Use of 14 MeV neutrons for activation analysis of five elements in man-like models

NUCLEAR

INSTRUMENTS

AND METHODS

92 (I97I) 6oi-6o7; © N O R T H - H O L L A N D

PUBLISHING

CO.

USE OF 14 MeV N E U T R O N S F O R A C T I V A T I O N ANALYSIS OF FIVE E L E M E N T S IN M A N - L I K E M O D E L S t C. K. B A T T Y E

Department of Medical Physics, King's College Hospital, London SE 5, England V. K N I G H T , T. O. M A R S H A L L , A. K N I G H T a n d B. E. G O D F R E Y

Radiological Protection Service, Clifton Avenue, Belmont, Sutton, Surrey, England

M e t h o d s using n e u t r o n activation analysis for the d e t e r m i n a t i o n o f the whole-body c o n t e n t o f an element in a living subject are n o w well established. T h e preliminary experiments described here relate to a m e t h o d of analysis which allows 5 body elements to be estimated simultaneously. 14 M e V n e u t r o n s p r o d u c e d by a low energy accelerator using the 3H(d,n)4He reaction, were used to activate a n u m b e r o f man-like models. T h e )'-ray spectra of activity f r o m the m o d e l s were then recorded a n d c o m p a r e d with those f r o m similar activated models each containing only one of the elements of interest. T h e irradiation was arranged to p r o d u c e

the same effect as that which would have been produced by an a r r a n g e m e n t suitable for irradiating h u m a n s , i.e. one which produces u n i f o r m fluence o f slow n e u t r o n s t h r o u g h o u t the body by m o d e r a t i o n of the incident 14 M e V neutrons. A m e t h o d of analysing the spectra is described which takes account o f the activity produced by the m a n y interfering reactions which occur between the fast n e u t r o n s a n d body elements. Results obtained by activation analysis of man-like models are c o m p a r e d with the k n o w n contents of the models and s h o w n to be within 18% of the correct values in every case.

1. Introduction

occurring with the unmoderated neutrons when using a 14 MeV neutron source. The use of whole-body neutron activation analysis to assess changes in the whole-body content of an element in a given patient requires only that the method should give repeatable results which are proportional to the whole-body content in that patient. Extreme uniformity of neutron fluence is not necessary if the element to be determined has a constant or uniform distribution, though the pattern of neutron fluence in the patient should be reproducible. In these circumstances incident neutrons with energies of a few MeV may be used for the irradiation and will produce a greater activity per unit dose-equivalent to the subject than will 14 MeV neutrons. Chamberlain et a l ) ) used neutrons of 3 MeV mean energy to measure wholebody calcium relative to whole-body sodium and also to measure changes in calcium content in patients following treatment [-Chamberlain and Peters4)]. If, however, studies are to be carried out on groups of subjects and comparisons are to be made between individuals or if the elements under investigation have not a fixed or uniform distribution within the subject, then the method must produce a similar, highly uniform neutron fluence in each subject. In these circumstances, it is advantageous to use a primary source of neutrons of 5 MeV or above [Newton et al.7)], in which case the actual mass of the whole-body content of an element can be obtained by comparison with a model or p h a n t o m of known content. Previous workers have estimated the accuracy of

The use of neutron activation analysis to determine the whole-body content of various elements in live men has been reported or discussed by a number of work,zrs 1-8). The analyses have usually been based on (n,y) reactions produced by slow neutrons* in the elements under investigation since the products of these reactions contribute the major part of the resuiting y-ray activity which is measured to determine the whole body contents. To obtain a sufficiently uniform distribution of slow neutron fluence throughout the human body, primary sources of fast neutrons have been used together with moderating material surrounding the subject. Roughly half of the irradiation has been delivered to the front of the subject and the remainder to the back (bi-lateral irradiation). Anderson et al. 1) irradiated two living subjects with 14 MeV neutrons. A man-like model containing a known amount of sodium chloride was similarly irradiated and the resulting activity compared with that from the live subjects to estimate their sodium and chlorine contents. The possibility of determining calcium by a similar method was also demonstrated. These methods have since been discussed by Battye et al. 2) with particular reference to the uniformity of the fluence of slow neutrons produced throughout the body and the effects of interfering (n,~), (n,p) and (n,2n) reactions t Proofread by the Publisher. * T h e term "slow n e u t r o n s " as used here m e a n s t h o s e h a v i n g energies up to a few tens o f electron volts, i.e. those in the energy range in which the (n,)') cross sections are significant.

601 VI. A P P L I C A T I O N S

TO M E D I C I N E

AND BIOLOGY

602

c. K, BATTYE et al.

their m e a s u r e m e n t s of single elements, but so far, no r e p o r t has been published which c o m p a r e s values for several elements, m e a s u r e d simultaneously, with the k n o w n contents in individual cadavers or man-like models. We therefore carried out this e x p e r i m e n t using m o d e l s a n d a p r i m a r y source o f 14 M e V neutrons. The analysis was p e r f o r m e d to d e t e r m i n e simultaneously the w h o l e - b o d y contents o f s o d i u m , p o t a s s i u m , phosphorus, chlorine a n d calcium, all o f which could p r o v i d e useful i n f o r m a t i o n to the clinician. Particular attention was given to the effects of interfering reactions, the statistical errors in the m e a s u r e m e n t of the 7-ray activity, a n d the errors resulting f r o m the m e t h o d o f analysis o f the 7-ray spectrum. It was decided n o t to investigate the effects of size a n d shape o f the m a n like m o d e l a n d one m o d e l with various contents was used t h r o u g h o u t the experiment.

2. Method A m o d e l simulating a m a n was s u r r o u n d e d by m o d e r a t i n g material a n d i r r a d i a t e d so as to deliver a total r a d i a t i o n dose-equivalent, a v e r a g e d t h r o u g h o u t the model, of a b o u t 1 rein. This value was selected as a dose-equivalent which, it was t h o u g h t , might be acceptable if this technique were a p p l i e d to humans. The s p e c t r u m o f 7-ray activity induced in the m o d e l was recorded, and analysed by c o m p a r i s o n with 6 s t a n d a r d spectra each o b t a i n e d by i r r a d i a t i n g the same m o d e l c o n t a i n i n g only one o f the b o d y elements, as shown in table 1. The e x p e r i m e n t was carried out 4 times with m a n - l i k e models with the c o m p o s i t i o n s also shown in table 1. By using different c o m p o s i t i o n s a check was m a d e on the m e t h o d o f allowing for interfering

reactions when these occur in varying p r o p o r t i o n s . 2.1. DESCRIPTION OF THE MAN-LIKE MODEL The m o d e l man, the c o n s t r u c t i o n of which was based on d a t a p u b l i s h e d by Bush9), consisted o f a n u m b e r o f polyethylene containers of elliptical or circular cross section representing the trunk, limbs etc., t h r o u g h which polyethylene tubes h a d been fitted, parallel to the long axis o f the body, to contain solid or p o w d e r e d m a t e r i a l representing bone. The total volume of the tubes in each c o n t a i n e r was based on published d a t a on the distribution by weight o f bone in the b o d y [Spiers and Butch ~o); Trotter, B o w m a n and Peterson I ~)]. The positions o f the tubes in each c o n t a i n e r were based on the a n a t o m i c a l d i s t r i b u t i o n of bone in man, subject to the restriction that the a r r a n g e m e n t in each vessei was symmetrical a b o u t the c o r o n a l plane t h r o u g h its axis (see fig. 1). To simulate the body c o m p o s i t i o n o f m a n , the containers were filled with an a q u e o u s solution o f salts representing soft tissue a n d the tubes were filled with a p o w d e r consisting o f a mixture of high purity c h a l k and calcium h y d r o g e n o r t h o - p h o s p h a t e , representing bone (see table 1). 2.2. METHOD OF IRRADIATION 2.2.1. Irradiation geometry It has been shown [ B a t t y e et al.2)], t h a t if the total thickness o f subject a n d m o d e r a t o r is the same along the whole length of the subject a highly u n i f o r m fluence o f slow neutrons can be o b t a i n e d t h o u g h o u t subjects o f all but the largest a n t e r i o r - p o s t e r i o r d i m e n s i o n by giving 2 similar i r r a d i a t i o n s f r o m front a n d back. S o m e w h a t artificial conditions were used in these

TABLE 1

Composition of man-like models and standards (g). Compound

Sodium chloride; NaCI Sodium nitrate; NaNOa Potassium nitrate; KNOa Magnesium sulphate; MgSO4, 7H20 Ammonium sulphate; (NH4)2SO4 Hydrochloric acid; HCI Sucrose; C12Hz2Oll Calcium carbonate; CaCO3 Calcium hydrogen ortho-phosphate; CaHPO4 Sodium carbonate; Na2CO3 Potassium carbonate; K2CO3 Magnesium acetate; (CHa.COO)o_Mg, 4H20 Ammonium di-hydrogen ortho-phosphate; NH4H2PO4 Polyethylene; (CH2)n (in kg) Water; HzO (in kg)

Man-like model A B CandD 173 147 147 137 116 116 363 308 308 355 301 301 532 451 451 4.6 27 885 439 439 999 2496 2 4 9 6 2200 1092 1 0 9 2 . . . . . . . . . . . . . . 12.9 1 2 . 9 12.9 52.7 52.7 52.7

Cl

Standard Mg K

Na

. . . . . 997 .

.

. . .

.

. .

. .

. . .

.

. . . .

. . . . . . . 2037 . .

. . .

15.5

52.7

. . . .

.

Ca

.

. .

. .

P

. . .

. .

. . 3016

.

. 1502 1686 -

. .

. 2748 15.5 15.5 15.5 12.9 52.7 52.7 52.7 52.7

12.9 52.7

A C T I V A T I O N ANALYSIS OF FIVE ELEMENTS IN M A N - L I K E MODELS

603

of the work over a period of several weeks the neutron yield fell from an initial value of approximately 1.4x 101° neutrons/sec to a value of approximately 4 x 10 9 neutrons/sec. All irradiations were carried out over a period of exactly 15 min keeping the neutron output from the generator as steady as possible.

Thigh~

Head@

Lurn~

Arm

Fig. 1. Cross sections of the parts of the man-like model illustrating the positions of the polyethylene tubes.

experiments which would not be suitable in the case of live men: the effect, however, was to produce the same activity as would have been produced by any method giving the same uniform slow neutron fluence throughout the subject. During the irradiations the parts of the model were encased within paraffin wax. Rectangular blocks were moulded to fit the polyethylene containers, the thickness of the moderator being nowhere less than 3.5 cm. The total anterior-posterior thickness of the blocks was 32 cm. The blocks were arranged as in fig. 2 so that the front faces lay along an arc of a circle centred on the source and of 1 m radius, the gaps between the blocks being filled with paraffin-wax wedges. The various parts of the man-like model were arranged to lie symmetrically about the plane parallel to the front face and halfway between the front and back of the blocks. Thus, all the material in the model was distributed so that the total activity produced by a single irradiation was the same as that which would be produced by irradiations from front and back. It was, therefore, not necessary to turn the model halfway through the irradiation as would be necessary in the case of a man. 2.2.2. 14 M e V neutron source The source of neutrons for the experiment consisted of a 150 kV deuteron accelerator supplied by a 150 kV electrostatic generator, and produced 14 MeV neutrons by the aH(d,n)4He reaction. During the course

2.2.3. The fluence and radiation dose-equivalent Those models filled to simulate the body composition of a man all received dose-equivalents of 14 MeV neutrons of approximately 2.2 rem at the front face of the moderating block, since experiments with nuclear emulsions [Marshall and Knight12)] showed that these conditions gave rise to a mean doseequivalent throughout the chest section of the model which was approximately 1 rem. It was also shown that if the same irradiation had been given in 2 equal halves from front and back, then the dose-equivalent would have been substantially constant throughout the model with a value of approximately 1 rein. Those models used for producing standard spectra of irradiated elements were given dose-equivalents several times those of the man-like models, the values depending on the maximum output available from the neutron generator at the time. The 14 MeV neutron yield, and hence the neutron

Coun Protc Reco

-*-)~ Tar

Fig. 2. Plan showing the arrangement of man-like model, moderator and detectors in relation to the neutron generator target. VI. A P P L I C A T I O N S TO M E D I C I N E AND BIOLOGY

604

c.K. BATTYE et al.

fluence at the front face of the wax moderator, was measured by means of a simplified proton-recoil telescope [Bame et al.'3)] at a fixed distance from the target. The neutron dose-equivalent at the front surface of the moderator was calculated from the incident neutron fluence, on the basis of 7× 10 -8 rem neutron- 1 cm 2 [Auxier et al.'*)]. A check on the doseequivalent at the front face of the moderator was made by attaching a nuclear emulsion plate to the front face, adjacent to the thorax, for the duration of the irradiation. The slow neutron fluence at the centre df the thorax section was measured by a small boron trifluoride (BF3) proportional counter introduced into a tube sealed into this section. The readings of the BF3 counter were proportional to those of the protonrecoil telescope for all irradiations except that of the model used for obtaining the standard spectrum for chlorine, when the reading of the BF 3 counter was approximately 20% less than expected. This was attributed to the absorption of slow neutrons by the large chlorine content of the model [10 x that of the I C R P standard man15)] due to the reaction 35Cl(n,y)36C1 which does not lead to a y-emitting product but which has a high reaction cross section of 30 b. As it was necessary to use this chlorine content to obtain sufficient activity from 38C1 the resulting flux depression was allowed for in the analysis of the y-ray spectra (see table 2). TABLE 2

Physical characteristics of the principal radionuclides of interest. Element

Reaction

H a l f life o f induced radionuclide

Na

23Na(n,y)24Na

15.0 h

CI

'~vCI(n,v)3sCI

37.3 m

37Cl(n,p)avS 3~Cl(n,2n)Z4Cl

5.0 m 32.4 m

z~CI(n,v)a~CI

Ca

natural 4°K 41K(n,~)4ZK a.~K(n,2n)aSK 48Ca(n,7)49Ca

3.0 × 105Y 1.3 × 1 0 9 y 12.5 h 7.7 m 8.7 m

P Mg

alP(n,ct)2SA1 24Mg(n,p)Z4Mg

2.3 m 15.0 h

N

14N(n,2n)laN

10 m

K

Energy of gamma-ray emittedbyproduct (MeV) 2.75 1.37 2.16 1.60 3.10 2.13 1.16 no v-ray 1.46 1.52 2.16 3.09 4.05 1.78 2.75 1.37 0.51

2.3. MEASUREMENTS OF GAMMA-RAY EMISSION INDUCED IN THE MODELS

The whole-body counter used for measurements of y-activity in the models comprised an array of scintillation counters installed in an underground room excavated from virgin sub-soil chalk [International Atomic Energy Agency16)]. Eight thallium-activated sodium iodide crystals each 10 cm diameter and 5 cm thick were arranged so that 4 were above and 4 below the supine subject. After irradiation, each model was transported to the whole-body counter and reassembled on a jig which was used to minimise errors in positioning. The y-ray measurements were started 5 rain after the irradiation was finished and continued for six 10 rain counting periods separated by 1.2 rain intervals, during which the data were recorded. The counts were accumulated in real time and a correction was made for the dead time where this was significant. Representative y-ray spectra from a man-like model are shown in fig. 3 and from sodium and chlorine standards in fig. 4. 2.4. ANALYSIS OF DATA The analysis was carried out in 2 stages; firstly the spectrum accumulated during the last 20 rain of counting was used to estimate the sodium, potassium and chlorine contents of the models, that is, those elements giving rise to activity with a relatively long half life. The proportions of the standard spectra which when summed gave the best fit (in a least squares sense) to the spectrum being analysed, were found by the method of Bentley'7). The analysis was limited to y-rays with energies between 1.0 and 4.0 MeV in order to exclude as much as possible of the activity from nuclides of elements for wich the spectra were not being analysed, such as the annihilation radiation at 0.51 MeV due to the decay of 13N and other positron emitting radionuclides produced by reactions with fast neutrons. In the second stage of the analysis, the activity produced by the irradiation of calcium and phosphorus was estimated by total energy peak analysis. For the calcium estimation the 3.1 MeV total energy peak from the spectrum accumulated during the first 20 min of counting was selected and that path of the activity which was calculated to have been due to sodium and chlorine was subtracted. The remaining activity was then compared with that from the calcium standard in the same energy region (2.95-3.35 MeV). In the case of the phosphorus estimation the spectrum used was that accumulated in the first 10 min of counting and the energy range selected, 1.45-2.2 MeV, was that

ACTIVATION

K-44

ANALYSIS

II

A1-28

OF FIVE ELEMENTS

IN MAN-LIKE

MODELS

605

]

I00.

_~

0

co (7, 300I00

z 0 O

O~ '

5'5

2,5 ENERGY

GAMMA - RAY

3

3.5"

4

M~tV

F i g . 3. Upper: The 7-ray spectrum for the man-like model A obtained from the first l0 min count. Lower: The corresponding

spectrum for the last 30 min of counting.

containing the 28A1 total energy peak at 1.79 MeV. In this case activity estimated to arise from the irradiation of sodium, chlorine, potassium and calcium was subtracted before the remainder was compared with the standard spectrum of irradiated phosphorus. Before the spectra were analysed, each was corrected to compensate for any gain shift in the pulse height analyser (normally less than 1%), accurate background spectra were subtracted and the residual spectra normalised to a given slow neutron fluence. The standard

spectrum of irradiated potassium was corrected to allow for differences in the ratio of activity from naturally occurring 4°K and induced 42K when different neutron fluences were used. After analysis the interference due to the average magnesium content of the adult human body [[CRP15) -] was estimated from the activity produced in the magnesium standard and this was subtracted from the results for sodium. Although it would have been possible to allow for the exact amount of magnesium in each model in these

Nai24

I00 .J

z<

112 >

0

Q-

~ soo.

38 CL-38

z 0 U

o.

" 1.5

L_ 2

GAMMA - RAY

......

2.S £NEIqGY

..... ....... 3

3.5

4

Iv~v

F i g . 4. Upper: The ;e-ray spectrum of the irradiated magnesium acetate standard, recorded during the last 30 min of counting.

Lower: The y-ray spectrum of the irradiated hydrochloric acid standard recorded during the first 10 min of counting. VI. APPLICATIONS

TO MEDICINE

AND

BIOLOGY

606

C. K. BATTYE et al. TABLE 3 Results of analyses of man-like models compared with known contents. a

Man-like model

A

D

b Elements

c Actual mass of element (g)

d Measured mass of element (g)

e Estimated standard deviation x 100 c (+_ %)

Na K C1 Ca P

105 140 105 1047 502

109 130 110 1036 487

2.0 3.7 5. I 8.2 3.5

3.8 -7.1 4.8 - 1.0 - 3.0

Na K CI Ca P

89 119 94 1320 249

94 126 104 1256 206

2.3 4.3 5.1 7.7 4.3

5.6 5.9 10.6 - 4.8 - 17.3

Na K Cl Ca P

89 119 120 1320 249

102 ll8 117 1187 269

2.6 4.2 4.5 8.4 4.1

14.6 -0.8 -2.5 - 10.1 8.0

Na K CI Ca P

89 119 120 1320 249

98 124 104 1218 259

2.6 4.2 4.4 7.7 4.2

10.1 4.2 - 13.7 -7.7 4.0

experiments this w o u l d n o t be possible when a p p l y i n g the technique to h u m a n s where the m a g n e s i u m content w o u l d not be known. The results o f some p r e l i m i n a r y experiments are given in table 3. 3. D i s e u s s i o n

In previous experiments [ A n d e r s o n et al.l), N e w t o n et al.7)], the s t a n d a r d spectra used in the analysis were those o f the individual r a d i o n u c l i d e s p r o d u c e d . A different m e t h o d o f analysis was used in this w o r k in t h a t some allowance was m a d e for the interfering reaction p r o d u c t s [Battye et al.Z)]. F o r example, activity f r o m 37S p r o d u c e d f r o m 3VCI by a r e a c t i o n with fast n e u t r o n s (see table 2) is included in the s t a n d a r d spect r u m f r o m i r r a d i a t e d chlorine a n d will not be m i s t a k e n for 49Ca, which has a ~-ray o f similar energy. The e s t i m a t e d a m o u n t s o f the elements in the m a n like m o d e l s are all within + 18% o f the actual a m o u n t s dispensed. The deviations f r o m the actual values for p o t a s s i u m a n d calcium are within the range expected f r o m a study o f c o u n t i n g statistics (col. e o f table 3).

f d-c --x c

100

The errors o b t a i n e d in the calcium d e t e r m i n a t i o n (root m e a n square of values in col. f o f table 3 = 6.8%) suggest t h a t in 100 single d e t e r m i n a t i o n s 95 would lie within 4- 13% o f the correct value. This figure m a y be c o m p a r e d with t h a t o f + 8% [given by P a l m e r et al.5)] for a single d e t e r m i n a t i o n of the w h o l e - b o d y calcium content a n d t h a t of a p p r o x i m a t e l y _+ 5% obt a i n e d f r o m the results o f C o h n et al.8). P a l m e r a n d colleagues were using NaI(TI) detectors of a b o u t 8 times the v o l u m e o f those available to us, whilst the w h o l e - b o d y c o u n t e r used by C o h n a n d colleagues cont a i n e d 54 NaI(TI) detectors, each 15 x 3 c m - a p p r o x i m a t e l y 15 x the v o l u m e o f o u r own detectors. In the case o f s o d i u m , chlorine a n d p h o s p h o r u s , however, the errors a r e larger t h a n w o u l d be expected on a basis o f counting statistics alone. The e s t i m a t e d s o d i u m contents are, on average, 8% greater t h a n the a c t u a l a m o u n t s . This discrepancy is n o t likely to be due to activity f r o m the interfering r e a c t i o n Z4Mg(n,p)24Na, since an a p p r o x i m a t e allowance was m a d e for this in the analysis, a n d it was shown t h a t in any case the 24Na activity arising f r o m the i r r a d i a t e d m a g n e s i u m in a

A C T I V A T I O N ANALYSIS OF FIVE ELEMENTS IN M A N - L I K E MODELS

standard man amounted to only 2.8°Jo of that from the irradiated sodium. This figure confirms the value of 3.3% calculated previously [Battye et al.2)] and may be compared with that of 30% measured by Palmer et al.5). Chamberlain and colleagues 3) reported values for the ratio of 24 h exchangeable sodium to total sodium which were 17.7% lower than those quoted by Anderson et al.1). The former group estimated total sodium by neutron-activation analysis using 3 MeV incident neutrons. It might therefore be construed that the use of 14 MeV neutrons gives a value for total body sodium which is too low. The results reported here, however, do not support this contention. It is possible that the systematic errors in the measurements of sodium and chlorine were due to errors in the measurements of the neutron fluences or to the curve-fitting procedure, which was found to be very sensitive to siight changes occurring in the gain of the multi-channel anaiyser. The phosphorus cc,ntents of the models were estimated by measuring the 2SA1 produced by activation with fast neutrons (those above 2 MeV). Although a method of irradiation producing uniform slow neutron fluence in the subject does not necessarily produce a uniform fast neutron fluence, work reported by Palmer, Nelp and Williams 6) suggests that a useful uniformity of phosphorus activation may nevertheless be achieved. For this reason phosphorus was included in the analysis as one of the elements it may be possible to estimate in humans. The results of these experiments indicate that if multi-element analysis is to be carried out with errors of about 5% or less, for any of the elements considered here, more sensitive counting techniques will be required. There would seem to be no reason, however, why this accuracy should not be achieved given a satisfactory whole-body counting system and an ir-

607

radiation facility suitable for irradiating humans in such a way as to produce a uniform slow neutron fluence throughout the body. Work on the design of an irradiatiop facility is being completed at present and it is hoped to report progress shortly (CKB). References a) j. Anderson, S. B. Osborn, R. W. S. Tomlinson, D. Newton, J. Rundo, L. Salmon and J. W. Smith, Lancet 2 (1964) 1201. ~) C. K. Battye, R. W. S. Tomlinson, J. Anderson and S. B. Osborn, Naclear activation techniques in the life sciences (I.A.E.A., Vienna, 1967) p. 573. 3) M. J. Chamberlain, J. H. Fremlin, D. K. Peters and H. Philip, Brit. Med. J. 2 (1968) 581, 583. 4) M. J. Chamberlain and D. K. Peters, Paper presented at Conf. Progress and problems of IN VIVO activation analysis (Glasgow, 1969). 5) H. E. Palmer, W. B. Nelp, R. Murano and C. Rich, Phys. Med. Biol. 13 (1968) 269. 6) H. E. Palmer, W. B. Nelp and J. Williams, Paper presented at Conf. Progress and problems of IN VIVO activation analysis (Glasgow, 1969). 7) D. Newton, J. Anderson, C. K. Battye, S. B. Osborn and R. W. S. Tomlinson, Int. J. Appl. Rad. Isotopes 20 (1969) 61. s) S. H. Cohn, C. S. Dombrowski and R. G. Fairchild, Int. J. Appl. Rad. Isotopes 21 (1970) 127. •~) F. Bush, Brit. J. Radiol. 19 (1946) 14. 10) F. W. Spiers and P. J. Burch, Brit. J. Radiol. Suppl. 7 (1957) 81. 11) M. Trotter, G. E. Broman and R. R. Peterson, J. Bone Joint Surg. 42A (1960) 50. 12) T. O. Marshall and A. Knight, these Proceedings. 13) S. J. Bame, Jr., E. Haddad, J. E. Perry, Jr. and R. K. Smith, Rev. Sci. Instr. 29 (1958) 652. 14) j. A. Auxier, W. S. Snyder and T. D. Jones, Radiation dosimetry 1 (eds. R. H. Attix and W. C. Roesch; Academic Press, New York and London, 1968) p. 275. 15) ICRP 1959 report of committee 2 on permissible dose for internal radiation (Pergamon Press, Oxford, 1959) p. 146. 16) IAEA directory of whole-body radioactivity monitors (I.A.E.A., Vienna, 1964) p. 335. 17) R. E. Bentley, Radioaktive Isotope in Klinik und Forschung, V (Urban and Schwarzenburg, Mtinchen und Berlin, 1963) p. 8

VI. A P P L I C A T I O N S TO M E D I C I N E AND B I O L O G Y