Thick-target yield for the production of yttrium-87 by the irradiation of natural rubidium chloride with alpha particles

International Journal of Applied Radiation and Isotopes, 1971, Vol. 22, pp. 85-88. Pergamon Press. Printed in Northern Ireland

Thick-target Yield for the Production of Yttrium-87 by the Irradiation of Natural Rubidium Chloride with Alpha Particles J. S T E Y N , B. R . M E Y E R a n d J . M. J . B A R E N D S M A National Physical Research Laboratory, P.O. Box 395, Pretoria, South Africa (Received 26 June 1970) A method is described for the standardization of the nuclide sty, based on 4~r X-gamma coincidence counting with the use of all internal liquid scintillation counter as 4~r detector. The thick-target yield for the production of s t y by irradiation of natural rubidium chloride with 32 MeV a-particles was measured as 174 I~Ci/ttA-hr at the peak of the s t y growth/decay curve after a 3 hr bombardment. LE R E N D E M E N T DE CIBLE EPAISSE P O U R LA P R O D U C T I O N D ' Y T T R I U M - 8 7 PAR L ' I R R A D I A T I O N DU C H L O R U R E DE R U B I D I U M N A T U R E L AVEC DES P A R T I C U L E S A LP H A On dficrit une mdthode pour la normalisation de la nucldide s t y bas6e sur le comptage coincidanee 47r X-gamma avec l'emploi d'un compteur de scintillation liquide intdrieure comme ddtecteur 4rr. Le rendement de cible fipaisse pour la production de st y par l'irradiation du chlorure de rubidium naturelavec des particules a de 32 MeV fut mesurfi comme 6tre 174 /,Ci//,A°hr au pic de la courbe de croissance/ddch6ance de st y suivant un bombardement de trois heures. ]3LIXO~ TOJICTOITI MHII/EHH ~JIH IIPOH3BO~ICTBA HTTPHH-87 IIOCPE~CTBOM O B g I V t t E H I I t t ECTECTBEItHOFO X,rIOPH~A P V B I I ~ H I t AJIbq~A-ttACTHILAMH 3 ~ e e B o m 4 e b l B a e T e g MeTO~ C T a H ~ a p T H 3 a I I H H ~ISOTOHa 8 7 y , OCHOBaHHblfl H a c q e T e eOBIIaJiegilif

4~r X-raMMa e NpHMeHeHneMB KaqecTBe/IeTeHwopac reoMeTp~efi 4rr H~H~HOCTHOFOCI~HHTHJIJIHI~IIOHHOFO c q e w q n g a c BHyTpeHHHM HCTOqHIIHOM. BbIXO~ TOJICTOI~ MIImeHIl ~JIH IIpoI~SBO/ICTBa

s t y o6~yqeHgeM ecTecTBegt~oro xzopnaa py6naHn ~-qacTntlaM~ B 32 MOB, ~sMepeHHNfl y BepmnH~i RpHBOti 87y pocw/pacnaa ~ocne 3-x qacoB 60M6api~Hp0BaHnn, 6Hn 174/zCi//~A-qac. AUSBEUTE DICKER TREFFPLATTEN FUR DIE HERSTELLUNG VON YTTRIUM-87 DURCH BESTRAHLUNG VON N A T ~ R L I C H E M RUBIDIUMCHLORID MIT ALPHATEILCHEN Ein Verfahren wird beschrieben ftir die Normung des sTy-Nuklids an Hand einer 4~r-R6ntgen Gamma-Koinzidenzziihlung unter Verwendung eines internen Fltissigkeits-SzintiUationsziihlers als 47r-Detektor. Die Ausbeute einer dicken Treffplatte ftir die Herstellung yon st y durch Bestrahlung nattirlichen Rubidiumchlorids mit 32 MeV-Alphateilchen wurde zu 174 #Ci[/,Ah gemessen und zwar auf dem Scheitel der aTY-Anwachs/Zerfallskurve nach einem Beschuss von drei Stunden. 85

86

J. Steyn, B. R. MJeyerand . I . M . J . Barendsma

INTRODUCTION AN AGGURATE knowledge of thick-target yields for production of s t y is of practical importance since the p a r e n t - d a u g h t e r system sTy-sTmSr forms an established nuclide generator for use in medical diagnostics. SSRb(~, 2n)STy is the most favourable reaction for production of s t y at the Pretoria cyclotron. T h e preferred method of target manufacture at present, is the preparation of a melt of natural rubidium chloride on a copper backing plate. For the determination of the thick-target yield, rubidium chloride targets were irradiated with the 32 M e V external alpha-beam of the cyclotron for time intervals short compared with the 80-hr half-life of s7y. After chemical separation, the s t y activity was measured absolutely by 47r X - g a m m a coincidence counting with the use of an internal liquid scintillation counter as 47r detector. T h e method is fully discussed in this paper. Directly after irradiation the s t y activity was found not to decay with the characteristic half-life, but in fact to increase initially. This was caused by 13-hr s7my which was also produced and which decays almost exclusively to s t y . A further target was therefore irradiated and a specific g a m m a ray observed with a Ge(Li) detector and a multichannel analyser in order to follow the growth and decay of 87y. THE STANDARDIZATION METHOD svy can be regarded for practical purposes as a pure electron-capture nuclide since the intensity of the positron branching is only 0" 16 per cent. m T h e standardization of electroncapture nuclides by 47r X - g a m m a coincidence counting has already been described, 12) but measurement of the svy disintegration rate is complicated by the presence of the 2 " 8 h r svmSr, (see Fig. 1). A further complication in the present work was caused by 13-hr STray which was produced in excess of SVy. T o achieve the absolute measurement of s t y under these conditions, care must be taken not to observe those radiations which are not coincident with the events of interest, i.e. the decaying s t y nuclei. I n the g a m m a channel, therefore, the 485 keV g a m m a - r a y peak was observed in a window which completely rejected the 388 keV

9/2*

keY

381

13,2h

IT 9 8 . 3 %

8o~ 87y

0

I/2-

Ec 3/2-

2.83

I/2-

l

IT

0.30% 9/2 +

8ZSr

~*

%

%

92,5

--

7.3

0.16

9.7%

0

Fro. 1. Decay schemes of stray, s?y and s7mSr (after ZOLLERet al.Q)). aT'"Sr g a m m a - r a y as well as the 381 keV g a m m a ray from the s7my isomeric transition to sty. Both these rays show internal conversion of about 22 per cent, resulting in conversion electrons with energies of 365 keV or higher, which are very effectively detected by the internal liquid scintillation counter. T h e rays of interest to be counted in the liquid scintillation counter, however, are the characteristic X-rays and Auger electrons following the electron-capture transition of sTy-87mSr. These rays, with energy about 14 keV, were therefore observed to the exclusion of the much more energetic conversion electrons by suitable differential discrimination in the 4rr channels. The characteristic radiations involved with the internal conversion events are not detected in the 4~r channels, because these radiations, being coincident with the conversion electrons just contribute to the mutual pulse height and are therefore electronically rejected together with the conversion electrons. Let N be the disintegration rate of a source dissolved in the liquid scintillation counter, p the probability that the 485 keV ray interacts with the liquid scintillator, and tI~ the intensity of the electron-capture branch to the 873 keV level. U n d e r the counting conditions as

Thick-target yield for the production of Yttrium-87 IO

described above the counting rate in the X-rayplus-Auger-electron window can then be written :

- - p ) N E m + (1 - - W ) N E , ,

X=~(1

where E,~ is the X-plus-Auger counting efficiency. It is assumed here that no pulses are caused in the X-ray-Auger-electron window by radiations from nuclear transitions. The g a m m a counting rate is given by: G --~ tF'(l -- p ) N E r where E~ is the g a m m a counting efficiency, and the coincidence counting rate is: C ~---rE(1 -- p ) N E m E v.

C/G = Era, and

N--

8

g

6

~,, 5 ~ ,

i;

XG

1

\

>:

,~

2

~>" ® 1

2,~

It follows that:

87

,

I

48

,

1

72

,

I

96

,

I

120

I

144

,

I

,

168

I

'192

TIME AFTER END OF IRRADIATION, HOURS

FIG. 2. Growth and decay of s7y activity; the end of irradiation was taken as zero time.

C "(1 --tFp) "

Growth and decay of s w directly after irradiation EXPERIMENTAL

Irradiations Chemically pure rubidium chloride was melted in a radiofrequency furnace in an argon atmosphere to form a layer of 1-2 m m thickness in a depression in a copper disk. Three of these targets were irradiated with 32 MeV particles for 3 hr each at mean beam currents of about 0"27 ttA. The experimental arrangement during irradiations, and the accurate measurement of integrated beam current were similar to what had been described before. ~3) The accuracy of the integrated beam current measurement was estimated as 0.2 per cent. The beam energy was not remeasured during this work, but it was known to be between 32.0 MeV and 32.5 MeV with a most probable value of 32-2 MeV.

Chemical separations The rubidium chloride was dissolved in 0.5 N HC1 after irradiation and the yttrium separated by ion exchange using a modified version of the method of STRELOW et al. ~4~ One milligram of yttrium chloride carrier was added before separation and the final solution was 10 ml of I N H C I . The recovery of s7y was estimated to be better than 99 per cent.

One of the irradiated targets was put in unprocessed form in front of a Ge(Li) detector, the 485 keV gamma-ray of 87y was isolated from other radiations by means of multichannel analysis, and its intensity measured as a function of time for a period of about 190 hr. Initially an interfering peak with energy 5-1 keV lower than the ray of interest was in evidence, most probably the isomeric g a m m a transition from 3.1 hr 90my. The intensity of the interfering gamma-ray was determined and subtracted by means of a peakfitting computer program.

The 4rr X-gamma measurements The counting cells containing about 15 ml of liquid scintillator were viewed by two bialkali phototubes in coincidence, while the g a m m a detector was a conventional 3 × 3 in. sodium iodide crystal. The overall amplification was set to make the impulse distributions in the two 4rr-channels more or less the same, and to bring the X-ray and Auger-electron pulses into the electronic windows, to the complete exclusion of the much higher amplitude pulses caused by the internal conversion electrons and g a m m a rays. The X-Augercounting efficiency was 0.463 on the average,

88

J. Steyn, B. R. Meyer and J. 3i. J. Barendsma

while the background was less than 0-05 per cent in this channel. The window in the g a m m a channel ensured that only the 485 keV photopeak was observed; the g a m m a counting channel background was less than 0-2 per cent of the g a m m a counting rate. A coincidence resolving time of 0.5 #sec was used. The composition of the liquid scintillator was the same as had been used previously, ~2~ but it contained yttrimn and strontium carrier at concentrations of about 20 mg/l. The samples of s t y solution dissolved readily in the scintillator and no adsorption or precipitation difficuhies were encountered. RESULTS AND DISCUSSION Figure 2 shows the growth and decay of s7y activity. The absolute radioactivity measurements were made about 140 hr after the end of irradiation, and the curve in Fig. 2 was used to correct the 87y activity to the time instant of maximum activity. This value was then used in the calculation of the thick target yields. It is possible to calculate the svy/87my activity ratio at the end of irradiation from the position of the maximum on the time axis, The activity ratio thus found was 0.066 at the end of a 3-hr irradiation. Small amounts of s6y, ssy, 90,,W and 90y were also observed in the solutions to be standardized. The 14.6hr s6y and 3.1 hr ~*~Y had completely disappeared by the time the radioactivity measm'ements were made. The relatively small amount of 90y in the solutions did not intert~re with the measurements--it is practically a pure beta emitter and impulses caused by its high energy beta radiation were rejected by the electronic discrimination. A correction had to be applied, however, tbr the 108 day 88y. The 8sy/sTy activity ratio at the time of the counting measurements was determined as 0"035 by means of comparative gamma-ray measurements. The effect of the ssy g a m m a radiation in the g a m m a counting window centered on the 485 keV peak of 87y could be neglected, but the say X-rays and Auger electrons were of course counted with the same efficiency as was the s7y characteristic radiation, so that the X-Auger counting rate had to be corrected by about 3.5 per cent. The factors xF and p appearing in the formula

TABLE 1. Thick-target yield, at time of maximum 87y activity Irradiation No.

pCi/t~A-hr

1 2

172-9 :k 0'6 174.6 !: 0"5

for the disintegration rate can be a source of systematic error. The intensity uF was taken from ZOLLER et al. m as 0"925. Should a better value become available in future, our results will have to be corrected accordingly. The probability p that the 485 keV g a m m a ray interacted with the liquid scintillator was not determined directly. This factor was measured for the g a m m a ray of a~Mn, an electroncapture nuclide with a simple decay scheme. The relative Compton photon scattering cross sections were then used to calculatep at 485 keV and the value arrived at in this way was 0"038 :k 0"004. Thick-target yields as determined by two different experiments are given in Table 1. The errors included in the table are the random errors associated with the counting measurements only. The overall relative systematic error, determined by combination of individual systematic errors in quadrature, is estimated as 0.5 per cent. The yield does not agree with the value of 59 #Ci]/tA-hr reported by HILLMAN et al. ~5) determined at 35 MeV, but since these authors neither described their method of radioactivity measurements nor stated at what time instant after irradiation the s t y activity was measured, the results arc perhaps not directly comparable. REFERENCES 1. ZOLLER W. H., WALTERS W. B. a n d CORYELL C.

D. Phys. Rev. 185, 1537 (1969). 2. STEYN, J. IAEA Syrup. Standardization of Radionuclides. Paper SM-79/16 (1966). 3. R6HM H. F., Vnp.wF,Y C. J., STEYN J. and RAUTENBACHW . L. J. inorg, nud. Chem. 31, 3345 (1969). 4. STRELOWF. W. E., ConTz~n J. H. J. and VAy ZVL C. R. Analyt. Chem. 40, 196 (1968). 5. HILLMANM., GREENEM. W., BisHoP ~,V. N. and RmHhRDS P. Int. J. appl. Radiat. Isotopes 17, 9 (1966).