Measurement of 125I by liquid scintillation methods

N U C L E A R I N S T R U M E N T S AND METHODS

133 (1976) 2 9 3 - 3 o i ;

© NORTH-HOLLAND

P U B L I S H I N G CO.

M E A S U R E M E N T OF 12sI BY L I Q U I D SCINTILLATION M E T H O D S DONALD L. HORROCKS

ScienHfic Instruments Div&ion, Beckman Instruments, Inc., Irvine, Ca. 92664, U.S.A. Received 18 November 1975 The decay scheme of 1251 is examined for the explanation of the observed pulse height spectrum and counting efficiency measured by liquid scintillation methods. The pathways of energy release in the form of kinetic energy of electrons (conversion or Auger electrons) is explained. Two major energy groups are predicted at 12 and 40 keV. These two groups were observed in the liquid scintillation systems. The measured counting efficiency in an unquenched emulsifier scintillator solution and a coincidence type liquid scintillation counter was 76%. The effects of quenching on the counting efficiency and pulse height spectrum of 125I were measured.

1. Introduction Several investigations have been reported on the measurement of 125I by internal sample liquid scintillation method¢-4). In addition Ashcroft 5) reported on counting ~zsI samples external to a heavy metal loaded liquid scintillator solution actually measuring the gamma and X-rays emitted by rE51. The counting efficiencies for internal samples were reported between 14 and 72%. However, in all of these reports no detailed explanation was given to explain the pulse height distributions obtained or the measured counting efficiencies. Rhodes/) only refers to an unpublished communication 6) which states that the decay energy was deposited in two energy groups of 13 and 40 keV. In one of these report¢) the distribution was erroneously attributed to 28 and 35 keV X-rays and gamma rays being stopped in the liquid scintillation solution. In this work the decay scheme of 1251 is detailed to show the various pathways of decay, the energy released, and the probability of each pathway. The measured pulse height spectrum is related to the decay scheme. The counting efficiency of 1251 w a s measured to be 76% in an emulsion type liquid scintillator solution. 2. Experimental 2.1. SAMPLE PREPARATION The samples were prepared from a calibrated (_+2.3%) source (Radiochemical Centre, Ltd., Amersham, England) which consisted of an aqueous solution of Nal containing Na1251. This source was also calibrated by the X-ray-X-ray coincidence technique7-9). The aqueous samples were emulsified into a liquid scintillator solution (Beckman Ready-Solv VI) which contained an emulsifier (Beckman BBS-3) at 16% v/v

in toluene and the fluors butyl-PBD (8g/l) and PBBO (0.5 g/l). 2.2. COUNTING EQUIPMENT All counting was performed on one of two commercial coincidence liquid scintillation counters (Beckman LS-250 or Beckman LS-100C). Both of these counters convert the summed coincident pulses from each multiplier phototube into a pulse which is proportional to the logarithm of the summed pulses. Thus the measured pulses are proportional to the logarithm of the energy of the exciting electrons1°). The pulse height spectra were measured with a multi-channel analyzer (Nuclear Data Model l l00). The log-summed pulses were directed into an amplifier (Nuclear Data ND-532ARC) without modification. The coincidence determining pulses of the liquid scintillation counter were shaped (by stretching to 3/~s duration) to satisfy the input requirements of the multichannel analyzer and then used as a coincidence gating pulse for the analyzed pulses 11). 3. Theory 3.1. DECAY SCHEME The decay s c h e m e 12) is shown in fig. 1 along with some additional data on the modes of decay. Other data that are used in determining the energy released by the various modes of decay are: Binding energy of Te K electron 31.8 keV, Binding energy of Te L electron 4.8 keV, Energy of Te K X-ray 27.5 keV, K fluorescence yield, o~K 0.86, Auger yield, ~OAu 0.14. The fluorescence yield, ~K, is the probability that a vacancy in the K-shell will be filled followed by the

294

DONALD

L. H O R R O C K S

emission of a K X-ray. The remaining fraction of the time the energy is released by the production of one or more monoenergetic electrons called Auger electrons. The total probability is of course unity: EOK'I- (DAu =

TABLE 1 Energy deposited in liquid scintillator solution from 1251 decay. Following Following K capture L capture (keV) (keV)

1.00.

3.2. PATHWAYSOF DECAY First, it was assumed that no Te K X-rays (27.5 keV) or 35.5 keV g a m m a rays were stopped in 15 ml of the liquid scintillator solution and that all lower energy X-rays were completely stopped in the solution. (Later, evidence will be presented on the fraction of Te K X-rays that are absorbed.) Fig. 2 shows the various pathways of 1251 decay, the probability of each pathway and the total energy deposited in the liquid scintillator solution by each pathway. The decay is divided into two parts for simplifying the discussion of the pathways: (a) the electron capture and (b) decay of the 35.5 keV excited nuclear energy level. The electron capture process involves the capture of an extranuclear electron by the nucleus resulting in the production of a vacancy in the captured electron's shell. This vacancy will be filled by re-arrangement of the orbital (extranuclear) electrons leading to the emission of characteristic X-rays and/or one or more monoenergetic electrons called Auger electrons. The electron capture process involves capture of a K-shell electron in 80% of the decays and an L-shell electron in the remaining 20%. Since the Te L X-ray is only 3.5 keV in energy it is totally stopped in the liquid scintillator solution. The Te K X-ray is 27.5 keV in energy and has a high probability of escape from the solution (the actual probability is 0.92 in 15 ml of toluene as will be derived later). The decay of the 35.5 keV excited nuclear energy 125I (60d)

E.C.

(100%)

Total energy per decay One K X-ray escape/decay T w o K X-rays escape/decay ~/-ray escape per decay O n e K X-ray & y-ray escape/decay

67.3 39.8 12.3 31.8 4.3

40.3 12.8 a 4.8 a

a N o t possible.

level is somewhat more complicated. The nucleus can undergo two modes of decays; (a) emission of a 35.5 keV g a m m a ray which actually occurs in only 7% of the decays and (b) internal conversion where the nucleus gives its excess energy to an extranuclear electron (no g a m m a ray is emitted) ejecting the electron with an energy equal to the difference between 35.5 and the binding energy of that electron. The resulting vacancy from the ejected electron is filled again either by emission of a characteristic X-ray and/or one or more Auger electrons; the same process as that following electron capture. The pathways shown in fig. 2 show all the possible combinations of the various modes of decay of the 125[ nucleus. As a means of explanation, one possible pathway is shown in fig. 3 which will be explained in detail. First the electron capture occurs by capture of a K-shell electron in 80% of the decays. This vacancy is filled 14% of the time when the binding energy is released by the emission of one or more Auger electrons. This is followed by 93% of the 35.5 keV excited '25Te atoms undergoing internal conversions. Of these 87.5% occur by emission of a K-shell electron. This K-shell vacancy is filled by emission of a Te K X-ray 86% of the time. The total probability of this pathway is given by the product of each of the probabilities: Probability = (0.80) (0.14) (0.93) (0.875) (0.86) -- 0.078. Of the total decays, 7.8% follow this particular pathway. Each of the other pathways are explained by the same logic.

0 125Te E.C. (L + M + . . . ) / E . C . (K) = 0.254 0.0355

e/~ = 12.6

Fig. 1. Decay scheme for 12~I.

3.3. ENERGY RELEASE The total energy released per decay of '251 will be determined by the shell in which the initial electron capture occurs plus the 35.5 keV from the excited

M E A S U R E M E N T

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PROBABILITY

ENERGY, KeV

0.078

39.3

(0.80) (0.14) (0.93)(0.875)(0.86) K K-~ (0.875) X-RAY~0.86)

Fig. 3. One pathway for 1251 decay showing the method of obtaining probability and energy release to liquid scintillator solution. TABLE 2 Energy groups of electrons produced during decay of 125I. Probabilities assuming no Te K X-rays stopped in LS solution

Energy (keV)

Probabilities assuming 8% of 27.5 keV Te K X-rays and 8% of 35.5 keV gamma rays are stopped in 15 ml of LS solution

4.3- 4.8 12.3-12.8

0.062"~ 0.622} 0.684

0,054"~ 0.537} 0.591

318

0.008 /

0.Oil /

39.8-40.3 67.3

0.282~ 0.316 0.026}

0.349~ 0.409 0.049 /

TABLE 3 Effect of amount of water on the 1251 counting efficiency and Compton edge quench monitor.

Na1251

0.05 0.05 0.05 0.05 0.05 0.05 0.05

Volume a (ml) Water Liquid scintillator solution

0 0.5 1.0 1.5 2.0 2.5 3.0

15 14.5 14 13.5 13 12.5 12

Counting Compton efficiency edge (%)

76.8 76.0 75.2 76.0 75.6 76.3 75.5

185 183.5 183 182.5 182 184 183.5

deposited per decay will be the total energy less the energy which escapes. Table l shows the various energy depositions for various combinations of decay. These decay pathways can be divided into several groups of approximately equal energy. The probabilities of each of these groups (assuming no absorption of 27.5 keV X-rays or 35.5 keV gamma rays) are obtained from fig. 2. Table 2 summarizes these groups. Further grouping into two main groups is also indicated in table 2.

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(b) 3 H (TRITIATEDWATER)

/

a Total volume maintained at 15.05 ml. Z

125Te atom. The electron capture occurs by of a K electron 80% of the time and an L 20% of the time. The energy released after the capture will be 31.8 keV for K capture and for L capture. The total energy release is thus:

(a) 1251 (AQUEOUSSOLUTION OF Nail

capture electron electron 4.8 keV

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(a)

2O

/

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Following K capture Following L capture

31.8+35.5 = 67.3 keV, 4.8+ 35.5 = 40.3 keV.

Assuming only the 27.5 keV Te K X-rays and the 35.5 keV g a m m a rays are energetic enough to escape from the 15 ml of scintillator solution, the energy

(b)

'\

\ 50 100 RELATIVE PULSE HEIGHT

150

Fig. 4. Pulse height spectra of 1251 and aH in an emulsion type liquid scintillator solution.

MEASUREMENT TABLE

297

OF t251

4

Value o f ratio as function o f fraction o f Te K X-rays and 35.5 keV g a m m a rays stopped in 15 ml o f toluene scintillator solution. Fraction o f X-rays and ),-rays absorbed

Fraction o f events which deposit a m o u n t s o f energy equal to: ~< 12.8 keV > 12.8 keV

0 0.07 0.08 0.09 0.10 0.13 0.15

0.683 0.606 0.591 0.581 0.568 0.544 0.521

4. Results 4.1. t 251 PULSE HEIGHT SPECTRUM A typical pulse height spectrum of a 125I containing liquid scintillator solution is shown in fig. 4 compared to the 3H pulse spectrum. The 1251 pulse spectrum is further compared to the l ° g C d - l ° 9 m A g pulse spectrum for energy calibration in fig. 5. The counting efficiency obtained with this calibrated source was (76_+2)%. The counting efficiency did not change for changes in water content of the solution from 0.05 to 3.0 ml while keeping the total volume of scintillator solution and sample at 15 ml. The measured counting efficiencies at different water content are given in table 3 along with the quench monitor value (Compton edge of 137Cs-137mBa source) with total volume maintained at 15 ml. 4.2. X - R A Y ABSORPTION In liquid scintillation counting the lower energy electrons are often not detectable because they produce too few photons to be measured TM 14). The probability of measuring the higher energy electrons is 1.00. Thus the reason for counting efficiency of less than 100 % for 125I decays is due to non-detection of electrons in the low energy groups, 4.3-12.8 keV. The ratio of counts in the two parts of the pulse height spectrum should be given by:

(fraction > 12.8 keV) Ratio = (fraction < 12.8 keV) - 0.24

0.317 0.394 0.409 0.419 0.432 0.456 0.479

0.707 1.077 1.165 1.229 1.311 1.500 1.701

spectrum for ten different samples was between 1.132 and 1.153. The difference between the expected value of the ratio (assuming no X-ray or g a m m a rays are stopped in the toluene solution) and the experimental value of the ratio is due to the fraction 27.5 keV X-rays and 35.5 keV g a m m a rays which are stopped in the scintillator solution by scattering or the photoelectric effect. This process in effect converts events which were assumed to deposit less than 12.8 keV energy into events which deposited more than 12.8 keV energy since the extra absorbed energy is coincident with the lower energy event. Table 4 lists the calculated ratios for assumed different fractions of the 27.5 keV X-rays % x

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(fraction > 12.8 keV) (fraction < 12.8 keV) - 0.24 0.316

0.684 - 0.240

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where 0.24 is the fraction of undetected events (i.e., 76% counting efficiency). The experimental ratio obtained from the two parts of the pulse height

1O0

150

RELATIVEPULSEHEIGHT Fig. 5. Pulse height spectra of 1251and l°gCd-l°gmAg in an emulsion type liquid scintillator solution,

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MEASUREMENT

and 35.5 keV g a m m a rays being absorbed in the liquid scintillator solution. From this data it can be seen that the absorption of 8% of the X-rays and g a m m a rays gives the experimentally obtained value of the ratio. The g a m m a ray attenuation equation is:

1/Io

giving It = 0.080 cm2/mg. This value of the attenuation coefficient can be used to predict the absorption in different volumes of toluene solutions containing ~25].

= e -"'a,

where I is the intensity of the transmitted radiation, Io is the intensity of the incident radiation, p is the mass absorption coefficient (cruZ/rag), p is the density of the absorbing medium (mg/cm3), and d is the thickness of the medium (cm). it is not necessary to consider those X- or gamma rays which are stopped in the walls of the glass vial since these have been essentially carried out of the liquid scintillator solution and will not produce scintillation events. From the experimental data:

4.3. PATHWAYS OF 125I DECAYS ASSUMING 8 % GAMMA CONTRIBUTION

Fig. 6 shows the various pathways of decay, probabilities of each pathway and energies deposited in the liquid scintillation solution when 8% of the 27.5 keV X-rays and 35.5 keV gamma rays are stopped in the 15 ml of the liquid scintillator solution. The main groups and the fraction of decays are given in table 2. The value of the calculated ratio is given by:

where p is the density of toluene, 0.867 mg/cm 3, and the average thickness of toluene, d, is the inside radius of the counting vial, 1.2 cm. The equation reduces to: 0.083 = p(0.867) (1.2),

-

1.165,

0.591 - 0.24

4.4. QUENCHING EFFECTS 4.4.1. W a t e r The amount of water in the counting sample was

CH3NO 2

80

0.409

R-

which is essentially the same as the average of the experimentally found ratio of I. 143.

0.92 = e - ' p a ,

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PULSE HEIGHT OF COMPTON EDGE

Fig. 7. Effect o f increasing a m o u n t o f a chemical type quench n i t r o m e t h a n e (CH,~NO2) on the 1251 counting efficiency as a function o f pulse height o f C o m p t o n edge o f 137Cs-137mBa external source for an emulsion type liquid scintillator solution.

1;0

125

150

175

200

PULSE HEIGHT OF COMPTON EDGE

Fig. 8. Effects o f increasing a m o u n t s o f two color type quenches, methyl-orange (yellow) a n d Buffalo blue-black, on the re51 counting efficiency as a function o f pulse height o f C o m p t o n edge o f 137Cs-137mBa external source for an emulsion type liquid scintillator solution,

300

DONALD L. HORROCKS

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Efficiency (%)

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VOLUME OF

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25

50 75 100 RELATIVE PULSE HEIGHT

0

25

50 75 100 RELATIVE PULSE HEIGHT

Fig. 9. Change in pulse height spectrum of 125I in an emulsion type liquid scintillator solution (1 ml of aqueous 1'251 solution) with different amounts of CH3NO,,.

Fig. 10. Change in pulse height spectrum of 1251 in an emulsion type liquid scintillator solution (I ml of aqueous 1'25I solution) with different amounts of Buffalo blue-black.

varied from 0.05 to 3.05 ml, keeping the total volume of the sample and scintillator solution constant at 15.05 ml, with no change in the '25I counting efficiency, table 3. The quench level was also monitored by measure of the pulse height of the Compton edge of the 662 keV gamma rays from a 137Cs-137mBa source'5). The Compton edge pulse height value was essentially constant over the total variation of the water content.

was a very dark blue color. A color quench agent acts as an absorber of the photons after they have been emitted from the scintillation solutes. Thus the scintillation response is reduced by decreasing the number of photons which escape from the scintillator solution. Fig. 8 shows the relationship between the Compton edge pulse height and the 125I counting efficiency for increasing amounts of the two color quenchers. The concentration of color quencher was such that the total volume added to give maximum quench caused an insignificant change in total volume. There was no apparent difference in the effect of the two colors as monitored by the Compton edge pulse height.

4.4.2. Chemical quench ( C H 3 N O 2 ) Nitromethane ( C H 3 N O 2 ) w a s used as a quenching agent because it is soluble in the toluene part of the emulsion system where it can complete with the energy transfer processl6). Fig. 7 shows the counting efficiency of '251 as related to the quench level as monitored by the Compton edge pulse height. The two different tests show the reproducibility of the quench curve. 4.4.3. Color quench Two color agents were used to study the effects of color on the '251 counting efficiency. Methyl-orange is a yellow material whereas the Buffalo blue-black

4.5. QUENCHED PULSE HEIGHT SPECTRUM Fig. 9 and 10 show the effect of chemical (CH3NO2) and color (Buffalo blue-black) quench on the pulse height spectrum of 1251 in the emulsion scintillator solution. Increasing amounts of quench caused a decrease in the response and a shift of the spectrum to lower pulse heights. At the highest quench only the most energetic events (40 keV) are measured,

MEASUREMENT OF 1251

5. Conclusions F r o m the t h e o r e t i c a l a n d e x p e r i m e n t a l d a t a p r e s e n t e d here, the i n t e r p r e t a t i o n o f t h e liquid s c i n t i l l a t i o n c o u n t i n g o f ~2Sl has been m a d e , T h e e x c i t a t i o n s are m a i n l y due to A u g e r a n d c o n v e r s i o n e l e c t r o n s p r o d u c e d d u r i n g the d e c a y p r o c e s s e s o f 125I. O n l y a m i n o r c o n t r i b u t i o n ( ~ 8 % ) is the result o f X - r a y s o r g a m m a rays p r o d u c e d d u r i n g the d e c a y p r o c e s s o f '251. T h e c o u n t i n g efficiency o f an u n q u e n c h e d s o l u t i o n ( 7 6 % ) is essentially the e x p e c t e d efficiency.

References 1) R. E. Yerick and H. H. Ross, Proc. Oak Ridge Isotope Conf. Gatlinburg, Tenn., USA (April 1965) p. 20. 2) B. A. Rhodes, Anal. Chem. 37 (1965) 995. 3) H. F. Polesky and D. Seligson, Anal. Biochem. 10 (1965) 347.

301

4) E. D. Bransome, Jr., and S. E. Sharpe 1I!, Anal. Biochem. 49 (1972) 343. 5) j. Ashcroft, Anal. Biochem. 37 (1970) 268. 6) R. Cavanaugh, private communications, quoted in ref. 2. 7) j. S. Eldridge and P. Crowther, Nucleonics 22 (6) (1964) 56. s) D. L. Horrocks and P. R. Klein, Nucl. Instr. and Meth. 124 (1975) 585. 9) D. L. Horrocks, Nucl. Instr. and Meth. 125 (1975) 105. 10) D. L. Horrocks, Int. J. Appl. Radiat. Isotopes 24 (1973) 49. 11) D. L. Horrocks, Nucl. Instr. and Meth. 117 (1974) 589. 12) C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes, (6th ed; J. Wiley, New York, 1968). lz) D. L. Horrocks and M. H. Studier, Anal. Chem. 33 (1961) 615. 14) D. L. Horrocks, in The current status o f liquid scintillation counting (ed. E.D. Bransome, Jr.; Grune and Stratton, New York, 1970) pp. 25-42. 15) D. L. Horrocks, Nature 202 (1964) 78. 16) D.L. Horrocks, Applications o f liquid scintillation counting (Academic Press, New York, 1974).