Unfortunately, the half-lives of the most energetic /?-emittersthat would exhibit the best detection efficiencies-are a fraction of a second, seconds or at the best a few minutes, the least inappropriate being 44K (T,,, = 22 min, E, = 4-9 MeV). However, potential applications exist for numerous isotopes of a physical half-life above a few hours. As LA~~cHLI(~~) does, we feel certain the method could be applied to emitters having maximum beta energies in the range O-5-1 MeV. It would be even better suited to nearly 40 emitters of maximum beta energy above 1 MeV such as s6Co, 8sSr, DlY, 97Zr, gsM~, lllAg, ls%b, ls21,

14OLa, 142Pr, 143Ce, 166H~, leeRe and lg41r. There is no doubt that the method looks most attractive for currently available radioisotopes of beta maximum energy above 2 MeV such as ssMn (2.89 MeV, 50 per cent) lssRe (Z-12 MeV, 79 per cent. and 1.96 MeV, 20 per cent) 76As (see Table 1) and s’JY (2.27 MeV). The Cerenkov detection of radionuclides in human urine may help tackle Health Physics problems.(s*) Inour laboratory, a comprehensive check-up of personnel likely to be contaminated by radionuclides includes whole body counting of y-emitters, counting of unprocessed urine in a NaI crystal, liquid scintillation counting of vacuum distilled urine for 3H as tritiated water and multichannel Cerenkov counting (Fig. 3) of hard p-emitters i.n decolorized wine. The advantages of Cerenkov counting over other counting techniques of hard B-emitters in solutions are numerous and as yet little appreciated: (1) Fairly high detection efficiencies may be expected, a great asset to the nuclear physician (Tables 1 and 8). (2) Relatively large volumes of samples, up to 22 ml, can be measured: one

14

B. Francois

of the major reasons for advocating the method. (3) Thanks to coincidence circuitry, a low background is achieved, thus improving counting statistics: a property giving liquid scintillation spectrometers a decisive edge over NaI well-type counters. (4) No sample preparation is required for colorless solutions, no further chemical procedures and no costly scintillators are needed. (5) The samples, unaltered by counting, may be completely recovered for other assays. (6) Chemical quenching being of no consequence in most situations entitles the radio chemist to use, if necessary, high concentrations of acids or oxidizing agents. Acknowledgements-The

author is indebted to C. J. the manuscript and providing helpful suggestions. He wishes to express his gratitude to MICHELLE LIMANDASand MARGUERITE PIERSON for their expert technical assistance. PALAIS for reading

REFERENCES 1. BELCHER E. H. Proc. R. Sot. A216,90 (1953). 2. JELLEY J. V. Cerenkov Radiation and its Ajjrlication (2nd edn). Pergamon Press (1959). 3. HABERER K. Atom Wirtschaft 10, 36 (1965). 4. HABERER K. and K~LLE W. Atompraxis 11, 664 (1965). 5. DE VOLPI A. and PORGES K. G. A. Int. J. apP1. Radiat. Isotobes 16, 496 (1965). 6. PARKER R. P. and ELRICK R. H. Int. J. a@$. Radiat. Isotoges 17, 361 ( 1966). 7. FRANCOISB. Int. J. a& Radiat. Isotopes 18, 525. (1967). 8. ELRICK R. H. and PARKER R. P. Int. J. apP1. Radiat. Isotopes 19,263 (1968). 9. Ross H. H. Analyt. Chem. 41, 1260 (1969).

10. PARKER R. P. and ELRICK R. H. in The Current Status of Liquid Scintillation Counting (Edited by E. D. BRANSOME, JR). Grune Stratton, London (1970). 11. HAVILAND R. T. and BIEBER L. L. Analyt. Biochem. 33, 323 (1970). 12. CLAUSEN T. Analyt. Biochem. 22, 70 (1968). 13. GARRAHAN P. J. and GLYNN I. M. J. Physiol. (Lond.) 186, 55 P (1966). 14. LA~~CHLI A. Int. J. appl. Radiat. Isotopes 20, 265. (1969). 15. PARKER R. P. Hlth Phys. 18, 175 (1970). 16. JOHNSON M. K. Biochem. J. 111, 348 (1969). 17. KRABIXH L. and BORGSTR~M B. Scand. J. Gastroent. 3, 458 (1968). 18. STUBBS R. D. and JACKSON A. Int. J. appl. Radiat. Isotopes 18, 857 (1967). 19. MIZUNO S., ECUCHI H., YANO K. and YAMAGUCHI H. Radioisotopes 18, 19 (1969). 20. HEIBERG E. and MARSHALL J. Rev. Scient. Instrum. 27,618 (1956). 21. JOHNSON J. E. and HART~UCK J. M. Hlth Phys. 16, 755 (1969). 22. FRANCOIS B. Physiopathalogy of Adipose Tissue, 3rd International Meeting of Endorrinologists. Excerpta Medica Monograph 349 (1969). 23. V E M M E R H. and G~~TTE J. 0. Atompraxis 10,475 (1964). 24. BRAUNSBERG H. and GUYVER A. Analyt. Biochem. 10, 86 (1965). 25. HINE G. J. and BROWNELL G. L. Radiation Dosimetry. Academic Press, London (1956). 26. WIDMAN J. C., MANTEL J., HORWITZ N. H. and POW~NER E. R. Int. J. appl. Radiat. Isotopes 19, 1 (1968). 27. DILLMAN L. T. J. nucl. Med. (Su@l. 2) 10, 5 (1969). 28. NARROG J. Personnel Dosimetry for Radiation Accidents, 427 IAEA, Vienna (1965).