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Studies relating to in vivo activation analysis using a source of 14 MeV neutrons
NUCLEAR
INSTRUMENTS
AND
METHODS
92
(I97I) 5 9 5 - 5 9 9 ;
© NORTH-HOLLAND
PUBLISHING
CO.
Part VI. Applications to medicine and biology~595-609 S T U D I E S R E L A T I N G T O IN VIVO A C T I V A T I O N ANALYSIS U S I N G A S O U R C E O F 14 MeV N E U T R O N S t T. O. M A R S H A L L a n d A. K N I G H T
Radiological Protection Service, Clifton Avenue, Bebnont, Sutton, Surrey, England It is s h o w n , in carrying o u t in vivo activation analysis with a source of m o d e r a t e d 14 M e V n e u t r o n s , that the choice of slow n e u t r o n activation detector to m e a s u r e the resultant distribution o f slow n e u t r o n fluence within the subject being analysed or in a s y s t e m designed to simulate the subject is relatively u n i m p o r t a n t . Similar results are obtained using either i n d i u m or s o d i u m activation detectors or a BF3 tube. Moreover, the presence o f b o d y cavities does n o t appreciably upset the distribution of slow
n e u t r o n fluence within the subject. However, in preparing a s t a n d a r d against which to c o m p a r e the activity produced in the subject, if the concentration of the element in question is appreciably greater t h a n that in the body, then depression of fluence m a y occur. The results of m e a s u r e m e n t s with nuclear e m u l s i o n s of the fast n e u t r o n dose equivalents t h r o u g h o u t a water-filled model of the chest are reported.
1. Introduction
others concerned with in vivo activation analysis. The studies in question concern the choice of detector to measure the distribution of slow neutron fluence through the phantom, the distribution of slow neutron fluence in phantoms containing different amounts of C1 (i.e. an element with a large slow neutron absorption cross section), and the distribution in a water-filled p h a n t o m containing voids (i.e. to simulate body cavities) has been measured. Finally, results of measurements of fast neutron dose equivalents delivered to various depths within a water-filled p h a n t o m are reported.
Several workers 1-7) have used neutron activation analysis to determine the whole body content of various elements in man. In some cases, the results have been used in the diagnosis of certain diseases and in evaluating the efficacy of treatment. Patients, volunteers, cadavers and man-like models (phantoms) have been irradiated with neutrons and analyses have been based on measurements of the resultant 7-rays with whole body counters. In the main the nuclear reactions of interest have been produced by slow neutrons* but in order to obtain a uniform distribution of slow neutron fluence through the subject, a primary source of fast neutrons has been used with a suitable moderator. The subject has been turned round approximately half-way through the irradiation (bilateral irradiation). The work reported here is related to the work of Battye et al. 7) in which man-like models containing solutions similar in composition to that of tissues in the human body, were analysed for Na, C1, Ca, K and P using 14 MeV neutrons, with paraffin wax as a moderator. The resulting y-ray spectrum of each manlike model was compared, using a whole body counter s ) with the y-ray spectra of standards (i.e. phantoms containing a known amount of each element). This paper is concerned with a number of studies which were carried out in order to demonstrate the validity of the method of Battye et al. 7) if applied to the analysis of live men, and these studies are reported here in the hope that the results will be of interest to t P r o o f r e a d by the Publisher. * T h e term "slow n e u t r o n s " , as used here, m e a n s those h a v i n g energies up to a few tens o f electron volts, i.e. those in the energy r a n g e in which the (n,F) cross sections are significant.
2. Experimental arrangement The experimental arrangement used was similar to that of Battye et al. 7) but only the chest section of the p h a n t o m was used. This section, made from polyethylene, was an elliptic cylinder with major and minor axes of 30 cm and 20 cm respectively, and was arranged with its axis vertical together with the paraffin-wax moderator to form a cuboid 56 cm wide x 31 cm thick x 40 cm high. The arrangement is shown in figs. l(a) and l(b). The experiment was carried out in a basement r o o m and the moderator and accelerator were positioned such that the target and the centre of the phantom were approximately 1 m from the floor, approximately 1 m from the ceiling and approximately 3 m from each of the side walls. The wall behind the moderator was approximately 6 m away. The upper end of the model-chest section was removed to allow the introduction of detectors into the liquid (water or HC1 solution in these experiments) it contained. Voids, consisting of perspex cylinders, could also be introduced into the liquid. The minimum thick-
595 VI. A P P L I C A T I O N S
TO MEDICINE
AND BIOLOGY
596
T. O. MARSHALL AND A. K N I G H T Paraffin
wax
moderator
BF3 counte r ~ ' - - ~ .-~,:
~
100cm
~ P r o t r~"~ec on oi I Water
I
counter
filled
chest
section
probability of being activated. This means that for a bilateral irradiation with the experimental arrangements used here, the reaction yield for a given element should be constant throughout its volume. This should be verified by experimental measurements. The reaction yield for an element of concentration C at a given point within the subject is proportional to
of p h a n t o m
a)
C
Water filled c h e s t Target
II
s e c t i o n of p h a n t o m
BF3 c
o
~
]
Proton recoil c o u n t e r fi b) Fig. 1. a) Plan view of experimental arrangement, b) Side elevation of experimental arrangement.
ness of moderator around any part of the chest section was 6 cm. The source of neutrons for these experiments consisted of a S.A.M.E.S. 150 kV deuteron accelerator which produced 14 MeV neutrons at the target by the 3H(d,n)4He reaction. Throughout this work, irradiations were monitored by means of a simplified proton recoil telescope 9) which measured the fluence of neutrons with energies greater than about 10 MeV. Detectors used for the various investigations reported here were a small boron trifluoride (BF3) tube with a sensitive volume having dimensions 0.5 cm dia. x 5 cm long, polystyrene containers (6.4 cm dia. x 2.2 cm thick) each containing 70 ml of sodium nitrate solution, indium foils 1.5 cm dia. x 0.05 cm thick, nuclear emulsions and photographic emulsions. When using the BF3 tube and when measuring the induced activities of the sodium and the indium detectors, the number of counts recorded was such that the statistical standard deviation was, in all cases, less than 1%. When using the nuclear emulsions the statistical standard deviation of counting tracks was between 4% and 15%. 3. Distribution of slow neutron fluenee
Since elements are not generally distributed uniformly throughout the body and the size and shape of human beings vary, it is desirable to ensure that each a t o m of the element under investigation has an equal
f
E oc cr(E)(o(E)dE,
E=O
where a(E) is the cross section for the reaction in question at energy E and qS(E) is the neutron fluence of energy E. The limits of the integral should, strictly speaking, extend over thetrange E = 0 to E = w. However, in practice, analyses are mainly based on reactions where a(E) is most significant in the slow neutron energy region. For there to be an equal probability of any atom of the element in question being activated the above integral must be constant throughout the subject. Since a(E) in the above integral is a characteristic of the element being analysed, the probability of activation is best measured by means of samples of this element. Another element can only be used if its variation of cross section with neutron energy is identical to that of the element being analysed or if the slow neutron part of the neutron spectrum remains substantially constant throughout the subject. In practice a slow neutron detector has usually been chosen on the basis of convenience and the significance of this has been investigated by measuring the distribution of the slow neutron fluence through a water-filled manphantom, irradiated with 14 MeV neutrons, with three detectors which had cross sections which varied differently with neutron energy. The detectors used were: 1. boron, a 1/v detector (i.e. cross section inversely proportional to neutron velocity); 2. sodium, an element often determined in in vivo activation studies, and an approximate 1Iv detector for neutrons below 100 eV; 3. indium, which has a number of large resonances in the slow neutron region. In the case of the activation detectors, sodium and indium, the 24Na activity arising from the 23Na(n,7)24Na reaction, and the 116in activity f r o m t h e 115in(n,?)116in reaction were measured. The results are shown in fig. 2(a) and it can be seen that there is little disagreement between the results for the three detectors. The results indicate that the
597
IN VIVO A C T I V A T I O N ANALYSIS
I 4O
----______%_
/
3O c
.~ 2 0 =
~: BF 3 T u b
.¢_
~ Indium :~ S o d i u m
Foils detectors
=
=~ tc
u.
I (a) Distribution front
for
14MeV
face
b) Distribution
for
hi-lateral
I from
incident
on
irradiation
I
5 Distance
neutrons
I
10 front
face
of c h e s t
15 section
0 (crn.)
Fig. 2. Distribution of slow neutron fluence through water-filled phantom.
choice of detector to measure the distribution of slow neutron fluence is relatively unimportant, which suggests that She proportion of the neutron spectrum up to at least an energy of a few 100 eV remains substantially constant throughout the phantom. Fig. 2(b) has been computed from fig. 2(a) to show the distribution which would be obtained for a bilateral irradiation. Thus a bilateral irradiation gives a substantially uniform distribution across the section and departures from the mean value are about +__4%. This figure agrees well with the value of __+3.5% obtained by Battye et al. 2) and shows that if a bilateral irradiation is used in the case of live men, the probability of a reaction occurring is substantially constant throughout the body.
The distributions of slow neutron ftuence through the p h a n t o m for different HC1 solutions were measured using the BF 3 tube. Results are shown in fig. 3. Figs. 2 and 3 show that when sufficient chlorine was added to the water-filled p h a n t o m to give the same C1 concentration as in standard man (fig. 3), the slow neutron fluence was not significantly different from that of a water-filled p h a n t o m (fig. 2). Fig. 3 shows that the slow neutron fluence fell by approximately 3% when the chlorine concentration was doubled, fell by approximately 9% when it was increased to 5 times that of the standard man, and finally, it fell by about 17% when the CI concentration was 10 times that of the standard man. Thus the presence of a C1 concentration equal to that of the I.C.R.P. standard man does not lead to significant depression of the slow neutron fluence and, therefore, does not present a difficulty if the technique is applied to live men. However, when preparing the solution for the C1 standard, care must be taken to use a concentration which does not lead to a significant depression of slow neutron fluence, or the experiment must be carried out in such a way that the effect is allowed for. 5. The effect on the distribution of slow neutron fluence of voids within the phantom
In order to observe the importance of voids, such as the lungs and other body cavities, the distribution of the slow neutron fluence was measured using the BF 3 tube, with two different void configurations introduced into the water-filled chest section of the phantom. The
30
4. Depression of slow neutron fluenee by chlorine, an element with a large slow neutron absorption cross section
20 c
One of the most important elements in m a n with regard to depression of slow neutron fluence in neutron activation analysis is chlorine, which has a total slow neutron absorption cross section of 33.8 b. However, the cross section for the 3vCI(n,y)3aC1 reaction which is used when analysing for chlorine is only 430 mb and when irradiating standards containing chlorine it is advantageous to increase the chlorine concentration to well above that of the I.C.R.P. standard man 1°) (i.e. more than 105 g of C1) in order to improve the yield of 3sC1 and hence the counting statistics.
= 10
a)
Chlorine
g
b)
2x
Chlorine concentration
in s t a n d a r d
man
c)
5x
Chlorine
in s t a n d a r d
man
d)
1Ox C h l o r i n e
~.
o
concentration
in s t a n d a r d
concentration tration
I from
in s t a n d a r d
I
5 Distance
face
man
I
10 f r ont
man
15 of
chest
section
20 (cm.)
Fig. 3. Distribution of slow neutron fluence through phantom containing different concentrations of chlorine. VI. A P P L I C A T I O N S TO MEDICINE AND BIOLOGY
598
T. O. MARSHALL
AND
which the fiuence, outside the voids, returns to approximately the value which would exist in the absence of the voids. The increase in fluence in the water adjacent to the void is greater (i.e. approximately 10% increase) for the 6.4 cm void, i.e. the larger the void, the greater departure from the fluence distribution that would exist in the absence of the void. Since actual body cavities will in general be smaller than the large void used in these experiments, it is concluded that any perturbation in fluence due to body cavities will be small and should not appreciably affect the accuracy of whole-body measurements.
30
= 20 ¢ =
. .5 t O eu c
~
Distribution
Measured
in a b s e n c e
of v o i d
A. KNIGHT
[//1
distribution
,7
6. Fast neutron dose equivalents within the phantom 0
5
Distance
trom
10
front
lace
of c h e s t
15
section
20
[cm.)
Fig. 4. Distribution of slow neutron ftuence through water-filled phantom containing a 6.4 cm diameter cylindrical void. first consisted of a water-tight perspex cylinder of internal diameter 6.4 cm and 40 cm long set with its axis vertical and positioned centrally within the phantom. The second consisted of two such vessels with internal diameters of 3 cm and positioned with their axes vertical and at distances of 6.5 cm and 14.5 cm from the front face of the phantom. Results are shown in figs. 4 and 5. It can be seen that a marked increase in the slow neutron fluence occurred within the voids. In the case of the 6.4 cm diameter void, there was a 50% increase above the value which would be present at the position of the centre of the void in its absence. In the case of the two 3 cm diameter voids the increase was approximately 20%. However, it is interesting to note the way in
'E 2O
o
10
g u.
0
31
I- -
I
Moderator
--I
~ .......
Phantom
Moderator
~- M e a s u r e d --Theoretical
g
value~ values
-o
z
o 0
Distance
I
I
10
20
from
front
surface
30 (cm.)
Fig. 6. Variation of fast neutron dose equivalent through paraffin wax moderator and water-filled phantom.
3O
E
The fast neutron dose equivalents were measured throughout the water-filled chest section of the phantom with nuclear emulsions. Proton recoil tracks in the
0
5
10
15
D i s t a n c e from front face of chest s e c t i o n
20 (cm.)
Fig. 5. Distribution of slow neutron fluencethrough water-filled phantom containing two cylindrical voids of diameter 3 cm.
emulsion were viewed through a microscope and counted. The slow neutron and 7-ray dose equivalents were measured with photographic emulsions but were found to be only a few percent of the fast neutron dose equivalents and were ignored. N o attempt was made to measure the dose equivalents due to intermediate energy neutrons but their contribution to the total neutron dose equivalent is assumed to be small. This assumption is based on studies by Marshall 11) on the attenuation of 14 MeV neutrons in water, which have indicated that intermediate energy neutrons account for' only a few percent of the total dose equivalent for water thicknesses up to 20 cm. The nuclear emulsion results are shown in fig. 6. A solid smooth line has been drawn through the
IN VlVO A C T I V A T I O N ANALYSIS
experimental points (plotted + 1 standard deviation) and the results have been normalised so that the dose equivalent at the front of the paraffin wax is 2.25 rem, i.e. the irradiation conditions used by Battye et al.7). Fig. 6 shows that this gives a mean dose equivalent through the water-filled p h a n t o m of 1 rem. If the water-filled p h a n t o m had been turned round half-way through the irradiation (bilateral irradiation) the neutron does equivalent throughout the water-filled phantom would have been practi, ally constant with a value of approximately 1 rein. The experimental results have been compared with the calculated values of Auxier, Snyder and Jones'2). These calculations were for a cylindrical homogeneous anthropomorphic phantom of 30 cm diameter irradiated with a parallel beam of neutrons. An inverse square law correction has been made to the calculated values so that they can be compared with the experimental results which were obtained using a source of neutrons 1 m from the front of the paraffin wax. The calculated values have been normalised so that the dose equivalent at the front of the cylindrical phantom is 2.25 rem and are shown as a dotted line in fig. 6. 7. Conclusions It is concluded that, in carrying out in vivo activation analysis using a source of 14 MeV neutrons, the variation of cross section with neutron energy of the detector chosen to measure the distribution of slow neutron fluence throughout water-filled phantoms is relatively unimportant, and that voids within the subject should not significantly affect the fluence in parts other than the voids. It is further concluded that in preparing standard solutions containing elements with large slow neutron absorption cross sections such as C1, care must be taken to either choose a solution strength which does not lead to a significant depression of the slow neutron fluence within the p h a n t o m or to design the experiment such that the effect may be allowed for. Finally, it has been shown that for an
599
irradiation in which a dose equivalent of 2.25 rein is delivered to the front face of the moderator and when the water-filled phantom is turned round half-way through the irradiation, the dose equivalent is substantially constant throughout the phantom having a value of about 1 rem. This work is part of a programme of work undertaken by the Radiological Protection Service which is supported by the Medical Research Council and the Department of Health and Social Security. The authors wish to thank Mr. C. L. Harvey, Mr. C. J. Bowles and Mrs. G. Flinn of the R. P. S. for their help with the experimental work. The authors are also indebted to Mr. C. K. Battye of King's College Hospital for much useful discussion. References 1) j. Anderson, S. B. Osborn, R. W. S. Tomlinson, D. Newton, J. Rundo, L. Salmon and J. W. Smith, Lancet 2 (1964) 1201. 2) C. K. Battye, R. W. S. Tomlinson, J. Anderson and S. B. Osborn, Nuclear activation techniques h2 the li]e scienees (I.A.E.A., Vienna, 1967) p. 573. a) M. J. Chamberlain, J. 1-[. Fremlin, D. K. Peters and H. Philip, Brit. Med. J. 2 (1968) 581, 583. 4) H. E. Palmer, W. B. Nelp, R. Murano and C. Rich, Phys. Med. Biol. 13 (1968) :269. ~) M. J. Chamberlain, J. H. Fremlin, D. K. Peters and H. Philip, 9th Intern. Syrup. Radioactive isotopes in clinical medicine and research, paper 4 (Bad Gstein, 1970). 6) S. H. Cohn, C. S. Dombrowski and R. G. Fairchild, Int. J. Appl. Rad. Isotopes 21 (1970) 127. 7) C. K. Battye, V. Knight, T. O. Marshall, A. Knight and B~ E. Godfrey, these Proceedings. ~) LA.E.A. directory o f whole-body radioactivity monitors (I.A.E.A., Vienna, 1964) p. 335. 9) S. J. Bame, Jr., E. Haddad, J. E. Perry, Jr. and R. K. Smith, Rev. Sci. Instr. 29 (1958) 652. lo) I.C.R.P. 1959 report o f committee 2 on permissible dose Jbr internal radiation (Pergamon Press, Oxford, 1959) p. 146. i1) T. O. Marshall, Health Phys., to be published. 12) j. A. Auxier, W. S. Snyder and T. D. Jones, Radiation dosimetry 1 (eds. F. H. Attix and W. C. Roesch, Academic Press, New York and London, 1968) p. 275.
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
INSTRUMENTS
AND
METHODS
92
(I97I) 5 9 5 - 5 9 9 ;
© NORTH-HOLLAND
PUBLISHING
CO.
Part VI. Applications to medicine and biology~595-609 S T U D I E S R E L A T I N G T O IN VIVO A C T I V A T I O N ANALYSIS U S I N G A S O U R C E O F 14 MeV N E U T R O N S t T. O. M A R S H A L L a n d A. K N I G H T
Radiological Protection Service, Clifton Avenue, Bebnont, Sutton, Surrey, England It is s h o w n , in carrying o u t in vivo activation analysis with a source of m o d e r a t e d 14 M e V n e u t r o n s , that the choice of slow n e u t r o n activation detector to m e a s u r e the resultant distribution o f slow n e u t r o n fluence within the subject being analysed or in a s y s t e m designed to simulate the subject is relatively u n i m p o r t a n t . Similar results are obtained using either i n d i u m or s o d i u m activation detectors or a BF3 tube. Moreover, the presence o f b o d y cavities does n o t appreciably upset the distribution of slow
n e u t r o n fluence within the subject. However, in preparing a s t a n d a r d against which to c o m p a r e the activity produced in the subject, if the concentration of the element in question is appreciably greater t h a n that in the body, then depression of fluence m a y occur. The results of m e a s u r e m e n t s with nuclear e m u l s i o n s of the fast n e u t r o n dose equivalents t h r o u g h o u t a water-filled model of the chest are reported.
1. Introduction
others concerned with in vivo activation analysis. The studies in question concern the choice of detector to measure the distribution of slow neutron fluence through the phantom, the distribution of slow neutron fluence in phantoms containing different amounts of C1 (i.e. an element with a large slow neutron absorption cross section), and the distribution in a water-filled p h a n t o m containing voids (i.e. to simulate body cavities) has been measured. Finally, results of measurements of fast neutron dose equivalents delivered to various depths within a water-filled p h a n t o m are reported.
Several workers 1-7) have used neutron activation analysis to determine the whole body content of various elements in man. In some cases, the results have been used in the diagnosis of certain diseases and in evaluating the efficacy of treatment. Patients, volunteers, cadavers and man-like models (phantoms) have been irradiated with neutrons and analyses have been based on measurements of the resultant 7-rays with whole body counters. In the main the nuclear reactions of interest have been produced by slow neutrons* but in order to obtain a uniform distribution of slow neutron fluence through the subject, a primary source of fast neutrons has been used with a suitable moderator. The subject has been turned round approximately half-way through the irradiation (bilateral irradiation). The work reported here is related to the work of Battye et al. 7) in which man-like models containing solutions similar in composition to that of tissues in the human body, were analysed for Na, C1, Ca, K and P using 14 MeV neutrons, with paraffin wax as a moderator. The resulting y-ray spectrum of each manlike model was compared, using a whole body counter s ) with the y-ray spectra of standards (i.e. phantoms containing a known amount of each element). This paper is concerned with a number of studies which were carried out in order to demonstrate the validity of the method of Battye et al. 7) if applied to the analysis of live men, and these studies are reported here in the hope that the results will be of interest to t P r o o f r e a d by the Publisher. * T h e term "slow n e u t r o n s " , as used here, m e a n s those h a v i n g energies up to a few tens o f electron volts, i.e. those in the energy r a n g e in which the (n,F) cross sections are significant.
2. Experimental arrangement The experimental arrangement used was similar to that of Battye et al. 7) but only the chest section of the p h a n t o m was used. This section, made from polyethylene, was an elliptic cylinder with major and minor axes of 30 cm and 20 cm respectively, and was arranged with its axis vertical together with the paraffin-wax moderator to form a cuboid 56 cm wide x 31 cm thick x 40 cm high. The arrangement is shown in figs. l(a) and l(b). The experiment was carried out in a basement r o o m and the moderator and accelerator were positioned such that the target and the centre of the phantom were approximately 1 m from the floor, approximately 1 m from the ceiling and approximately 3 m from each of the side walls. The wall behind the moderator was approximately 6 m away. The upper end of the model-chest section was removed to allow the introduction of detectors into the liquid (water or HC1 solution in these experiments) it contained. Voids, consisting of perspex cylinders, could also be introduced into the liquid. The minimum thick-
595 VI. A P P L I C A T I O N S
TO MEDICINE
AND BIOLOGY
596
T. O. MARSHALL AND A. K N I G H T Paraffin
wax
moderator
BF3 counte r ~ ' - - ~ .-~,:
~
100cm
~ P r o t r~"~ec on oi I Water
I
counter
filled
chest
section
probability of being activated. This means that for a bilateral irradiation with the experimental arrangements used here, the reaction yield for a given element should be constant throughout its volume. This should be verified by experimental measurements. The reaction yield for an element of concentration C at a given point within the subject is proportional to
of p h a n t o m
a)
C
Water filled c h e s t Target
II
s e c t i o n of p h a n t o m
BF3 c
o
~
]
Proton recoil c o u n t e r fi b) Fig. 1. a) Plan view of experimental arrangement, b) Side elevation of experimental arrangement.
ness of moderator around any part of the chest section was 6 cm. The source of neutrons for these experiments consisted of a S.A.M.E.S. 150 kV deuteron accelerator which produced 14 MeV neutrons at the target by the 3H(d,n)4He reaction. Throughout this work, irradiations were monitored by means of a simplified proton recoil telescope 9) which measured the fluence of neutrons with energies greater than about 10 MeV. Detectors used for the various investigations reported here were a small boron trifluoride (BF3) tube with a sensitive volume having dimensions 0.5 cm dia. x 5 cm long, polystyrene containers (6.4 cm dia. x 2.2 cm thick) each containing 70 ml of sodium nitrate solution, indium foils 1.5 cm dia. x 0.05 cm thick, nuclear emulsions and photographic emulsions. When using the BF3 tube and when measuring the induced activities of the sodium and the indium detectors, the number of counts recorded was such that the statistical standard deviation was, in all cases, less than 1%. When using the nuclear emulsions the statistical standard deviation of counting tracks was between 4% and 15%. 3. Distribution of slow neutron fluenee
Since elements are not generally distributed uniformly throughout the body and the size and shape of human beings vary, it is desirable to ensure that each a t o m of the element under investigation has an equal
f
E oc cr(E)(o(E)dE,
E=O
where a(E) is the cross section for the reaction in question at energy E and qS(E) is the neutron fluence of energy E. The limits of the integral should, strictly speaking, extend over thetrange E = 0 to E = w. However, in practice, analyses are mainly based on reactions where a(E) is most significant in the slow neutron energy region. For there to be an equal probability of any atom of the element in question being activated the above integral must be constant throughout the subject. Since a(E) in the above integral is a characteristic of the element being analysed, the probability of activation is best measured by means of samples of this element. Another element can only be used if its variation of cross section with neutron energy is identical to that of the element being analysed or if the slow neutron part of the neutron spectrum remains substantially constant throughout the subject. In practice a slow neutron detector has usually been chosen on the basis of convenience and the significance of this has been investigated by measuring the distribution of the slow neutron fluence through a water-filled manphantom, irradiated with 14 MeV neutrons, with three detectors which had cross sections which varied differently with neutron energy. The detectors used were: 1. boron, a 1/v detector (i.e. cross section inversely proportional to neutron velocity); 2. sodium, an element often determined in in vivo activation studies, and an approximate 1Iv detector for neutrons below 100 eV; 3. indium, which has a number of large resonances in the slow neutron region. In the case of the activation detectors, sodium and indium, the 24Na activity arising from the 23Na(n,7)24Na reaction, and the 116in activity f r o m t h e 115in(n,?)116in reaction were measured. The results are shown in fig. 2(a) and it can be seen that there is little disagreement between the results for the three detectors. The results indicate that the
597
IN VIVO A C T I V A T I O N ANALYSIS
I 4O
----______%_
/
3O c
.~ 2 0 =
~: BF 3 T u b
.¢_
~ Indium :~ S o d i u m
Foils detectors
=
=~ tc
u.
I (a) Distribution front
for
14MeV
face
b) Distribution
for
hi-lateral
I from
incident
on
irradiation
I
5 Distance
neutrons
I
10 front
face
of c h e s t
15 section
0 (crn.)
Fig. 2. Distribution of slow neutron fluence through water-filled phantom.
choice of detector to measure the distribution of slow neutron fluence is relatively unimportant, which suggests that She proportion of the neutron spectrum up to at least an energy of a few 100 eV remains substantially constant throughout the phantom. Fig. 2(b) has been computed from fig. 2(a) to show the distribution which would be obtained for a bilateral irradiation. Thus a bilateral irradiation gives a substantially uniform distribution across the section and departures from the mean value are about +__4%. This figure agrees well with the value of __+3.5% obtained by Battye et al. 2) and shows that if a bilateral irradiation is used in the case of live men, the probability of a reaction occurring is substantially constant throughout the body.
The distributions of slow neutron ftuence through the p h a n t o m for different HC1 solutions were measured using the BF 3 tube. Results are shown in fig. 3. Figs. 2 and 3 show that when sufficient chlorine was added to the water-filled p h a n t o m to give the same C1 concentration as in standard man (fig. 3), the slow neutron fluence was not significantly different from that of a water-filled p h a n t o m (fig. 2). Fig. 3 shows that the slow neutron fluence fell by approximately 3% when the chlorine concentration was doubled, fell by approximately 9% when it was increased to 5 times that of the standard man, and finally, it fell by about 17% when the CI concentration was 10 times that of the standard man. Thus the presence of a C1 concentration equal to that of the I.C.R.P. standard man does not lead to significant depression of the slow neutron fluence and, therefore, does not present a difficulty if the technique is applied to live men. However, when preparing the solution for the C1 standard, care must be taken to use a concentration which does not lead to a significant depression of slow neutron fluence, or the experiment must be carried out in such a way that the effect is allowed for. 5. The effect on the distribution of slow neutron fluence of voids within the phantom
In order to observe the importance of voids, such as the lungs and other body cavities, the distribution of the slow neutron fluence was measured using the BF 3 tube, with two different void configurations introduced into the water-filled chest section of the phantom. The
30
4. Depression of slow neutron fluenee by chlorine, an element with a large slow neutron absorption cross section
20 c
One of the most important elements in m a n with regard to depression of slow neutron fluence in neutron activation analysis is chlorine, which has a total slow neutron absorption cross section of 33.8 b. However, the cross section for the 3vCI(n,y)3aC1 reaction which is used when analysing for chlorine is only 430 mb and when irradiating standards containing chlorine it is advantageous to increase the chlorine concentration to well above that of the I.C.R.P. standard man 1°) (i.e. more than 105 g of C1) in order to improve the yield of 3sC1 and hence the counting statistics.
= 10
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man
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10 f r ont
man
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chest
section
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Fig. 3. Distribution of slow neutron fluence through phantom containing different concentrations of chlorine. VI. A P P L I C A T I O N S TO MEDICINE AND BIOLOGY
598
T. O. MARSHALL
AND
which the fiuence, outside the voids, returns to approximately the value which would exist in the absence of the voids. The increase in fluence in the water adjacent to the void is greater (i.e. approximately 10% increase) for the 6.4 cm void, i.e. the larger the void, the greater departure from the fluence distribution that would exist in the absence of the void. Since actual body cavities will in general be smaller than the large void used in these experiments, it is concluded that any perturbation in fluence due to body cavities will be small and should not appreciably affect the accuracy of whole-body measurements.
30
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Fig. 4. Distribution of slow neutron ftuence through water-filled phantom containing a 6.4 cm diameter cylindrical void. first consisted of a water-tight perspex cylinder of internal diameter 6.4 cm and 40 cm long set with its axis vertical and positioned centrally within the phantom. The second consisted of two such vessels with internal diameters of 3 cm and positioned with their axes vertical and at distances of 6.5 cm and 14.5 cm from the front face of the phantom. Results are shown in figs. 4 and 5. It can be seen that a marked increase in the slow neutron fluence occurred within the voids. In the case of the 6.4 cm diameter void, there was a 50% increase above the value which would be present at the position of the centre of the void in its absence. In the case of the two 3 cm diameter voids the increase was approximately 20%. However, it is interesting to note the way in
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o
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0
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Phantom
Moderator
~- M e a s u r e d --Theoretical
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-o
z
o 0
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I
I
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20
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Fig. 6. Variation of fast neutron dose equivalent through paraffin wax moderator and water-filled phantom.
3O
E
The fast neutron dose equivalents were measured throughout the water-filled chest section of the phantom with nuclear emulsions. Proton recoil tracks in the
0
5
10
15
D i s t a n c e from front face of chest s e c t i o n
20 (cm.)
Fig. 5. Distribution of slow neutron fluencethrough water-filled phantom containing two cylindrical voids of diameter 3 cm.
emulsion were viewed through a microscope and counted. The slow neutron and 7-ray dose equivalents were measured with photographic emulsions but were found to be only a few percent of the fast neutron dose equivalents and were ignored. N o attempt was made to measure the dose equivalents due to intermediate energy neutrons but their contribution to the total neutron dose equivalent is assumed to be small. This assumption is based on studies by Marshall 11) on the attenuation of 14 MeV neutrons in water, which have indicated that intermediate energy neutrons account for' only a few percent of the total dose equivalent for water thicknesses up to 20 cm. The nuclear emulsion results are shown in fig. 6. A solid smooth line has been drawn through the
IN VlVO A C T I V A T I O N ANALYSIS
experimental points (plotted + 1 standard deviation) and the results have been normalised so that the dose equivalent at the front of the paraffin wax is 2.25 rem, i.e. the irradiation conditions used by Battye et al.7). Fig. 6 shows that this gives a mean dose equivalent through the water-filled p h a n t o m of 1 rem. If the water-filled p h a n t o m had been turned round half-way through the irradiation (bilateral irradiation) the neutron does equivalent throughout the water-filled phantom would have been practi, ally constant with a value of approximately 1 rein. The experimental results have been compared with the calculated values of Auxier, Snyder and Jones'2). These calculations were for a cylindrical homogeneous anthropomorphic phantom of 30 cm diameter irradiated with a parallel beam of neutrons. An inverse square law correction has been made to the calculated values so that they can be compared with the experimental results which were obtained using a source of neutrons 1 m from the front of the paraffin wax. The calculated values have been normalised so that the dose equivalent at the front of the cylindrical phantom is 2.25 rem and are shown as a dotted line in fig. 6. 7. Conclusions It is concluded that, in carrying out in vivo activation analysis using a source of 14 MeV neutrons, the variation of cross section with neutron energy of the detector chosen to measure the distribution of slow neutron fluence throughout water-filled phantoms is relatively unimportant, and that voids within the subject should not significantly affect the fluence in parts other than the voids. It is further concluded that in preparing standard solutions containing elements with large slow neutron absorption cross sections such as C1, care must be taken to either choose a solution strength which does not lead to a significant depression of the slow neutron fluence within the p h a n t o m or to design the experiment such that the effect may be allowed for. Finally, it has been shown that for an
599
irradiation in which a dose equivalent of 2.25 rein is delivered to the front face of the moderator and when the water-filled phantom is turned round half-way through the irradiation, the dose equivalent is substantially constant throughout the phantom having a value of about 1 rem. This work is part of a programme of work undertaken by the Radiological Protection Service which is supported by the Medical Research Council and the Department of Health and Social Security. The authors wish to thank Mr. C. L. Harvey, Mr. C. J. Bowles and Mrs. G. Flinn of the R. P. S. for their help with the experimental work. The authors are also indebted to Mr. C. K. Battye of King's College Hospital for much useful discussion. References 1) j. Anderson, S. B. Osborn, R. W. S. Tomlinson, D. Newton, J. Rundo, L. Salmon and J. W. Smith, Lancet 2 (1964) 1201. 2) C. K. Battye, R. W. S. Tomlinson, J. Anderson and S. B. Osborn, Nuclear activation techniques h2 the li]e scienees (I.A.E.A., Vienna, 1967) p. 573. a) M. J. Chamberlain, J. 1-[. Fremlin, D. K. Peters and H. Philip, Brit. Med. J. 2 (1968) 581, 583. 4) H. E. Palmer, W. B. Nelp, R. Murano and C. Rich, Phys. Med. Biol. 13 (1968) :269. ~) M. J. Chamberlain, J. H. Fremlin, D. K. Peters and H. Philip, 9th Intern. Syrup. Radioactive isotopes in clinical medicine and research, paper 4 (Bad Gstein, 1970). 6) S. H. Cohn, C. S. Dombrowski and R. G. Fairchild, Int. J. Appl. Rad. Isotopes 21 (1970) 127. 7) C. K. Battye, V. Knight, T. O. Marshall, A. Knight and B~ E. Godfrey, these Proceedings. ~) LA.E.A. directory o f whole-body radioactivity monitors (I.A.E.A., Vienna, 1964) p. 335. 9) S. J. Bame, Jr., E. Haddad, J. E. Perry, Jr. and R. K. Smith, Rev. Sci. Instr. 29 (1958) 652. lo) I.C.R.P. 1959 report o f committee 2 on permissible dose Jbr internal radiation (Pergamon Press, Oxford, 1959) p. 146. i1) T. O. Marshall, Health Phys., to be published. 12) j. A. Auxier, W. S. Snyder and T. D. Jones, Radiation dosimetry 1 (eds. F. H. Attix and W. C. Roesch, Academic Press, New York and London, 1968) p. 275.
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