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The photodeuteron yield from 30 MeV Bremsstrahlung irradiation
2.I
I
Nuclear Physics 23 (1961) 269--284;~)North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE PHOTODEUTERON BREMSSTRAHLUNG BENGT
YIELD F R O M 30 MeV IRRADIATION
FORKMAN
Department of Physics, University o/ Lurid, Lund, Sweden Received 28 N o v e m b e r 1960
Abstract:
A n i n v e s t i g a t i o n of t h e (F, d) r e a c t i o n in s u l p h u r , c o b a l t a n d copper h a s been carried out. B r e m s s t r a h l u n g of 30 M e V m a x i m u m e n e r g y w a s used. T h e e m i t t e d c h a r g e d p h o t o p a r t i c l e s were deflected b y a m a g n e t i c field a n d recorded in n u c l e a r emulsions. T h e r a n g e a n d t h e o r i e n t a t i o n of t h e t r a c k s in t h e e m u l s i o n s were m e a s u r e d a n d t h e trajectories of t h e p h o t o particles were c a l c u l a t e d b a c k w a r d s for p r o t o n s , d e u t e r o n s a n d a l p h a particles. T h e intersection p o i n t of e a c h t r a j e c t o r y w i t h t h e t a r g e t p l a n e g a v e i n f o r m a t i o n a b o u t t h e kind of p h o t o - p a r t i c l e w h i c h h a d p r o d u c e d t h e track. T h e ratio Rap b e t w e e n t h e (F, d) yield a n d t h e (F, P) yield w a s d e t e r m i n e d . No s t r o n g f l u c t u a t i o n in t h e ratio b e t w e e n t h e a d j a c e n t e l e m e n t s c o b a l t a n d copper w a s observed. T h e ratios f o u n d were all zero for t h e t h r e e e l e m e n t s s u l p h u r , c o b a l t a n d copper, w i t h a s t a n d a r d d e v i a t i o n of r e s p e c t i v e l y 0.009, 0.012 a n d 0.018. A calcul a t i o n b a s e d o n t h e s t a t i s t i c a l m o d e l of t h e ratio Rap w a s m a d e . T h e ratios o b t a i n e d are, w i t h i n t h e limits of t h e errors, in a g r e e m e n t w i t h t h e e x p e r i m e n t a l results. T h e p r e s e n t r e s u l t s also were c o m p a r e d w i t h t h e p i c k - u p process t h e o r y , w i t h earlier (7, d) e x p e r i m e n t s a n d w i t h t h e i n v e r s e reaction. T h e o b t a i n e d ratio b e t w e e n t h e (~, a) yield a n d t h e (7, P) yield was in a g r e e m e n t w i t h earlier (F, co) e x p e r i m e n t s .
1. I n t r o d u c t i o n
During a long time the (y, d) reaction has been a point of question in connection with photonuclear reactions 1). Both the statistical model and the direct interaction model predict that photodeuterons should be quite rare compared to photoprotons. However, a number of investigators that have used bremsstrahlung with a maximum energy of about 30 MeV have observed the existence of photodeuterons. The reported yields of photodeuterons vary considerably from element to element. Especially for copper a high yield was obtained, which amounted to about a hundred times the yield predicted from the statistical model 2,3, 4). Photodeuterons have also been observed using sulphur ~) and zinc 6). It is, however, difficult to separate deuterons from protons, especially if they are of low energy. Different techniques have been used to attack the problem. In the case of copper the deuterons and the protons have been recorded in nuclear emulsions, and the separation has been done b y grain-counting of the tracks. As the tracks are short a good separation is hard to obtain. In the case of sulphur and zinc the photodeuteron yield has been determined by measuring the activity of the daughter nuclei in the 2 MeV region above the 269
270
BENGT
FORKMAN
(y, d) threshold. Above this region the (y, np) reaction will compete and it is therefore difficult to measure the (y, d) yield only. As the accessible energy region is small this method too gives only crude information about the (7, d) reaction. Several investigators, that have used a maximum bremsstrahlung energy of about 80 MeV report, too, the existence of photodeuterons. In copper ~) and sulphur s) the photodeuterons have been separated from tile photoprotons b y a cloud chamber traversed b y a magnetic field. The method shows a good resolution of protons and deuterons. Again a surprisingly high yield of photodeuterons from copper is reported. The irradiated targets were thick and tile cloud chamber favoured high-energy particles. Therefore it is difficult to compare the results with those from the 30 MeV irradiation. The cloud-chamber experiments can, however, be compared with a recently made investigation which thoroughly studied a lot of elements, separating the photodeuterons from tile photoprotons b y a two-crystal telescope 9, 10). This method shows, too, a good resolution of protons and deuterons but studies only photodeuterons down to 15 MeV. The yields of photodeuterons are considerably smaller than those obtained with the cloud chamber. No strong fluctuations in the yields are observed. The yield increases continuously with mass number. Even these results are difficult to compare with those from the 30 MeV irradiation as the photodeuterons are of high energy. An attempt has been made to study the low-energy photodeuterons from gold n) b y the method of grain-counting but again it is difficult to obtain a good resolution with this method. The yield of photodeuterons has been investigated also with a maximum bremsstrahhing energy of 300 MeV 12,13,14).The high-energy deuterons have been studied b y a two-crystal telescope. In this energy range too, a continuous increase of tile yield with the mass number has been observed. The ratio of the photodeuteron to the photoproton yield was here about 4 times larger than tile corresponding ratio obtained with the telescope investigation at the lower irradiation energy. In order to explain the high yield of photodeuterons it has been assumed that the deuterons are formed in a two-stage process. The incident photon is absorbed b y a single nucleon which in its turn picks up another nucleon with the right charge and momentum and forms an outgoing deuteron. Such a pick-up process must occur on the surface of the nucleus. A semi-quantitative calculation of this pick-up process has been made b y Sawicki and Czy~ 15). The aim of this work has been to investigate the photodeuteron yield from some elements with a maximum bremsstrahlung energy of 30 MeV using a new technique worked out in Lund 16) that is able to separate single deuterons from protons down to 2 MeV. Fluctuations in the yield were looked for. It was also tried to test the proposed theoretical model experimentally. The same
THE
PHOTODEUTERON
271
YIELD
technique has recently been used b y the Iowa group with a -~aximum bremsstrahlung energy of 45 MeV 17). It is well known that the ()', ~) reaction in medium weight nuclei is satisfactorily explained b y the statistical model 18). This reaction is much easier to study than the ()', d) reaction, since the alpha particles are easy to separate from singly charged particles and because the stability of the alpha particle ensures the reliability of the activity measurement of the daughter nucleus. In the present work the (y, ~) reaction has been studied, since the power of the experimental arrangement to separate photo-alpha particles is a proof of its stfitability for observing photodeuterons. The observed ()', ~) yields have been compared with earlier measurements.
2. Experimental Arrangement and Measurements In order to obtain a good separation between protons and deuterons a magnetic field was used to deflect the particles. The photo-particles were recorded in nuclear emulsions. The method is fully described in ref. 16). The experimental arrangement is shown in fig. 1. As the source of the photons a 30 MeV electron synchrotron was used. The machine was energy stabilized and was run with maximum energy. The ),-beam was collimated b y a conic lead collimator and passed then through an evacuated camera. In the camera the photons hit a thin target. The diameter of the ?,-beam at the position of the target was 1.5 cm. Three different targets were used. At the first exposure a copper foil rolled out to 10 #m was irradiated. The second exposure was made without target in order to determine the background. Next a sulphur target 34 #m thick, obtained b y allowing sulphur to evaporate on a 3 ,urn thick gold foil, was irradiated. At the fourth exposure a cobalt foil, 10/zm thick, produced b y electrolysis on a Lead collimator
/
Synchr'ot ron
Boron-paraffin
:~ /'~
Ionization
Nuclear plates
L
0
I
10
;0
I
30
4;
emulsion
510 cm
Fig. 1. E x p e r i m e n t a l a r r a n g e m e n t to a n a l y z e photo-particles. T h e e v a c u a t e d c a m e r a was placed in t h e g a p of a n e l e c t r o m a g n e t w i t h t h e m a g n e t i c field parallel w i t h t h e z-axis. T h e s c a n n e d a r e a in t h e e m u l s i o n s is s h a d e d in t h e figure.
272
BENGT
FORKMAN
gold foil, also 3 / t m thick, was used. The camera was placed in the gap of an electromagnet. The current to the electromagnet was stabilized and gave a constant magnetic field of 1.5 W b/ m ~. Two Ilford C2 emulsion plates with the size 5.1 cm by 10.2 cm were used to record the photo-particles. The two plates were mounted oblique to each other with an opening angle of 23 °. In order to shield the nuclear plates from background radiation the camera was covered with lead and the whole magnet was surrounded with boron-paraffin blocks. Furthermore a lead wall was piled up between the camera and the synchrotron. The camera was watercooled in order to keep the nuclear plates at a safe temperature. The total dose given to the targets at the different exposures amounted to about 3000 r. In order to test that the experiment was not disturbed by fast neutrons, a phosphorus sample was placed in the beam at the target position. No neutron induced phosphorus activity was observed. As already has been mentioned a background exposure was performed without any target. Unexpectedly m a n y background tracks were obtained in the plates. These originated from the entrance window of the camera which was a 50/~m thick Al-foih The background from the gold foil in the suphur and cobalt exposures was negligible both through the thinness of the foil and through the high coulomb barrier for proton and deuteron emission. The nuclear emulsions were developed in the usual way and were scanned and measured by a Leitz microscope. The entrance co-ordinates, the length and the orientation of the tracks were measured. As the position of the nuclear plates in relation to the target was known and also the strength of the magnetic field, it was possible to calculate backwards the trajectories of the photoparticles from the target to the emulsion. Such a calculation was made first assuming th at all the recorded particles were protons and then assuming t hat the particles were deuterons. Finally a calculation was made assuming them to be alpha particles. These calculations were performed using SMIL (the University of Lu n d digital computer). The co-ordinates of the intersection points between the trajectories of the particles and the plane of the target were calculated. In the calculations a right-handed system of co-ordinate axes was placed so that the y-axis was parallel with the y-beam and the z-axis parallel with the direction of the magnetic field. The co-ordinate system is drawn in fig. 1. The length of the target in the x-direction was 2.0 cm and in the z-direction 2.8 cm. In the chosen co-ordinate system the centre of the target had the co-ordinates x = -- 10.1 cm, y = -- 10.9 cm and z = 0.0 cm. The errors in the measurements of the dip of the tracks give an uncertainty in the z co-ordinates (zt) of the intersection points. The distribution of the zt values of the points in the interval -- 8.4 > x t > -- 11.6 is shown in fig. 2. Owing to the broadness of this distribution all measured tracks giving a ]ztl < 3.0 have been accepted in the following analysis.
THE
PHOTODEUTERON
273
YIELD
The nuclear plates were scanned in the area shaded in fig. 1. To this area only those photo-particles came t h a t h a d a small angle Y with the p h o t o n direction. Fig. 3 shows the distribution of the angle Y of the particles recorded in the plates. In the nuclear emulsions all tracks h a v i n g a length more t h a n 30 a m were measured. This corresponds to a p r o t o n e n e r g y of 1.6 MeV, a d e u t e r o n e n e r g y of 2.1 MeV and an alpha particle e n e r g y of 6.5 MeV. However, in the area m e a s u r e d only protons with an energy above 2.5 MeV will be recorded. P r o t o n s with lower energies will cut the c a m e r a wall before t h e y reach the plates. The efficiency goes up v e r y steeply a n d above 3 MeV all protons will be recorded o o
o. 150. E
o
?-
~6 xa
E
E i0c
\
_p-
o
/ _~ [~
_~
G
~
i ~
Z¢:
50l
100-
°0" (cm)
Fig. 2. Experimental distribution from the cobalt exposure of the z co-ordinates zt of the intersection p o i n t s of t h e p r o t o n t r a j e c t o r i e s w i t h t h e p l a n e o f t h e foil f a l l i n g i n t h e i n t e r v a l - - 8 . 4 > x t > - - 1 1 . 6 . T h e b a c k g r o u n d is s u b t r a c t e d . T h e c e n t r e o f t h e t a r g e t h a s t h e z c o - o r d i n a t e z t = 0.0.
L
10"
Go"
90' Y
Fig. 3. Experimental distribution from the cobalt exposure of the angles Y of the emitted photoprotons with the y-axis.
with the same efficiency. The corresponding value for the deuterons is 2 MeV a n d for alpha particles 3 MeV le).
3. R e s u l t s T h e value of the x co-ordinate (xt) of the intersection point between the calculated t r a j e c t o r y a n d the plane of the t a r g e t depends on w h e t h e r the t r a c k in the emulsion is assumed to originate from a proton, a d e u t e r o n or an alpha particle. In one of these cases x t m a y fall in the region of the target and it will be possible to i d e n t i f y the t r a c k as t h a t of a proton, a d e u t e r o n or an
274
BENGT FORKMAN
alpha particle. The alpha tracks, however, m a y also be distinguished directly from singly charged particle tracks as the alpha tracks are denser. Figs. 4, 5 and 6 show the experimental distribution of x t from the three main exposures after the background has been subtracted. In section A of the figures it is assumed that all the singly charged particle tracks are due to photoprotons, while in section B the assumption is made that the same tracks are due to photodeuterons. There are two reasons, w h y t h e number of points in sections A and B is not the same. The first is that the computer rejected trajectories which cut the walls Cobalt
'11
E
.,n
"6
Sulphur
20
E o
.~
E
z
0
i
20
z
r-'-I i-I
100
100
80
80.
60.
60-
40
40.
20.
20-
0
~
0
~
,-r'l._~, -5
] ~
f~f-"~ -I0
0 -I'5 x t (cm)
Fig. 4. E x p e r i m e n t a l d i s t r i b u t i o n from t h e s u l p h u r e x p o s u r e of the x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t falling in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. The b a c k g r o u n d is s u b t r a c t e d . I n section A all t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s are a s s u m e d to o r i g i n a t e from protons. hx s e c t i o n B t h e y are a s s u m e d to o r i g i n a t e from deuterons. I n section C all t h e dense t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . The c e n t r e of t h e t a r g e t h a s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 13.2 cm 2.
0
~ - 5
-10
-15
X t (cm)
Fig. 5. E x p e r i m e n t a l d i s t r i b u t i o n from t h e c o b a l t e x p o s u r e of t h e x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t fa l l i ng in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. T h e b a c k g r o u n d is s u b t r a c t e d . I n s e c t i o n A a l l t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s a re a s s u m e d to o r i g i n a t e from prot ons . I n sectioi1 B t h e y a re a s s u m e d to o r i g i n a t e from d e u t e r o n s . Iix s e c t i o n C all t h e de ns e t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . The c e n t r e of t h e t a r g e t ha s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 18.0 c m ~.
THE PHOTODEUTERON YIELD
~75
of the camera. The second is t h a t m a n y of the tracks h a d rz nges which corresponded to such high d e u t e r o n energies, t h a t t h e y were in conflict with the law of e n e r g y conservation. In the d e u t e r o n calculations such tracks were rejected. In section C the intersection points of the alpha tracks are shown. Section A of all the figures shows a n a r r o w distribution of the n u m b e r of intersection points a r o u n d the centre value x t = --10.1 of the targets. These points therefore are starting points for photoprotons. Such peaks are not observed in the sections B. Copper
10 c D.
C 0
~
-~ 20, z 10,
0l
~
r
r ~ r ]
I
S
A
40-
30.
20"
10-
O. ~ _ . . _ . _ ~ : : z _ ~ _ _ 0
-5
-10
-I 5
,4"t (cm)
Fig. 6. E x p e r i m e n t a l d i s t r i b u t i o n from t h e c o p p e r e x p o s u r e of t h e x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t f a l l i n g in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. The b a c k g r o u n d is s u b t r a c t e d . I n s e c t i o n A all t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s are a s s u m e d t o o r i g i n a t e fro m p r o t o n s . I n s e c t i o n B t h e y are a s s u m e d to o r i g i n a t e from d e u t e r o n s . I n s e c t i o n C all t h e dense t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . T h e c e n t r e of t h e t a r g e t h a s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 9.8 cmz.
Only v e r y few tracks give intersection points t h a t fall a r o u n d x t = --10.1. In sections C again the points originating from alpha particles are c o n c e n t r a t e d a r o u n d x t = -- 10.1. In fig. 7 the b a c k g r o u n d is shown in the same way. It m a i n l y consists of fast p h o t o p r o t o n s originating from the entrance window. The position of the b a c k g r o u n d peak accidentally falls on almost the same place as the peak of the p h o t o p r o t o n s from the main exposures. Fig. 7B shows t h a t the b a c k g r o u n d
276
BENGT
~ORKMAN
f r o m the e n t r a n c e window does not d i s t u r b the s t u d y of p h o t o d e u t e r o n s . T h e few b a c k g r o u n d points f r o m x t = 0 d o w n to x t = - - 12 o r i g i n a t e d m o s t l y f r o m such recoil p r o t o n s t h a t are f o r m e d in the emulsions a n d pass t h r o u g h the surface of the emulsions. I n m o s t cases t h e y can be sorted out, as t h e y do not h a v e a characteristic t r a c k end, b u t for l o w - e n e r g y p r o t o n s this s e p a r a t i o n is h a r d to perform. No b a c k g r o u n d points originating f r o m a l p h a particles were obtained. The energy distributions of the p h o t o p r o t o n s from the different exposures t h a t h a v e an x t value in the i n t e r v a l - - 8 . 4 > x t > --11.6 are shown in fig. 8. T h e distributions are c o r r e c t e d for the b a c k g r o u n d . T h e p h o t o p r o t o n distributions f r o m sulphur, cobalt a n d c o p p e r h a v e been r e p o r t e d earlier with m u c h Background B
.5 20-
o
z
r-'ra--Y~ ,
f-h A
30-
20
0:
----a r~ [7
,, -5
"~ -1'0
-1'5
Fig. 7. E x p e r i m e n t a l distribution from the b a c k g r o u n d exposure of the x co-ordinate of the intersection points of the trajectories w i t h the plane of the t a r g e t failing in the interval - - 3 . 0 < zt < 3.0. In section A all the singly charged particle tracks are a s s u m e d to originate from protons. I n section B t h e y are a s s u m e d to originate from deuterons. No dense tracks gave intersection points in the x region s h o w n in the figure when. t h e y were a s s u m e d to be a l p h a particle tracks. The area scanned is 7.1 cm z.
b e t t e r statistics 19,2o,21). T h e d a s h e d h i s t o g r a m s d r a w n in the figure are the expected distributions. I t is e v i d e n t t h a t the distributions here f o u n d are in agreem e n t with the e x p e c t e d ones. This a g r e e m e n t shows t h a t the present investigation does not f a v o u r a n y p r o t o n energy. T h e figure gives also the energy distrib u t i o n of the b a c k g r o u n d tracks. T h e figs. 4, 5 a n d 6 give the following ratios R0p b e t w e e n the (7, d) yield and the (7, P) yield with the c o r r e s p o n d i n g s t a n d a r d d e v i a t i o n s s: S: Co: Cu:
Rap = 0.000, Rap = 0.000, Rop = 0.000,
s = 0.009; s = 0.012; s = 0.018.
THE
PHOTODEUTERON
YIELD
277
The present experiment cannot settle a lower limit of the ratio Rdp as no photodettterons with certainty have been observed in the three exposures. Fig. 9A shows tile sum of the three x t co-ordinate distributions with the assumption t h a t all the singly charged particle tracks were deuteron tracks. This has been done in order to get better statistics. In the figure the background is not subtracted. It is still impossible to observe a peak around x t = --10.1 above
0
4O
Background
i
20:
f~
O~ Copper
E 4oi Z
20 0 120
Cobalt
100~ 80 r-
60 40 ! 20 0 120 100 80
F•
Sulphur
-q
60
L~--
-\-,
~0 20 0
0
5
10
15 20 Proton energy (MeV)
Fig. 8. E n e r g y d i s t r i b u t i o n of t h e o h o t o p r o t o n t r a c k s g i v i n g i n t e r s e c t i o n p o i n t s i n tile a r e a - - 8 . 4 > x t > - - 1 1 . 6 a n d - - 3 . 0 < zt - 3.0. The l o w e s t s e c t i o n s h o w s t h e p h o t o p r o t o n s from t h e s u l p h u r e x p o s u r e . T h e d a s h e d line is t h e e x p e c t e d distrlL,~ltion from ref. 19). The second s e c t i o n s how s t h e p h o t o p r o t o n s f r o m t h e c o b a l t exposure. The d a s h e d line is t h e e x p e c t e d d i s t r i b u t i o n f r o m ref. ,0). T h e t h i r d s e c t i o n s h o w s t h e p h o t o p r o t o n s f r o m t h e c o p p e r e x p o s u r e . T h e d a s h e d line is t h e e x p e c t e d d i s t r i b u t i o n from ref. =1). The b a c k g r o u n d is s u b t r a c t e d in t h e s e t h r e e sections, b u t s h o w n in t h e u p p e r m o s t section. I t o r i g i n a t e s f r o m t h e e n t r a n c e w i n d o w .
~8
BENGT FORKMAN
t h e b a c k g r o u n d level. T h e figure gives for t h e r a t i o :
Y,Y (7, d) -
0.000,
-
with
s = 0.007.
~ Y (X, P) T h e r e f o r e t h e u p p e r limit is p r o b a b l y n o t m o r e t h a n 0.007. I n fig. 9B all t h e t r a c k s o r i g i n a t i n g b o t h f r o m s i n g l y a p 4 d o u b l y c h a r g e d particles were a s s u m e d to be a l p h a t r a c k s if t h e y h a d r a n g e s c o r r e s p o n d i n g to a l p h a - p a r t i c l e energies b e t w e e n 6.5 a n d 11.0 MeV. N o w a d i s t i n c t g r o u p of ul
E
20
X "~ .o K z
0 20 0 160 140 120 100 80 60 40 20 0 0
-5
-10
-1'5
X~ (cm)
Fig. 9. The sum of the experimental distributions from the three main exposures of the x co-ordinate of the intersection points of the trajectories with the plane of the target falling in the interval --3.0 < zt < 3.0. In section A all the singly charged particle tracks are assumed to originate from deuterons. The background is not subtracted. In section B all the tracks originating both from singly and doubly charged particles are assumed to be alpha particles. The background is not subtracted. In section C only the dense tracks are assumed to be alpha particle tracks. No background exists. p o i n t s a r o u n d x t = - - 10.1 is o b s e r v e d a b o v e t h e b a c k g r o u n d level s h o w i n g t h e e x i s t e n c e of p h o t o - a l p h a particles. I n fig. 9C t r a j e c t o r i e s were c a l c u l a t e d o n l y for t h o s e t r a c k s t h a t d u r i n g t h e s c a n n i n g of tile plates were r e c o g n i z e d as a l p h a t r a c k s . I n this s e c t i o n no b a c k g r o u n d exists, h o w e v e r t h e y i e l d of a l p h a particles d e t e r m i n e d f r o m figs. 9B a n d 9C is t h e same. I t is e v i d e n t t h a t t h e (y, d) y i e l d m u s t be less t h a n t h e (7, ~) yield. O t h e r w i s e a g r o u p of p o i n t s at t h e p o s i t i o n of t h e t a r g e t similar to t h a t f o u n d in fig. 9B s h o u l d h a v e b e e n o b s e r v e d in fig. 9A.
THE
PHOTODEUTERON
YIELD
279
Only alpha tracks with energies more than 6.5 MeV were measured. An 8 MeV alpha particle loses about 1.5 MeV when it passes through half the thickness of the cobalt or copper foil. The energy distribution of the emitted photoalpha particles from copper using a maximum bremsstrahlung energy of 30.5 MeV is peaked around 8 MeV 22). It is therefore evident that the lower energy part of the photo-alpha particles are lost in the present experiment. In order to get an estimate of the ratio R=, between the (y, ~) yield and the (y, p) yield, the number of observed alpha tracks in the case of cobalt and copper has been corrected for this loss. In the case of sulphur too m a n y alpha particles have been lost to get a fair estimate. The present experiment gives the following ratios: Co: R=p ----- 0.09;
Cu: R=p ---- 0.05.
These ratios may be compared with the reported ratios for cobalt at 21.5 MeV bremsstrahlung 20, 23) R=p = 0.047 and for copper at 24 MeV bremsstrahlung 2) R=p --~ 0.040. The ratio R=p will rise with the irradiation energy. The present ratios therefore are in agreement with earlier (y, =) experiments. All the identified alpha particles had energies between 6.5 and 10.5 MeV in agreement with the known energy distribution. 4. D i s c u s s i o n
It is well known i) that the statistical model satisfactorily gives an account of the energy distribution of the emitted photoneutrons. This is also the case for photoprotons from light and medium weight nuclei. For these nuclei the statistical model also predicts the yield of photoprotons compared with the yield of photoneutrons. The experimental deviations from the statistical model for heavy nuclei can be explained in terms of the direct photo-effect. The experimental photo-alpha particle yield is) and the photo-triton yield 24) follows also the statistical theory. It will therefore be of interest to compare the present results with this theory. To perform such a calculation it is necessary to know the level density of the daughter nuclei. It has been shown that low-energy nuclear temperatures are independent of the excitation energy 25). At higher excitation energies, however, the nuclei can be described as a Fermi gas and the excitation energy will be proportional to the temperature squared 26). The level density will therefore have one of the forms
p(E) =
E const x e x p -~
(low-energy excitation),
(1)
(high-energy excitation),
(2)
where T is a constant, or
P (E) = const x exp V'~E where ~ is a constant.
280
BENGT FORKMAN
The ratio Rap is sensitive to which of these formulas is used as the thresholds for the reactions (7, P) and (~, d) differ considerably. In the copper region T seems to be constant up to 10 MeV having a value of 1.16 MeV 27). At excitation energies between 10 and 20 MeV the level density looks more like eq. 2 with a value for ~ of 5.56 MeV -1 2s). In the sulphur region T has the value 1.6 MeV 25). As the level density in sulphur is lower than in copper at the same energy, eq. 1 should be expected to be valid for sulphur in the whole energy range of the present experiment. In the calculation of the ratio it has been taken into account that there is a difference in the total number of states for even, odd, and odd mass nuclei at a constant energy. There exists a close connection between the difference and the pairing energy 25). The calculation is based on the experimental cross-section curves for sulphur 29) and copper 3o) and Shapiro's penetrability values 31). Using eq. 1 the following ratios are obtained: $32:
Rap =
3 × 10-3;
Cu63: Rap = 6 × 10-4.
Using eq. 2 we have C u 63:
Rap =
7×10
-a.
In the case of copper the estimate from eq. 2 should be more reliable as the level densities in the copper region given by eqs. (1) and (2) with the experimental constants agree fairly well at low excitation energies, and eq. (2) is the best at high energies. Hence the statistical model predicts an Rap of 0.003--0.007 when the maximum bremsstrahlung energy is 30 MeV. This Rap shall be compared with the present experimental result Rap < 0.007. The present results m a y also be compared with the estimation of the photodeuteron emission by a pick-up process calculated by Sawicki and Czyz 15). T h e y give for sulphur a ratio of the cross sections a(7, d)/a(7, p) = 0.1 for a photon energy of 25 MeV. The photoproton cross section is that of the direct process only. The ratio a(7 , d)/(a(y, p)dlrect@a(y, P)compounO) must be much lower. The deuteron dissociation within the nucleus m a y also suppress the deuteron yield. From a nucleus with N -----Z the direct dipole emission of a deuteron is forbidden since the nuclear charge-to-mass ratio is the same as that of the deuteron. Photodeuterons from light and medium weight nuclei must therefore mainly be formed in an evaporation or a pick-up process. The experimental results from the Iowa group 17) are in close agreement with the present ones. T h e y report that in copper Rap is less than 1.6-4-1 ~/o. T hey have measured the emitted photo-particles in the angle range between 30 ° and 80 ° from the incident beam direction and therefore cover a different angular region than the present investigation made in the forward direction. As the anisotropy in the angular distribution of the direct emitted photoprotons is strong and peaked around 60 ° 32) it should be expected t h a t non-
THE PHOTODEUTERON YIELD
281
statistical photodeuterons will more easily be observed in forward and backward directions. Chizhov lo) has shown that the emitted photodeuterons of high energy from light elements have an angular distribution which is peaked around 0 °, which further argues in favour of measurements at small angles. The inverse reactions often give valuable information about the photoprocesses. The radiative deuteron capture process Zn~4(d, ~)Ga 66 has recently been studied by Carver and Jones 33) using deuteron energies of about 4 MeV. The magnitude of the cross section was determined b y activity measurements of Ga 66. As the captured 7-ray spectrum is not known non-statistical processes may contribute to the cross section. However, statistical calculations of the cross section were in good agreement with the experiment showing that in the energy range studied an anomalous yield of photodeuterons is not expected. This supports the present investigation. Byerly and Stephens 2) were the first who stated the existence of photodeuterons. They found in the energy distribution of photoprotons from copper an extra peak at 2.5 MeV which they assumed to originate from deuterons. They reported that it is possible to separate single deuterons from protons b y graincounting the last 40 #m of the photo-particle tracks in nuclear emulsions and got in this way a ratio Rop of 0.314-0.09. However, the author 3) tried to repeat the experiment of Byerly and Stephens and showed that a separation of single deuterons from protons cannot be expected if common nuclear emulsions are used. By a statistical processing of the data Rop for Cu was estimated to 0.16~: ±0.08. Grant et al. 4) later used the same technique recording the tracks in fine grain emulsions. Still a separation of single deuterons from protons was not obtained. They report for Cu and Rdp of 0.214-0.05. However, they did not succeed in repeating this value but got a considerably lower Rop 34). The existence of the extra peak in the energy distribution of the photoprotons from copper have not been observed in a recent investigation b y Lin'kova et al. 21). Probably it does not exist but is a background effect. The assumed photodeuterons in the experiment of Byerly and Stephens should have an energy of about 4 MeV and would easily have been detected in the present experiment. The estimate of the yield of photodeuterons from the present experiment is considerably smaller than that from the grain-counting experiments. The explanation is that the grain-counting method is not well adapted for the present problem. Surface effects in the emulsions cause variations of the grain density in the tracks, ~hich easily disturb the results and make them hard to reproduce. The errors in the i ~~asurements are so great that the reported fluctuations in the yield of the photodeuterons from element to element might be statistical deviations and not an effect of the nuclear structure. In the present investigation no difference in the yield of photodeuterons from the adjacent elements cobalt and copper is observed.
~82
B E N G T FORKMAN
The present investigation may also be compared with the activity measurements of the daughter nuclei in the 2 MeV region above the ()p, d) threshold. Katz and Penfold 5) have with this method reported a cross-section curve for the reaction S32(y, d)P3°. From this curve and from the total absorption crosssection curve 29) knowing the (~, p) and (y, n) branching ratios from statistical calculations an Rap of 0.02 is obtained which is about 10 times higher than the value predicted from the statistical model. Such a discrepancy m a y be explained by errors in the measurements of the (y, d) and the total absorption cross sections and in the estimates of the terms involved in the calculations. Still, however, it is difficult to explain the experimental result, that the photodeuteron emission starts just over the threshold of the reaction. According to the statistical model there will be no photodeuterons with an energy less than 1 MeV because of the coulomb barrier. Even in a pick-up process there must exist a barrier that keeps the photodeuteron emission down in the first MeV region above the threshold. Ferrero et al. 35) have repeated tile experiment of Katz and Penfold. In a first experiment they got some activity below 21 MeV but this was shown to originate from a small oxygen contamination in the sulphur target 36). 015 has a half-life nearly like the half-life of p30. In a second experiment with a fairly pure sulphur target the activation curve from p3o started at about 22 MeV well above the (V, np) threshold. Perhaps this is the explanation of the discrepancy between the Katz-Penfold experiment and the statistical model. The S~o Paulo group have in two papers 6, 3~) reported an anomalous yield of photodeuterons. They used the activation technique, too, and studied S32, Zn 84, Zn G~and Fe 54. Their results from sulphur are in agreement with those of Katz and Penfold but presumably the explanation of the discrepancy between these and the statistical model is the same. In the case of zinc the results from the S~o Paulo group and the Ztirich group 3s) are inconsistent. The S~o Paulo group reports a cross section of the reaction Zn66(y,ndp)eU64 that is about a hundred times higher than the value reported by Hofmann and Stoll. This discrepancy must be explained before an anomalous yield of photodeuterons from zinc is claimed. In the case of iron a much lower yield of the reaction (~, dp) was found than for zinc. In Ljubljana 39) recently a study has been done of the (),, ap) reaction on K a" with the same activation technique. No activity originating from photodeuterons was observed in the energy region between the (),, d) and the (y, np) thresholds. As is seen criticism can be directed against the experiments that report an anomalous yield of photodeuterons using the grain-counting method or the activation technique. It is very difficult to get information about the photodeuteron yield with these methods. Still there m a y exist an emission of photodeuterons by a pick-up mechanism but at a maximum bremsstrahlung irra-
THE PHOTODEUTERON YIELD
283
diation energy of about 30 MeV the cross section for such a process is of the order of or less than the cross section for the evaporation of a deuteron. It is difficult to compare the present results with the experiments at much higher irradiation energies as in these only photodeuterons of high energies have been studied. Smith and Laslett 7) irradiated a thick copper target with 65 MeV maximum bremsstrahlung. They separated deuterons from protons by a magnetic cloud chamber and got an Rap of 0.76 which is surprisingly high and hard to understand. Ring s) used the same experimental equipment and irradiated sulphur. He got an Rap of 0.15. Both the results are considerably higher than those obtained by Chizhov 10) with a telescope arrangement. He studied charged photo-particles in the energy range of 15.5--30 MeV and reports that the ratio Rap increases slowly with the mass number and goes from 0.02 for S to 0.07 for Au. No fluctuations in Rap from element to element is obtained. Only the lightest elements show deviations from this trend and have ratios of the same magnitude as the heaviest. Both the mass-dependence of Rap and the angular distribution of the photodeuterons is shown to be explained by the assumption that the photodeuterons are formed in a pick-up mechanism. The emission of photodeuterons formed in such a mechanism should increase with the irradiation energy as more combinations will be accessible for the formation of the deuteron. Chizhov has shown experimentally that the cross section for the (y, d) reaction in Li 8 is strongly dependent on the irradiation energy. The energy dependence of the (7, d) cross section is confirmed by the telescope measurements at 300 MeV maximum bremsstrahlung irradiation 12,13,1~). The ratio Rap still increases continuously with the mass number but is considerably higher than at 90 MeV and now goes from 0.12 for C to 0.24 for Pb. The yield ratio of photodeuterons to photoprotons with the same energy is experimentally found to increase when the energy of the compared photoparticles decreases 9), however this ratio must go through a maximum. At photo-particle energies below about 10 MeV the evaporated photoprotons will effect the ratio and press it down. Both this effect depending on evaporated photoprotons and the energy dependence of the (7, d) cross section cause very low Rap values at irradiation energies around 30 MeV. This is verified in the present experiment. The telescope experiments, the Iowa group results with nuclear emulsions and the present ones together, give knowledge of how the photodeuterons behave. The author wishes to express his deep gratitude to Dr. S. A. E. Johansson for m a n y helpful discussions and for advice during the course of this experiment. The author also wants to thank Dr. C. E. FrSberg for carrying out the coding of the calculations on SMIL.
284
BENGT
FORKMAN
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39)
J. S. Levinger, Nuclear Photo-disintegration (Oxford University Press, 1960) P. R. Byerly, Jr., and W. E. Stephens, Phys. Rev. 83 (1951) 54 B. Forkman, Ark. f. Fysik 11 (1956) 265 J. J. Grant, H. A. Medicus, J. Demers and P. Demers, Photographie Corpusculaire II. ed. by P. Demers (Les Presses Universitaires de Montrdal, 1959) p. 255 L. Katz and A. S. Penfold, Phys. Rev. 81 (1951) 815; 83 (1951) 169 J. Goldemberg and L. Marquez, Nuclear Physics 7 (1958) 202 W. H. Smith and L. J. Laslett, Phys. Rev. 86 (1952) 523 L. S. Ring, Jr., Phys. Rev. 99 (1955) 137 V. P. Chizhov and L. A. Kul'chitskii, J E T P 36 (1959) 345 V. P. Chizhov, J E T P 38 (1960) 809 E. D. Makhnovskii, J E T P 36 (1959) 739 J . F . DeWire, A. Silverman and B. Wolfe, Phys. Rev. 02 (1953) 519 D. A. Edwards, B. Wolfe, A. Silverman and J. W. DeWire, Phys. Rev. 95 (1954) 629 J. Rosengren and R. Wertheim, M.I.T. Ann. Progr. Rep., May 31 (1954) 128 J. Sawicki and W. Czy~, Nuclear Physics 4 (1957) 248, 695 B. Forkman and S. A. E. Johansson, Ark. f. Fysik (in press) G. Ho, E. L. Iloff and D. J. Zaffarano, Nuclear Physics (in press) F. Heinrich, H. W~ffler and M. Walter, Helv. Phys. Acta 29 (1956) 3 B. ~ujec-Dobovi~ek, Congr. Intern. de Physique Nucldaire (1958) 634 M. E. Toms and W. E. Stephens, Phys. Rev. 95 (1954) 1209 N . V . Lin'kova, R. M. Osokina, B. S. Ratner, R. Sh. Amirov and V. V. Akindinov, J E T P 38 (1960) 780 F. Bobard, G. Boul~gue and P. Chanson, Comptes Rendus 244 (1957) 1761 M. E. Toms and J. McElhinney, Congr. Intern. de Physique Nucldaire (1958) 660 F. Heinrich and H. W~ffler, Helv. Phys. Acta 29 (1956) 232 T. Ericson, Nuclear Physics 11 (1959) 481 J. M. Blatt and V. F. Weisskopf, Theoretical Nuclear Physics (John Wiley and Sons, New York, 1952) H. W. Fulbright, N. O. Lassen and N. O. Roy Poulsen, Mat. Fys. Medd. Dan. Vid. Selsk. 31, No. 10 (1959) R. M. Eisberg, G. Igo and H. E. Wegner, Phys. Rev. 100 (1955) 1309 J. Kockum and N. Starfelt, Nuclear Instruments and Methods 5 (1959) 37 A. I. Berman and K. L. Brown, Phys. Rev. 96 (1954) 83 M. M. Shapiro, Phys. Rev. 90 (1953) 171 S. A. E. Johansson, Phys. Rev. 97 (1955) 434 J. H. Carver and G. A. Jones, Nuclear Physics 11 (1959) 400 H. A. Medicus (private communication) F. Ferrero, S. Ferroni, R. Malvano, S. Menardi and E. Silva, Nuclear Physics 15 (1960) 436 R. Malvano, (private communication) S. S. Villa~a and J. Goldemberg, An. Acad. Bras. Ciencias 27 (1955) 427 A. Hofmann and P. Stoll, Helv. Phys. Acta 31 (1958) 591 P. Horvat, J. Pahor and M. Vakselj, Nuclear Physics 16 (1960) 90
I
Nuclear Physics 23 (1961) 269--284;~)North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE PHOTODEUTERON BREMSSTRAHLUNG BENGT
YIELD F R O M 30 MeV IRRADIATION
FORKMAN
Department of Physics, University o/ Lurid, Lund, Sweden Received 28 N o v e m b e r 1960
Abstract:
A n i n v e s t i g a t i o n of t h e (F, d) r e a c t i o n in s u l p h u r , c o b a l t a n d copper h a s been carried out. B r e m s s t r a h l u n g of 30 M e V m a x i m u m e n e r g y w a s used. T h e e m i t t e d c h a r g e d p h o t o p a r t i c l e s were deflected b y a m a g n e t i c field a n d recorded in n u c l e a r emulsions. T h e r a n g e a n d t h e o r i e n t a t i o n of t h e t r a c k s in t h e e m u l s i o n s were m e a s u r e d a n d t h e trajectories of t h e p h o t o particles were c a l c u l a t e d b a c k w a r d s for p r o t o n s , d e u t e r o n s a n d a l p h a particles. T h e intersection p o i n t of e a c h t r a j e c t o r y w i t h t h e t a r g e t p l a n e g a v e i n f o r m a t i o n a b o u t t h e kind of p h o t o - p a r t i c l e w h i c h h a d p r o d u c e d t h e track. T h e ratio Rap b e t w e e n t h e (F, d) yield a n d t h e (F, P) yield w a s d e t e r m i n e d . No s t r o n g f l u c t u a t i o n in t h e ratio b e t w e e n t h e a d j a c e n t e l e m e n t s c o b a l t a n d copper w a s observed. T h e ratios f o u n d were all zero for t h e t h r e e e l e m e n t s s u l p h u r , c o b a l t a n d copper, w i t h a s t a n d a r d d e v i a t i o n of r e s p e c t i v e l y 0.009, 0.012 a n d 0.018. A calcul a t i o n b a s e d o n t h e s t a t i s t i c a l m o d e l of t h e ratio Rap w a s m a d e . T h e ratios o b t a i n e d are, w i t h i n t h e limits of t h e errors, in a g r e e m e n t w i t h t h e e x p e r i m e n t a l results. T h e p r e s e n t r e s u l t s also were c o m p a r e d w i t h t h e p i c k - u p process t h e o r y , w i t h earlier (7, d) e x p e r i m e n t s a n d w i t h t h e i n v e r s e reaction. T h e o b t a i n e d ratio b e t w e e n t h e (~, a) yield a n d t h e (7, P) yield was in a g r e e m e n t w i t h earlier (F, co) e x p e r i m e n t s .
1. I n t r o d u c t i o n
During a long time the (y, d) reaction has been a point of question in connection with photonuclear reactions 1). Both the statistical model and the direct interaction model predict that photodeuterons should be quite rare compared to photoprotons. However, a number of investigators that have used bremsstrahlung with a maximum energy of about 30 MeV have observed the existence of photodeuterons. The reported yields of photodeuterons vary considerably from element to element. Especially for copper a high yield was obtained, which amounted to about a hundred times the yield predicted from the statistical model 2,3, 4). Photodeuterons have also been observed using sulphur ~) and zinc 6). It is, however, difficult to separate deuterons from protons, especially if they are of low energy. Different techniques have been used to attack the problem. In the case of copper the deuterons and the protons have been recorded in nuclear emulsions, and the separation has been done b y grain-counting of the tracks. As the tracks are short a good separation is hard to obtain. In the case of sulphur and zinc the photodeuteron yield has been determined by measuring the activity of the daughter nuclei in the 2 MeV region above the 269
270
BENGT
FORKMAN
(y, d) threshold. Above this region the (y, np) reaction will compete and it is therefore difficult to measure the (y, d) yield only. As the accessible energy region is small this method too gives only crude information about the (7, d) reaction. Several investigators, that have used a maximum bremsstrahlung energy of about 80 MeV report, too, the existence of photodeuterons. In copper ~) and sulphur s) the photodeuterons have been separated from tile photoprotons b y a cloud chamber traversed b y a magnetic field. The method shows a good resolution of protons and deuterons. Again a surprisingly high yield of photodeuterons from copper is reported. The irradiated targets were thick and tile cloud chamber favoured high-energy particles. Therefore it is difficult to compare the results with those from the 30 MeV irradiation. The cloud-chamber experiments can, however, be compared with a recently made investigation which thoroughly studied a lot of elements, separating the photodeuterons from tile photoprotons b y a two-crystal telescope 9, 10). This method shows, too, a good resolution of protons and deuterons but studies only photodeuterons down to 15 MeV. The yields of photodeuterons are considerably smaller than those obtained with the cloud chamber. No strong fluctuations in the yields are observed. The yield increases continuously with mass number. Even these results are difficult to compare with those from the 30 MeV irradiation as the photodeuterons are of high energy. An attempt has been made to study the low-energy photodeuterons from gold n) b y the method of grain-counting but again it is difficult to obtain a good resolution with this method. The yield of photodeuterons has been investigated also with a maximum bremsstrahhing energy of 300 MeV 12,13,14).The high-energy deuterons have been studied b y a two-crystal telescope. In this energy range too, a continuous increase of tile yield with the mass number has been observed. The ratio of the photodeuteron to the photoproton yield was here about 4 times larger than tile corresponding ratio obtained with the telescope investigation at the lower irradiation energy. In order to explain the high yield of photodeuterons it has been assumed that the deuterons are formed in a two-stage process. The incident photon is absorbed b y a single nucleon which in its turn picks up another nucleon with the right charge and momentum and forms an outgoing deuteron. Such a pick-up process must occur on the surface of the nucleus. A semi-quantitative calculation of this pick-up process has been made b y Sawicki and Czy~ 15). The aim of this work has been to investigate the photodeuteron yield from some elements with a maximum bremsstrahlung energy of 30 MeV using a new technique worked out in Lund 16) that is able to separate single deuterons from protons down to 2 MeV. Fluctuations in the yield were looked for. It was also tried to test the proposed theoretical model experimentally. The same
THE
PHOTODEUTERON
271
YIELD
technique has recently been used b y the Iowa group with a -~aximum bremsstrahlung energy of 45 MeV 17). It is well known that the ()', ~) reaction in medium weight nuclei is satisfactorily explained b y the statistical model 18). This reaction is much easier to study than the ()', d) reaction, since the alpha particles are easy to separate from singly charged particles and because the stability of the alpha particle ensures the reliability of the activity measurement of the daughter nucleus. In the present work the (y, ~) reaction has been studied, since the power of the experimental arrangement to separate photo-alpha particles is a proof of its stfitability for observing photodeuterons. The observed ()', ~) yields have been compared with earlier measurements.
2. Experimental Arrangement and Measurements In order to obtain a good separation between protons and deuterons a magnetic field was used to deflect the particles. The photo-particles were recorded in nuclear emulsions. The method is fully described in ref. 16). The experimental arrangement is shown in fig. 1. As the source of the photons a 30 MeV electron synchrotron was used. The machine was energy stabilized and was run with maximum energy. The ),-beam was collimated b y a conic lead collimator and passed then through an evacuated camera. In the camera the photons hit a thin target. The diameter of the ?,-beam at the position of the target was 1.5 cm. Three different targets were used. At the first exposure a copper foil rolled out to 10 #m was irradiated. The second exposure was made without target in order to determine the background. Next a sulphur target 34 #m thick, obtained b y allowing sulphur to evaporate on a 3 ,urn thick gold foil, was irradiated. At the fourth exposure a cobalt foil, 10/zm thick, produced b y electrolysis on a Lead collimator
/
Synchr'ot ron
Boron-paraffin
:~ /'~
Ionization
Nuclear plates
L
0
I
10
;0
I
30
4;
emulsion
510 cm
Fig. 1. E x p e r i m e n t a l a r r a n g e m e n t to a n a l y z e photo-particles. T h e e v a c u a t e d c a m e r a was placed in t h e g a p of a n e l e c t r o m a g n e t w i t h t h e m a g n e t i c field parallel w i t h t h e z-axis. T h e s c a n n e d a r e a in t h e e m u l s i o n s is s h a d e d in t h e figure.
272
BENGT
FORKMAN
gold foil, also 3 / t m thick, was used. The camera was placed in the gap of an electromagnet. The current to the electromagnet was stabilized and gave a constant magnetic field of 1.5 W b/ m ~. Two Ilford C2 emulsion plates with the size 5.1 cm by 10.2 cm were used to record the photo-particles. The two plates were mounted oblique to each other with an opening angle of 23 °. In order to shield the nuclear plates from background radiation the camera was covered with lead and the whole magnet was surrounded with boron-paraffin blocks. Furthermore a lead wall was piled up between the camera and the synchrotron. The camera was watercooled in order to keep the nuclear plates at a safe temperature. The total dose given to the targets at the different exposures amounted to about 3000 r. In order to test that the experiment was not disturbed by fast neutrons, a phosphorus sample was placed in the beam at the target position. No neutron induced phosphorus activity was observed. As already has been mentioned a background exposure was performed without any target. Unexpectedly m a n y background tracks were obtained in the plates. These originated from the entrance window of the camera which was a 50/~m thick Al-foih The background from the gold foil in the suphur and cobalt exposures was negligible both through the thinness of the foil and through the high coulomb barrier for proton and deuteron emission. The nuclear emulsions were developed in the usual way and were scanned and measured by a Leitz microscope. The entrance co-ordinates, the length and the orientation of the tracks were measured. As the position of the nuclear plates in relation to the target was known and also the strength of the magnetic field, it was possible to calculate backwards the trajectories of the photoparticles from the target to the emulsion. Such a calculation was made first assuming th at all the recorded particles were protons and then assuming t hat the particles were deuterons. Finally a calculation was made assuming them to be alpha particles. These calculations were performed using SMIL (the University of Lu n d digital computer). The co-ordinates of the intersection points between the trajectories of the particles and the plane of the target were calculated. In the calculations a right-handed system of co-ordinate axes was placed so that the y-axis was parallel with the y-beam and the z-axis parallel with the direction of the magnetic field. The co-ordinate system is drawn in fig. 1. The length of the target in the x-direction was 2.0 cm and in the z-direction 2.8 cm. In the chosen co-ordinate system the centre of the target had the co-ordinates x = -- 10.1 cm, y = -- 10.9 cm and z = 0.0 cm. The errors in the measurements of the dip of the tracks give an uncertainty in the z co-ordinates (zt) of the intersection points. The distribution of the zt values of the points in the interval -- 8.4 > x t > -- 11.6 is shown in fig. 2. Owing to the broadness of this distribution all measured tracks giving a ]ztl < 3.0 have been accepted in the following analysis.
THE
PHOTODEUTERON
273
YIELD
The nuclear plates were scanned in the area shaded in fig. 1. To this area only those photo-particles came t h a t h a d a small angle Y with the p h o t o n direction. Fig. 3 shows the distribution of the angle Y of the particles recorded in the plates. In the nuclear emulsions all tracks h a v i n g a length more t h a n 30 a m were measured. This corresponds to a p r o t o n e n e r g y of 1.6 MeV, a d e u t e r o n e n e r g y of 2.1 MeV and an alpha particle e n e r g y of 6.5 MeV. However, in the area m e a s u r e d only protons with an energy above 2.5 MeV will be recorded. P r o t o n s with lower energies will cut the c a m e r a wall before t h e y reach the plates. The efficiency goes up v e r y steeply a n d above 3 MeV all protons will be recorded o o
o. 150. E
o
?-
~6 xa
E
E i0c
\
_p-
o
/ _~ [~
_~
G
~
i ~
Z¢:
50l
100-
°0" (cm)
Fig. 2. Experimental distribution from the cobalt exposure of the z co-ordinates zt of the intersection p o i n t s of t h e p r o t o n t r a j e c t o r i e s w i t h t h e p l a n e o f t h e foil f a l l i n g i n t h e i n t e r v a l - - 8 . 4 > x t > - - 1 1 . 6 . T h e b a c k g r o u n d is s u b t r a c t e d . T h e c e n t r e o f t h e t a r g e t h a s t h e z c o - o r d i n a t e z t = 0.0.
L
10"
Go"
90' Y
Fig. 3. Experimental distribution from the cobalt exposure of the angles Y of the emitted photoprotons with the y-axis.
with the same efficiency. The corresponding value for the deuterons is 2 MeV a n d for alpha particles 3 MeV le).
3. R e s u l t s T h e value of the x co-ordinate (xt) of the intersection point between the calculated t r a j e c t o r y a n d the plane of the t a r g e t depends on w h e t h e r the t r a c k in the emulsion is assumed to originate from a proton, a d e u t e r o n or an alpha particle. In one of these cases x t m a y fall in the region of the target and it will be possible to i d e n t i f y the t r a c k as t h a t of a proton, a d e u t e r o n or an
274
BENGT FORKMAN
alpha particle. The alpha tracks, however, m a y also be distinguished directly from singly charged particle tracks as the alpha tracks are denser. Figs. 4, 5 and 6 show the experimental distribution of x t from the three main exposures after the background has been subtracted. In section A of the figures it is assumed that all the singly charged particle tracks are due to photoprotons, while in section B the assumption is made that the same tracks are due to photodeuterons. There are two reasons, w h y t h e number of points in sections A and B is not the same. The first is that the computer rejected trajectories which cut the walls Cobalt
'11
E
.,n
"6
Sulphur
20
E o
.~
E
z
0
i
20
z
r-'-I i-I
100
100
80
80.
60.
60-
40
40.
20.
20-
0
~
0
~
,-r'l._~, -5
] ~
f~f-"~ -I0
0 -I'5 x t (cm)
Fig. 4. E x p e r i m e n t a l d i s t r i b u t i o n from t h e s u l p h u r e x p o s u r e of the x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t falling in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. The b a c k g r o u n d is s u b t r a c t e d . I n section A all t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s are a s s u m e d to o r i g i n a t e from protons. hx s e c t i o n B t h e y are a s s u m e d to o r i g i n a t e from deuterons. I n section C all t h e dense t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . The c e n t r e of t h e t a r g e t h a s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 13.2 cm 2.
0
~ - 5
-10
-15
X t (cm)
Fig. 5. E x p e r i m e n t a l d i s t r i b u t i o n from t h e c o b a l t e x p o s u r e of t h e x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t fa l l i ng in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. T h e b a c k g r o u n d is s u b t r a c t e d . I n s e c t i o n A a l l t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s a re a s s u m e d to o r i g i n a t e from prot ons . I n sectioi1 B t h e y a re a s s u m e d to o r i g i n a t e from d e u t e r o n s . Iix s e c t i o n C all t h e de ns e t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . The c e n t r e of t h e t a r g e t ha s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 18.0 c m ~.
THE PHOTODEUTERON YIELD
~75
of the camera. The second is t h a t m a n y of the tracks h a d rz nges which corresponded to such high d e u t e r o n energies, t h a t t h e y were in conflict with the law of e n e r g y conservation. In the d e u t e r o n calculations such tracks were rejected. In section C the intersection points of the alpha tracks are shown. Section A of all the figures shows a n a r r o w distribution of the n u m b e r of intersection points a r o u n d the centre value x t = --10.1 of the targets. These points therefore are starting points for photoprotons. Such peaks are not observed in the sections B. Copper
10 c D.
C 0
~
-~ 20, z 10,
0l
~
r
r ~ r ]
I
S
A
40-
30.
20"
10-
O. ~ _ . . _ . _ ~ : : z _ ~ _ _ 0
-5
-10
-I 5
,4"t (cm)
Fig. 6. E x p e r i m e n t a l d i s t r i b u t i o n from t h e c o p p e r e x p o s u r e of t h e x c o - o r d i n a t e s of t h e i n t e r s e c t i o n p o i n t s of t h e t r a j e c t o r i e s w i t h t h e p l a n e of t h e t a r g e t f a l l i n g in t h e i n t e r v a l - - 3 . 0 < z t < 3.0. The b a c k g r o u n d is s u b t r a c t e d . I n s e c t i o n A all t h e s i n g l y c h a r g e d p a r t i c l e t r a c k s are a s s u m e d t o o r i g i n a t e fro m p r o t o n s . I n s e c t i o n B t h e y are a s s u m e d to o r i g i n a t e from d e u t e r o n s . I n s e c t i o n C all t h e dense t r a c k s are a s s u m e d to be a l p h a p a r t i c l e t r a c k s . T h e c e n t r e of t h e t a r g e t h a s t h e x c o - o r d i n a t e x t = - - 1 0 . 1 . The a r e a s c a n n e d is 9.8 cmz.
Only v e r y few tracks give intersection points t h a t fall a r o u n d x t = --10.1. In sections C again the points originating from alpha particles are c o n c e n t r a t e d a r o u n d x t = -- 10.1. In fig. 7 the b a c k g r o u n d is shown in the same way. It m a i n l y consists of fast p h o t o p r o t o n s originating from the entrance window. The position of the b a c k g r o u n d peak accidentally falls on almost the same place as the peak of the p h o t o p r o t o n s from the main exposures. Fig. 7B shows t h a t the b a c k g r o u n d
276
BENGT
~ORKMAN
f r o m the e n t r a n c e window does not d i s t u r b the s t u d y of p h o t o d e u t e r o n s . T h e few b a c k g r o u n d points f r o m x t = 0 d o w n to x t = - - 12 o r i g i n a t e d m o s t l y f r o m such recoil p r o t o n s t h a t are f o r m e d in the emulsions a n d pass t h r o u g h the surface of the emulsions. I n m o s t cases t h e y can be sorted out, as t h e y do not h a v e a characteristic t r a c k end, b u t for l o w - e n e r g y p r o t o n s this s e p a r a t i o n is h a r d to perform. No b a c k g r o u n d points originating f r o m a l p h a particles were obtained. The energy distributions of the p h o t o p r o t o n s from the different exposures t h a t h a v e an x t value in the i n t e r v a l - - 8 . 4 > x t > --11.6 are shown in fig. 8. T h e distributions are c o r r e c t e d for the b a c k g r o u n d . T h e p h o t o p r o t o n distributions f r o m sulphur, cobalt a n d c o p p e r h a v e been r e p o r t e d earlier with m u c h Background B
.5 20-
o
z
r-'ra--Y~ ,
f-h A
30-
20
0:
----a r~ [7
,, -5
"~ -1'0
-1'5
Fig. 7. E x p e r i m e n t a l distribution from the b a c k g r o u n d exposure of the x co-ordinate of the intersection points of the trajectories w i t h the plane of the t a r g e t failing in the interval - - 3 . 0 < zt < 3.0. In section A all the singly charged particle tracks are a s s u m e d to originate from protons. I n section B t h e y are a s s u m e d to originate from deuterons. No dense tracks gave intersection points in the x region s h o w n in the figure when. t h e y were a s s u m e d to be a l p h a particle tracks. The area scanned is 7.1 cm z.
b e t t e r statistics 19,2o,21). T h e d a s h e d h i s t o g r a m s d r a w n in the figure are the expected distributions. I t is e v i d e n t t h a t the distributions here f o u n d are in agreem e n t with the e x p e c t e d ones. This a g r e e m e n t shows t h a t the present investigation does not f a v o u r a n y p r o t o n energy. T h e figure gives also the energy distrib u t i o n of the b a c k g r o u n d tracks. T h e figs. 4, 5 a n d 6 give the following ratios R0p b e t w e e n the (7, d) yield and the (7, P) yield with the c o r r e s p o n d i n g s t a n d a r d d e v i a t i o n s s: S: Co: Cu:
Rap = 0.000, Rap = 0.000, Rop = 0.000,
s = 0.009; s = 0.012; s = 0.018.
THE
PHOTODEUTERON
YIELD
277
The present experiment cannot settle a lower limit of the ratio Rdp as no photodettterons with certainty have been observed in the three exposures. Fig. 9A shows tile sum of the three x t co-ordinate distributions with the assumption t h a t all the singly charged particle tracks were deuteron tracks. This has been done in order to get better statistics. In the figure the background is not subtracted. It is still impossible to observe a peak around x t = --10.1 above
0
4O
Background
i
20:
f~
O~ Copper
E 4oi Z
20 0 120
Cobalt
100~ 80 r-
60 40 ! 20 0 120 100 80
F•
Sulphur
-q
60
L~--
-\-,
~0 20 0
0
5
10
15 20 Proton energy (MeV)
Fig. 8. E n e r g y d i s t r i b u t i o n of t h e o h o t o p r o t o n t r a c k s g i v i n g i n t e r s e c t i o n p o i n t s i n tile a r e a - - 8 . 4 > x t > - - 1 1 . 6 a n d - - 3 . 0 < zt - 3.0. The l o w e s t s e c t i o n s h o w s t h e p h o t o p r o t o n s from t h e s u l p h u r e x p o s u r e . T h e d a s h e d line is t h e e x p e c t e d distrlL,~ltion from ref. 19). The second s e c t i o n s how s t h e p h o t o p r o t o n s f r o m t h e c o b a l t exposure. The d a s h e d line is t h e e x p e c t e d d i s t r i b u t i o n f r o m ref. ,0). T h e t h i r d s e c t i o n s h o w s t h e p h o t o p r o t o n s f r o m t h e c o p p e r e x p o s u r e . T h e d a s h e d line is t h e e x p e c t e d d i s t r i b u t i o n from ref. =1). The b a c k g r o u n d is s u b t r a c t e d in t h e s e t h r e e sections, b u t s h o w n in t h e u p p e r m o s t section. I t o r i g i n a t e s f r o m t h e e n t r a n c e w i n d o w .
~8
BENGT FORKMAN
t h e b a c k g r o u n d level. T h e figure gives for t h e r a t i o :
Y,Y (7, d) -
0.000,
-
with
s = 0.007.
~ Y (X, P) T h e r e f o r e t h e u p p e r limit is p r o b a b l y n o t m o r e t h a n 0.007. I n fig. 9B all t h e t r a c k s o r i g i n a t i n g b o t h f r o m s i n g l y a p 4 d o u b l y c h a r g e d particles were a s s u m e d to be a l p h a t r a c k s if t h e y h a d r a n g e s c o r r e s p o n d i n g to a l p h a - p a r t i c l e energies b e t w e e n 6.5 a n d 11.0 MeV. N o w a d i s t i n c t g r o u p of ul
E
20
X "~ .o K z
0 20 0 160 140 120 100 80 60 40 20 0 0
-5
-10
-1'5
X~ (cm)
Fig. 9. The sum of the experimental distributions from the three main exposures of the x co-ordinate of the intersection points of the trajectories with the plane of the target falling in the interval --3.0 < zt < 3.0. In section A all the singly charged particle tracks are assumed to originate from deuterons. The background is not subtracted. In section B all the tracks originating both from singly and doubly charged particles are assumed to be alpha particles. The background is not subtracted. In section C only the dense tracks are assumed to be alpha particle tracks. No background exists. p o i n t s a r o u n d x t = - - 10.1 is o b s e r v e d a b o v e t h e b a c k g r o u n d level s h o w i n g t h e e x i s t e n c e of p h o t o - a l p h a particles. I n fig. 9C t r a j e c t o r i e s were c a l c u l a t e d o n l y for t h o s e t r a c k s t h a t d u r i n g t h e s c a n n i n g of tile plates were r e c o g n i z e d as a l p h a t r a c k s . I n this s e c t i o n no b a c k g r o u n d exists, h o w e v e r t h e y i e l d of a l p h a particles d e t e r m i n e d f r o m figs. 9B a n d 9C is t h e same. I t is e v i d e n t t h a t t h e (y, d) y i e l d m u s t be less t h a n t h e (7, ~) yield. O t h e r w i s e a g r o u p of p o i n t s at t h e p o s i t i o n of t h e t a r g e t similar to t h a t f o u n d in fig. 9B s h o u l d h a v e b e e n o b s e r v e d in fig. 9A.
THE
PHOTODEUTERON
YIELD
279
Only alpha tracks with energies more than 6.5 MeV were measured. An 8 MeV alpha particle loses about 1.5 MeV when it passes through half the thickness of the cobalt or copper foil. The energy distribution of the emitted photoalpha particles from copper using a maximum bremsstrahlung energy of 30.5 MeV is peaked around 8 MeV 22). It is therefore evident that the lower energy part of the photo-alpha particles are lost in the present experiment. In order to get an estimate of the ratio R=, between the (y, ~) yield and the (y, p) yield, the number of observed alpha tracks in the case of cobalt and copper has been corrected for this loss. In the case of sulphur too m a n y alpha particles have been lost to get a fair estimate. The present experiment gives the following ratios: Co: R=p ----- 0.09;
Cu: R=p ---- 0.05.
These ratios may be compared with the reported ratios for cobalt at 21.5 MeV bremsstrahlung 20, 23) R=p = 0.047 and for copper at 24 MeV bremsstrahlung 2) R=p --~ 0.040. The ratio R=p will rise with the irradiation energy. The present ratios therefore are in agreement with earlier (y, =) experiments. All the identified alpha particles had energies between 6.5 and 10.5 MeV in agreement with the known energy distribution. 4. D i s c u s s i o n
It is well known i) that the statistical model satisfactorily gives an account of the energy distribution of the emitted photoneutrons. This is also the case for photoprotons from light and medium weight nuclei. For these nuclei the statistical model also predicts the yield of photoprotons compared with the yield of photoneutrons. The experimental deviations from the statistical model for heavy nuclei can be explained in terms of the direct photo-effect. The experimental photo-alpha particle yield is) and the photo-triton yield 24) follows also the statistical theory. It will therefore be of interest to compare the present results with this theory. To perform such a calculation it is necessary to know the level density of the daughter nuclei. It has been shown that low-energy nuclear temperatures are independent of the excitation energy 25). At higher excitation energies, however, the nuclei can be described as a Fermi gas and the excitation energy will be proportional to the temperature squared 26). The level density will therefore have one of the forms
p(E) =
E const x e x p -~
(low-energy excitation),
(1)
(high-energy excitation),
(2)
where T is a constant, or
P (E) = const x exp V'~E where ~ is a constant.
280
BENGT FORKMAN
The ratio Rap is sensitive to which of these formulas is used as the thresholds for the reactions (7, P) and (~, d) differ considerably. In the copper region T seems to be constant up to 10 MeV having a value of 1.16 MeV 27). At excitation energies between 10 and 20 MeV the level density looks more like eq. 2 with a value for ~ of 5.56 MeV -1 2s). In the sulphur region T has the value 1.6 MeV 25). As the level density in sulphur is lower than in copper at the same energy, eq. 1 should be expected to be valid for sulphur in the whole energy range of the present experiment. In the calculation of the ratio it has been taken into account that there is a difference in the total number of states for even, odd, and odd mass nuclei at a constant energy. There exists a close connection between the difference and the pairing energy 25). The calculation is based on the experimental cross-section curves for sulphur 29) and copper 3o) and Shapiro's penetrability values 31). Using eq. 1 the following ratios are obtained: $32:
Rap =
3 × 10-3;
Cu63: Rap = 6 × 10-4.
Using eq. 2 we have C u 63:
Rap =
7×10
-a.
In the case of copper the estimate from eq. 2 should be more reliable as the level densities in the copper region given by eqs. (1) and (2) with the experimental constants agree fairly well at low excitation energies, and eq. (2) is the best at high energies. Hence the statistical model predicts an Rap of 0.003--0.007 when the maximum bremsstrahlung energy is 30 MeV. This Rap shall be compared with the present experimental result Rap < 0.007. The present results m a y also be compared with the estimation of the photodeuteron emission by a pick-up process calculated by Sawicki and Czyz 15). T h e y give for sulphur a ratio of the cross sections a(7, d)/a(7, p) = 0.1 for a photon energy of 25 MeV. The photoproton cross section is that of the direct process only. The ratio a(7 , d)/(a(y, p)dlrect@a(y, P)compounO) must be much lower. The deuteron dissociation within the nucleus m a y also suppress the deuteron yield. From a nucleus with N -----Z the direct dipole emission of a deuteron is forbidden since the nuclear charge-to-mass ratio is the same as that of the deuteron. Photodeuterons from light and medium weight nuclei must therefore mainly be formed in an evaporation or a pick-up process. The experimental results from the Iowa group 17) are in close agreement with the present ones. T h e y report that in copper Rap is less than 1.6-4-1 ~/o. T hey have measured the emitted photo-particles in the angle range between 30 ° and 80 ° from the incident beam direction and therefore cover a different angular region than the present investigation made in the forward direction. As the anisotropy in the angular distribution of the direct emitted photoprotons is strong and peaked around 60 ° 32) it should be expected t h a t non-
THE PHOTODEUTERON YIELD
281
statistical photodeuterons will more easily be observed in forward and backward directions. Chizhov lo) has shown that the emitted photodeuterons of high energy from light elements have an angular distribution which is peaked around 0 °, which further argues in favour of measurements at small angles. The inverse reactions often give valuable information about the photoprocesses. The radiative deuteron capture process Zn~4(d, ~)Ga 66 has recently been studied by Carver and Jones 33) using deuteron energies of about 4 MeV. The magnitude of the cross section was determined b y activity measurements of Ga 66. As the captured 7-ray spectrum is not known non-statistical processes may contribute to the cross section. However, statistical calculations of the cross section were in good agreement with the experiment showing that in the energy range studied an anomalous yield of photodeuterons is not expected. This supports the present investigation. Byerly and Stephens 2) were the first who stated the existence of photodeuterons. They found in the energy distribution of photoprotons from copper an extra peak at 2.5 MeV which they assumed to originate from deuterons. They reported that it is possible to separate single deuterons from protons b y graincounting the last 40 #m of the photo-particle tracks in nuclear emulsions and got in this way a ratio Rop of 0.314-0.09. However, the author 3) tried to repeat the experiment of Byerly and Stephens and showed that a separation of single deuterons from protons cannot be expected if common nuclear emulsions are used. By a statistical processing of the data Rop for Cu was estimated to 0.16~: ±0.08. Grant et al. 4) later used the same technique recording the tracks in fine grain emulsions. Still a separation of single deuterons from protons was not obtained. They report for Cu and Rdp of 0.214-0.05. However, they did not succeed in repeating this value but got a considerably lower Rop 34). The existence of the extra peak in the energy distribution of the photoprotons from copper have not been observed in a recent investigation b y Lin'kova et al. 21). Probably it does not exist but is a background effect. The assumed photodeuterons in the experiment of Byerly and Stephens should have an energy of about 4 MeV and would easily have been detected in the present experiment. The estimate of the yield of photodeuterons from the present experiment is considerably smaller than that from the grain-counting experiments. The explanation is that the grain-counting method is not well adapted for the present problem. Surface effects in the emulsions cause variations of the grain density in the tracks, ~hich easily disturb the results and make them hard to reproduce. The errors in the i ~~asurements are so great that the reported fluctuations in the yield of the photodeuterons from element to element might be statistical deviations and not an effect of the nuclear structure. In the present investigation no difference in the yield of photodeuterons from the adjacent elements cobalt and copper is observed.
~82
B E N G T FORKMAN
The present investigation may also be compared with the activity measurements of the daughter nuclei in the 2 MeV region above the ()p, d) threshold. Katz and Penfold 5) have with this method reported a cross-section curve for the reaction S32(y, d)P3°. From this curve and from the total absorption crosssection curve 29) knowing the (~, p) and (y, n) branching ratios from statistical calculations an Rap of 0.02 is obtained which is about 10 times higher than the value predicted from the statistical model. Such a discrepancy m a y be explained by errors in the measurements of the (y, d) and the total absorption cross sections and in the estimates of the terms involved in the calculations. Still, however, it is difficult to explain the experimental result, that the photodeuteron emission starts just over the threshold of the reaction. According to the statistical model there will be no photodeuterons with an energy less than 1 MeV because of the coulomb barrier. Even in a pick-up process there must exist a barrier that keeps the photodeuteron emission down in the first MeV region above the threshold. Ferrero et al. 35) have repeated tile experiment of Katz and Penfold. In a first experiment they got some activity below 21 MeV but this was shown to originate from a small oxygen contamination in the sulphur target 36). 015 has a half-life nearly like the half-life of p30. In a second experiment with a fairly pure sulphur target the activation curve from p3o started at about 22 MeV well above the (V, np) threshold. Perhaps this is the explanation of the discrepancy between the Katz-Penfold experiment and the statistical model. The S~o Paulo group have in two papers 6, 3~) reported an anomalous yield of photodeuterons. They used the activation technique, too, and studied S32, Zn 84, Zn G~and Fe 54. Their results from sulphur are in agreement with those of Katz and Penfold but presumably the explanation of the discrepancy between these and the statistical model is the same. In the case of zinc the results from the S~o Paulo group and the Ztirich group 3s) are inconsistent. The S~o Paulo group reports a cross section of the reaction Zn66(y,ndp)eU64 that is about a hundred times higher than the value reported by Hofmann and Stoll. This discrepancy must be explained before an anomalous yield of photodeuterons from zinc is claimed. In the case of iron a much lower yield of the reaction (~, dp) was found than for zinc. In Ljubljana 39) recently a study has been done of the (),, ap) reaction on K a" with the same activation technique. No activity originating from photodeuterons was observed in the energy region between the (),, d) and the (y, np) thresholds. As is seen criticism can be directed against the experiments that report an anomalous yield of photodeuterons using the grain-counting method or the activation technique. It is very difficult to get information about the photodeuteron yield with these methods. Still there m a y exist an emission of photodeuterons by a pick-up mechanism but at a maximum bremsstrahlung irra-
THE PHOTODEUTERON YIELD
283
diation energy of about 30 MeV the cross section for such a process is of the order of or less than the cross section for the evaporation of a deuteron. It is difficult to compare the present results with the experiments at much higher irradiation energies as in these only photodeuterons of high energies have been studied. Smith and Laslett 7) irradiated a thick copper target with 65 MeV maximum bremsstrahlung. They separated deuterons from protons by a magnetic cloud chamber and got an Rap of 0.76 which is surprisingly high and hard to understand. Ring s) used the same experimental equipment and irradiated sulphur. He got an Rap of 0.15. Both the results are considerably higher than those obtained by Chizhov 10) with a telescope arrangement. He studied charged photo-particles in the energy range of 15.5--30 MeV and reports that the ratio Rap increases slowly with the mass number and goes from 0.02 for S to 0.07 for Au. No fluctuations in Rap from element to element is obtained. Only the lightest elements show deviations from this trend and have ratios of the same magnitude as the heaviest. Both the mass-dependence of Rap and the angular distribution of the photodeuterons is shown to be explained by the assumption that the photodeuterons are formed in a pick-up mechanism. The emission of photodeuterons formed in such a mechanism should increase with the irradiation energy as more combinations will be accessible for the formation of the deuteron. Chizhov has shown experimentally that the cross section for the (y, d) reaction in Li 8 is strongly dependent on the irradiation energy. The energy dependence of the (7, d) cross section is confirmed by the telescope measurements at 300 MeV maximum bremsstrahlung irradiation 12,13,1~). The ratio Rap still increases continuously with the mass number but is considerably higher than at 90 MeV and now goes from 0.12 for C to 0.24 for Pb. The yield ratio of photodeuterons to photoprotons with the same energy is experimentally found to increase when the energy of the compared photoparticles decreases 9), however this ratio must go through a maximum. At photo-particle energies below about 10 MeV the evaporated photoprotons will effect the ratio and press it down. Both this effect depending on evaporated photoprotons and the energy dependence of the (7, d) cross section cause very low Rap values at irradiation energies around 30 MeV. This is verified in the present experiment. The telescope experiments, the Iowa group results with nuclear emulsions and the present ones together, give knowledge of how the photodeuterons behave. The author wishes to express his deep gratitude to Dr. S. A. E. Johansson for m a n y helpful discussions and for advice during the course of this experiment. The author also wants to thank Dr. C. E. FrSberg for carrying out the coding of the calculations on SMIL.
284
BENGT
FORKMAN
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39)
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