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The C13(d, n)N14 reaction
Nuclear Physics 24 (1961) 1 3 2 ~ I 3 7 ; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE Ct3(d, n)N 14 REACTION A. N. JAMES
Cavendish Laboratory, Cambridge Received 2 J a n u a r y 1961
Abstract: The time of flight method has been used to measure angular distributions and excitation functions for the neutrons emitted to form the ground and first six excited states of N x4 ill the C~3(d, n)N ~4reaction. The properties of the 4.91 and 5.69 MeV N 1. levels were consistent with tl~e weak coupling configuration (Cla2s½). The nature of the 5.10 and 5.83 MeV levels remains in doubt.
1. Introduction The properties of excited states of N ~4between 4: and 7 MeV are of interest in determining which states have shell model p~0 wave functions, and which states are formed predominantly by coupling a 2s or ld proton to a C13 core t). In the first category only a 2 + state is expected while four T = 0 states are expected in t he second. Warburton el al. ~) were able to make plausible the identification of ~hese four states, 0-, 2-, 1- and 3-, with the 4.91, 5.10, 5.69 and 5.83 N ~4 states. Their arguments were based on an analysis of gamma-rays from C l a + p ((2 ~ 7.55 MeV) and on data from C~8(d, n)N l*. Benenson has observed 1 = 0 stripping for the 4.91 MeV level ~) and Ranken et al. have measured relative gamma-ray yields a). Because of their unique parent character 4) these states are expected to be formed in C1~(d, n)N 14 with large stripping reduced widths, by I = 0 for the 0- and 1- states, and I = 2 for the 2- and 3- states. Since the Q value for these levels is low (see table 1), angular distributions typical of stripping reactions should be observed even at low deuteron energy 5).
2. Experimental Th~ time of flight apparatus of the Cavendish Laboratory's 1.4× 106 V Cockccoft-Walton ~,_netatol' ~ ~, ,~ ,6) has been used to measure angular distributions and excitation functions for the neutrons emitted to form the ground and first six excited sta,~es of N 14 in the Cla (d, n)N ~4 reaction. Scintillation counters incorporating neutron discriminating Nuclear Enterprise N.E. 213 liquid scintillator 7) were used with biases equivalent to neutron energies of 600 keV and 330 keV. For a bias of 600 keV and R.C.A. 7264 photomultiplier was used and gave a time resolution for the neutron groups in the time of flight spectrum of 6 ns. The lower noise current in an E.N. 1.6097 photomultiplier allowed a bias 132
THE C13(d, n)N 14 REACTION
133
of 330 keV with a neutron group time resolution s) of 8 ns. The flight paths were 3.5 and 1.5 m. The target was isotopically enriched carbon on 0.05 em copper. It was less than 20 keV thick to 1.16 MeV protons. The time of flight spectrum I
1
i'
4 ~3 .J UJ
;2
!
0
t !
I ,.
I
ANGLE
I
I
(C.O.a)
Fig. I. N e u t r o n a n g u l a r d i s t r i b u t i o n s in t h e centrc-of-ma.ss system formimz the gro~md s t ~ t e a n d first; e x c i t e d 2.,31 McV s t a t e (n 1) of N ~'t a t 1.'2 .'qeV d e u t e r o n energy.
(n~)
4
3
t n2
I
J hi ~'O
,,"r,, . . . . ~
,
or
0
I
J \
,
3o
,
-~0
,
_,__,__ 9o
A N G LE
J2o
(c.o.M)
ELi
i5o
~0o
Fig. 2. N e u t r o n a n g u l a r d i s t r i b u t i o n s in d i e centre-of-m~ss s y s t e m f o r m i n g tile 3.95 MeV (r@ ~md 5.10 M e V (n,} s t a t e s of N 1~ a t 1.2 MeV d e u t e r o n energy.
showed that the target contained significant amounts of Ct2 and dm!terium. Allowance was made for the cr"(d, n)N 13 and d(d, n)He 3 reactions in extracting the neutron intensities of die (;~:~(d,n)N ~ reaction..
A. N. J A M E S
I¸$4¸
Figs. 1, 2 and 3 show the measured angular distributions at 1.2 MeV deuteron energy. Errors arising from corrections for counter efficiency s) are less than i0 °/o within any one of the distributions. The relative intensities of the neutron
i!
Z-o
ro = 6,t
°°,: a o~
-
¢
0
-
nS÷nS÷CI2
J
Lu
>" 4 o
~
n6 contr" bution from CI2
~., I ~ 30Y-" '~_ ] ~
-
n5 / = 0 r°= 6"2
\ ,ot0
+ 2"
\ ao
60
90.
~aQ
ANm.E (C.O.M.)
iso
180
Fig. 3, N e u t r o n a n g u l a r d i s t r i b u t i o n s in the centre-of-mass system f o r m i n g t h e 4.91 MeV (n s) and 5.69 MeV (n s, see t e x t ) states of N '4 a t 1.2 MeV d e u t e r o n energy'.
groups forming the different levels are expected to be correct within 30 % (in particular the intensity ratio for groups n 3 and n s is expected to be within 10 %). The main features of the angular distributions are given in table 1. TABLE 1
E x p e r i m e n t a l results for t h e different n e u t r o n groups
! Group
E x in N xi {MeV)
Q (MeV)
Relative intensity at 0°
C h a r a c t e r of a n g u l a r d i s t r i b u t i o n
............
I].
!
=°
0 2.31 R95 4.91 5.10 5.69 5.83
5.32 3,01 1,37
0,41 0,22 --0.37 --0.51
1.5 1.0 3.0 12.5 2.5 33 3
:~zt
1 -- ( 0 . 7 : 0 . 1 ) cos 0 1 + (0.9--~0.2)P2(cos 0) Isotropic within 10 % l = 0 stripping, ro = 6.1 × 10 -13 c m Isotropic within 15 op /o l ---- 0 stripping, r o = 6. ° × 10 -18 cm Isotropic within 60 %
The experimental points for the 5.69 MeV level (ns) shown in fig. 3 included the neutron groups ClZ(d, n)N 13 and n~. At 0 ° the position of the peak in tile
TIlE Cla(d, n)N l~t REACTION
135
time of flight spectrum showed that the large intensity was due to the neutron group forming the 5.69 MeV level; less than 20 % of the intensity could have been from other groups. Four experimental points for the intensity of n8 were d e t e r m i n e d between 50 ° and 130 °. With the above limit on the i n t e n d t y near 0 °, the yield of neutrons forming this level was shown to have no str~ng variation with angle, it was assumed to be isotropic in obtaining the angular distribution of n 5. The rise in yield at backward angles could have come from the C~2(d, no)N ~3 reaction. The angular distribution for this reaction at 1.2 MeV s) is shown in fig. 3. The error in the measurements of the intensity k)r ns at 0 ° caused b y including the yield from this reaction was negligibly small. 3. The 4.91 and 5.69 MeV levels
The forward peaks in the angular distributions of both these levels were fitted by Butler stripping curves 9,1o) t with 1 =:- 0 and r 0 --- 6.1 × 10-~3 cm (fig. 3). The ratio of the reduced widths with which the levels were formed was found to be R = [(2J+1)°~']5~9 = 2.s~o.5. [ ( 2 J + 1)o-'j,.,, Angular distributions for these two levels were also measured in forward direztions at 1 MeV. Again the peaks at 0 ° could be fitted by Butler stripping eurw~s with 1 = 0, r o = 6.1 x 10 -~a cm. Fig. 4 shows excitation functions at 0 ° for these
n5 1
LI 0
{0 : S l~
¢
¢ --I W
L " O ro ~ 6,1 ÷ ¢
O
0.8
09 IiO t,I t.2 1,3 DEUTEP, ON ENERGY (MeV)
Fig, 4. Excitation functions (in the laboratory system) at 0 =for tile production of neutlons forming the 4.91 MeV (na) ~md 5.69 MeV (n~) states in N ~4 'the ct:rves are the predictions of Butler stripping theory (see text),
two groups; also shown are the theoretical excitation functions predicted by Butler stripping theory under the assumpti(ln that r 0 and 0~ are independent of t I n ref, g), a correction wlfich takes account of the Coulomb effect on the captured proton wa:~ used 4, lo).
136
A . N . JA~ES
the deuteron energy. The ratios of reduced widths evaluated from the experimental excitation functions are given in table 2. The ratio is constant within the limits of experiment, :indicating that the difference between theory and experiment shown in fig. 4 arises from neglecting the effects of barrier penetration by the deuteron, TABLE 2 Ratios of reduced widths E a (MeV)
R
0.88 0.99 1.10 1.20 ] .31
2 .0 ± 1 ,0 2.7:k0.6 3.0±0.6 2.8~0.5 3.3~0.6
Stripping reactions in ClS(d, n)N 14 with I ----- 0 restrict the spin and parity of the states produced in N 14 to 0- or 1-. Warburton et al. 1) considered the gamma-ray selection rules for transitions from 0- and l - states to the 1+, T = 0 ground state and the 0% T = 1 first excited state of N 14. They found that the decay of the 0- level had to be to the ground state, and, if T -----0, th ~ 1- level would decay mainly to the first excited state, since the ground state transition would be forbidden by the isobaric spin selection rule for E 1 radiation. The 5,69 MeV level was thought to be J = 1 from the strengths of the gamma-ray cascades from the 8.06 and 8.62 MeV levels 11). It decayed predominantly to the first excited state 1) and therefore would seem to be 1rather than 0-, with preference for T -----0. The gamma-ray decay of the 4.91 MeV level was to the ground state and 0- seemed the more likely assignment. The ratio of reduced widths evaluated from the data was consistent with the theoretical value 3, which would be expected for 0- and 1- states of the type (Cla, 2s~), since such states imply identical reduced widths 02 equal to the single particle width. This ratio of reduced widths also agreed with that found by Ranken et al. 3) from the relative yield of gamma-rays following deuteron bombardment of C13 at 4.5 MeV. Ranken et al. had to assume s wave proton capture: for the 5.69 MeV state. Warburton et al. 1) found no evidence in the gamrna~ray decay schemes that was against these assignments.
4. The ground, 2.31 and 3.95 MeV levels Excitation functions at 20 ° and 120 ° were measured for these three levels. They showed that the angular distributions of figs. 1 and 2 were in agreement with those of Green et al. 12) at 860 keV. i n particular the cos 0 term for n o changed sign near 1.1 MeV where all the excitation functions indicated a weak broad resonance la). If the assumption is made that these three levels (all
THE c l a ( d , n ) N 14 REACTION
137
belonging to the s4p 1° configuration) are populated by a compound nucleus process through the same resonance, then, from the forms of the angular distributions, that resonance must have J = -2a_~ and be formed by la r~. "> 1. 5. T h e 5.10 and 5.83 MeV levels
On the assumption that these levels are of the type (C'a, ld~) it is possible to predict, from the intensity of the groups n.3 and ns, that the intensity at the maximum of the I --= 2 stripping peak (fig. 2) would be four of the relatiw:~ units used in table 1. [The ratio of the 2s} to the ld~. single particle reduced widths 4) was taken as 2. No attempt was made to justify the assumption t l ~ t dist,rting effects (see fig. 4) affected 1 = 0 and I -- 2 stripping in the same way i. This inte~,sityis very similar to that expect,~d from compound nucleus formation (no, n I and n2). The shape of the experimental angular distributions, particularly for the 5.10 MeV level (ha), favours compotmd nucleus formation rather than l = 2 stripping. If the 5.10 MeV level is formed by a compound nucleus process through the same resonance as the ground and first excited states, then the isotropic distribution observed indicates positive parity for this lew,1; this interpretation must remain in doubt since levels of opposite parity are known to interfere in the compound nucleus (cos 0 term ill the ang,.flar distribution of no). The isotropic target was supplied by A.E.R.E. Harwell. This work wa.,+ performed while the author held a Research Fellowship awarded by the Department of Scientific and Industrial Research. The author al~,., wi.~hcs to ;tcknowledg(; 3Ir. D. D. Stewart's invaluable aid in running and maintaining the CockcroftWalton generator. References 1) E, K. '~Varburton, H, J. Rose and E. N. Hatch, Phys. Rev. 114 (1959) 214; E. K. XVarburton and \V. T. l~inkst, m, Phys. R~.,v. 118 (1960) 733 2"1 R. E, Benenson, Phys, Rev. 90 (1953) 420 3i W. A. Ranken, T. W. Bonner, J. 3I. McCrary zmd T. \ . Rabson, Phys. Roy. 109 (1958) 917 ,1) M. H. Macfarlane and J. B. French, Rcv. 3I,~d. Phys. 32 (1960) 567 5) D. H. Wilkinson, Phil. Mag. 3 (195S) 11S5 6) A. N. J a m e s and C. 3I. P, Johnson, Nuclear Instruments 10 (1961) 68 7) F. D. Brooks, Nuclear Instrunlcnts 4 (195~.1) 151 R. B. Owen, Nucleonics 17, no. 9 (1959) 92 8) A. N. James, Nuclear Physics 00 (1961) 0~)o 9) C. R. Lnbitz, Atomic Energy Commissicm Report .\t,2('U-3990 (1937) unp~fl~lished 10) S. T. Butler and O. 5!. Hittmair, Nuclear Stripping Reatfions (! [orowitz Publications, I957) 11) Do H. Wilkinson and S. l). Bloom, I'hit. Mug. 2 (1957) 63 12i L. L. Green, J. P. Scanlon and J. (?. Willmott, Prt;c. Phvs, Soc, A 68 (1955) 386 13) J. B. Marion and G. "Weber, Phvs. Rev, 102 (1956) 1355
THE Ct3(d, n)N 14 REACTION A. N. JAMES
Cavendish Laboratory, Cambridge Received 2 J a n u a r y 1961
Abstract: The time of flight method has been used to measure angular distributions and excitation functions for the neutrons emitted to form the ground and first six excited states of N x4 ill the C~3(d, n)N ~4reaction. The properties of the 4.91 and 5.69 MeV N 1. levels were consistent with tl~e weak coupling configuration (Cla2s½). The nature of the 5.10 and 5.83 MeV levels remains in doubt.
1. Introduction The properties of excited states of N ~4between 4: and 7 MeV are of interest in determining which states have shell model p~0 wave functions, and which states are formed predominantly by coupling a 2s or ld proton to a C13 core t). In the first category only a 2 + state is expected while four T = 0 states are expected in t he second. Warburton el al. ~) were able to make plausible the identification of ~hese four states, 0-, 2-, 1- and 3-, with the 4.91, 5.10, 5.69 and 5.83 N ~4 states. Their arguments were based on an analysis of gamma-rays from C l a + p ((2 ~ 7.55 MeV) and on data from C~8(d, n)N l*. Benenson has observed 1 = 0 stripping for the 4.91 MeV level ~) and Ranken et al. have measured relative gamma-ray yields a). Because of their unique parent character 4) these states are expected to be formed in C1~(d, n)N 14 with large stripping reduced widths, by I = 0 for the 0- and 1- states, and I = 2 for the 2- and 3- states. Since the Q value for these levels is low (see table 1), angular distributions typical of stripping reactions should be observed even at low deuteron energy 5).
2. Experimental Th~ time of flight apparatus of the Cavendish Laboratory's 1.4× 106 V Cockccoft-Walton ~,_netatol' ~ ~, ,~ ,6) has been used to measure angular distributions and excitation functions for the neutrons emitted to form the ground and first six excited sta,~es of N 14 in the Cla (d, n)N ~4 reaction. Scintillation counters incorporating neutron discriminating Nuclear Enterprise N.E. 213 liquid scintillator 7) were used with biases equivalent to neutron energies of 600 keV and 330 keV. For a bias of 600 keV and R.C.A. 7264 photomultiplier was used and gave a time resolution for the neutron groups in the time of flight spectrum of 6 ns. The lower noise current in an E.N. 1.6097 photomultiplier allowed a bias 132
THE C13(d, n)N 14 REACTION
133
of 330 keV with a neutron group time resolution s) of 8 ns. The flight paths were 3.5 and 1.5 m. The target was isotopically enriched carbon on 0.05 em copper. It was less than 20 keV thick to 1.16 MeV protons. The time of flight spectrum I
1
i'
4 ~3 .J UJ
;2
!
0
t !
I ,.
I
ANGLE
I
I
(C.O.a)
Fig. I. N e u t r o n a n g u l a r d i s t r i b u t i o n s in t h e centrc-of-ma.ss system formimz the gro~md s t ~ t e a n d first; e x c i t e d 2.,31 McV s t a t e (n 1) of N ~'t a t 1.'2 .'qeV d e u t e r o n energy.
(n~)
4
3
t n2
I
J hi ~'O
,,"r,, . . . . ~
,
or
0
I
J \
,
3o
,
-~0
,
_,__,__ 9o
A N G LE
J2o
(c.o.M)
ELi
i5o
~0o
Fig. 2. N e u t r o n a n g u l a r d i s t r i b u t i o n s in d i e centre-of-m~ss s y s t e m f o r m i n g tile 3.95 MeV (r@ ~md 5.10 M e V (n,} s t a t e s of N 1~ a t 1.2 MeV d e u t e r o n energy.
showed that the target contained significant amounts of Ct2 and dm!terium. Allowance was made for the cr"(d, n)N 13 and d(d, n)He 3 reactions in extracting the neutron intensities of die (;~:~(d,n)N ~ reaction..
A. N. J A M E S
I¸$4¸
Figs. 1, 2 and 3 show the measured angular distributions at 1.2 MeV deuteron energy. Errors arising from corrections for counter efficiency s) are less than i0 °/o within any one of the distributions. The relative intensities of the neutron
i!
Z-o
ro = 6,t
°°,: a o~
-
¢
0
-
nS÷nS÷CI2
J
Lu
>" 4 o
~
n6 contr" bution from CI2
~., I ~ 30Y-" '~_ ] ~
-
n5 / = 0 r°= 6"2
\ ,ot0
+ 2"
\ ao
60
90.
~aQ
ANm.E (C.O.M.)
iso
180
Fig. 3, N e u t r o n a n g u l a r d i s t r i b u t i o n s in the centre-of-mass system f o r m i n g t h e 4.91 MeV (n s) and 5.69 MeV (n s, see t e x t ) states of N '4 a t 1.2 MeV d e u t e r o n energy'.
groups forming the different levels are expected to be correct within 30 % (in particular the intensity ratio for groups n 3 and n s is expected to be within 10 %). The main features of the angular distributions are given in table 1. TABLE 1
E x p e r i m e n t a l results for t h e different n e u t r o n groups
! Group
E x in N xi {MeV)
Q (MeV)
Relative intensity at 0°
C h a r a c t e r of a n g u l a r d i s t r i b u t i o n
............
I].
!
=°
0 2.31 R95 4.91 5.10 5.69 5.83
5.32 3,01 1,37
0,41 0,22 --0.37 --0.51
1.5 1.0 3.0 12.5 2.5 33 3
:~zt
1 -- ( 0 . 7 : 0 . 1 ) cos 0 1 + (0.9--~0.2)P2(cos 0) Isotropic within 10 % l = 0 stripping, ro = 6.1 × 10 -13 c m Isotropic within 15 op /o l ---- 0 stripping, r o = 6. ° × 10 -18 cm Isotropic within 60 %
The experimental points for the 5.69 MeV level (ns) shown in fig. 3 included the neutron groups ClZ(d, n)N 13 and n~. At 0 ° the position of the peak in tile
TIlE Cla(d, n)N l~t REACTION
135
time of flight spectrum showed that the large intensity was due to the neutron group forming the 5.69 MeV level; less than 20 % of the intensity could have been from other groups. Four experimental points for the intensity of n8 were d e t e r m i n e d between 50 ° and 130 °. With the above limit on the i n t e n d t y near 0 °, the yield of neutrons forming this level was shown to have no str~ng variation with angle, it was assumed to be isotropic in obtaining the angular distribution of n 5. The rise in yield at backward angles could have come from the C~2(d, no)N ~3 reaction. The angular distribution for this reaction at 1.2 MeV s) is shown in fig. 3. The error in the measurements of the intensity k)r ns at 0 ° caused b y including the yield from this reaction was negligibly small. 3. The 4.91 and 5.69 MeV levels
The forward peaks in the angular distributions of both these levels were fitted by Butler stripping curves 9,1o) t with 1 =:- 0 and r 0 --- 6.1 × 10-~3 cm (fig. 3). The ratio of the reduced widths with which the levels were formed was found to be R = [(2J+1)°~']5~9 = 2.s~o.5. [ ( 2 J + 1)o-'j,.,, Angular distributions for these two levels were also measured in forward direztions at 1 MeV. Again the peaks at 0 ° could be fitted by Butler stripping eurw~s with 1 = 0, r o = 6.1 x 10 -~a cm. Fig. 4 shows excitation functions at 0 ° for these
n5 1
LI 0
{0 : S l~
¢
¢ --I W
L " O ro ~ 6,1 ÷ ¢
O
0.8
09 IiO t,I t.2 1,3 DEUTEP, ON ENERGY (MeV)
Fig, 4. Excitation functions (in the laboratory system) at 0 =for tile production of neutlons forming the 4.91 MeV (na) ~md 5.69 MeV (n~) states in N ~4 'the ct:rves are the predictions of Butler stripping theory (see text),
two groups; also shown are the theoretical excitation functions predicted by Butler stripping theory under the assumpti(ln that r 0 and 0~ are independent of t I n ref, g), a correction wlfich takes account of the Coulomb effect on the captured proton wa:~ used 4, lo).
136
A . N . JA~ES
the deuteron energy. The ratios of reduced widths evaluated from the experimental excitation functions are given in table 2. The ratio is constant within the limits of experiment, :indicating that the difference between theory and experiment shown in fig. 4 arises from neglecting the effects of barrier penetration by the deuteron, TABLE 2 Ratios of reduced widths E a (MeV)
R
0.88 0.99 1.10 1.20 ] .31
2 .0 ± 1 ,0 2.7:k0.6 3.0±0.6 2.8~0.5 3.3~0.6
Stripping reactions in ClS(d, n)N 14 with I ----- 0 restrict the spin and parity of the states produced in N 14 to 0- or 1-. Warburton et al. 1) considered the gamma-ray selection rules for transitions from 0- and l - states to the 1+, T = 0 ground state and the 0% T = 1 first excited state of N 14. They found that the decay of the 0- level had to be to the ground state, and, if T -----0, th ~ 1- level would decay mainly to the first excited state, since the ground state transition would be forbidden by the isobaric spin selection rule for E 1 radiation. The 5,69 MeV level was thought to be J = 1 from the strengths of the gamma-ray cascades from the 8.06 and 8.62 MeV levels 11). It decayed predominantly to the first excited state 1) and therefore would seem to be 1rather than 0-, with preference for T -----0. The gamma-ray decay of the 4.91 MeV level was to the ground state and 0- seemed the more likely assignment. The ratio of reduced widths evaluated from the data was consistent with the theoretical value 3, which would be expected for 0- and 1- states of the type (Cla, 2s~), since such states imply identical reduced widths 02 equal to the single particle width. This ratio of reduced widths also agreed with that found by Ranken et al. 3) from the relative yield of gamma-rays following deuteron bombardment of C13 at 4.5 MeV. Ranken et al. had to assume s wave proton capture: for the 5.69 MeV state. Warburton et al. 1) found no evidence in the gamrna~ray decay schemes that was against these assignments.
4. The ground, 2.31 and 3.95 MeV levels Excitation functions at 20 ° and 120 ° were measured for these three levels. They showed that the angular distributions of figs. 1 and 2 were in agreement with those of Green et al. 12) at 860 keV. i n particular the cos 0 term for n o changed sign near 1.1 MeV where all the excitation functions indicated a weak broad resonance la). If the assumption is made that these three levels (all
THE c l a ( d , n ) N 14 REACTION
137
belonging to the s4p 1° configuration) are populated by a compound nucleus process through the same resonance, then, from the forms of the angular distributions, that resonance must have J = -2a_~ and be formed by la r~. "> 1. 5. T h e 5.10 and 5.83 MeV levels
On the assumption that these levels are of the type (C'a, ld~) it is possible to predict, from the intensity of the groups n.3 and ns, that the intensity at the maximum of the I --= 2 stripping peak (fig. 2) would be four of the relatiw:~ units used in table 1. [The ratio of the 2s} to the ld~. single particle reduced widths 4) was taken as 2. No attempt was made to justify the assumption t l ~ t dist,rting effects (see fig. 4) affected 1 = 0 and I -- 2 stripping in the same way i. This inte~,sityis very similar to that expect,~d from compound nucleus formation (no, n I and n2). The shape of the experimental angular distributions, particularly for the 5.10 MeV level (ha), favours compotmd nucleus formation rather than l = 2 stripping. If the 5.10 MeV level is formed by a compound nucleus process through the same resonance as the ground and first excited states, then the isotropic distribution observed indicates positive parity for this lew,1; this interpretation must remain in doubt since levels of opposite parity are known to interfere in the compound nucleus (cos 0 term ill the ang,.flar distribution of no). The isotropic target was supplied by A.E.R.E. Harwell. This work wa.,+ performed while the author held a Research Fellowship awarded by the Department of Scientific and Industrial Research. The author al~,., wi.~hcs to ;tcknowledg(; 3Ir. D. D. Stewart's invaluable aid in running and maintaining the CockcroftWalton generator. References 1) E, K. '~Varburton, H, J. Rose and E. N. Hatch, Phys. Rev. 114 (1959) 214; E. K. XVarburton and \V. T. l~inkst, m, Phys. R~.,v. 118 (1960) 733 2"1 R. E, Benenson, Phys, Rev. 90 (1953) 420 3i W. A. Ranken, T. W. Bonner, J. 3I. McCrary zmd T. \ . Rabson, Phys. Roy. 109 (1958) 917 ,1) M. H. Macfarlane and J. B. French, Rcv. 3I,~d. Phys. 32 (1960) 567 5) D. H. Wilkinson, Phil. Mag. 3 (195S) 11S5 6) A. N. J a m e s and C. 3I. P, Johnson, Nuclear Instruments 10 (1961) 68 7) F. D. Brooks, Nuclear Instrunlcnts 4 (195~.1) 151 R. B. Owen, Nucleonics 17, no. 9 (1959) 92 8) A. N. James, Nuclear Physics 00 (1961) 0~)o 9) C. R. Lnbitz, Atomic Energy Commissicm Report .\t,2('U-3990 (1937) unp~fl~lished 10) S. T. Butler and O. 5!. Hittmair, Nuclear Stripping Reatfions (! [orowitz Publications, I957) 11) Do H. Wilkinson and S. l). Bloom, I'hit. Mug. 2 (1957) 63 12i L. L. Green, J. P. Scanlon and J. (?. Willmott, Prt;c. Phvs, Soc, A 68 (1955) 386 13) J. B. Marion and G. "Weber, Phvs. Rev, 102 (1956) 1355