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T = 32 levels in 17F
Volume 23, number 8
PHYSICS LETTERS
T
= ~ LEVELS
IN
21 November 1966
17F*
E. G. ADE L B E R G E R and C. A. BARNES
California Institute of Technology. Pasadena, California Received 24 October 1966
T = ~ levels in 17F have been seen at excitation energies of 11.195, 12.544 and 13.055 MeV in the reaction 15N(3He,n). Angular distributions lead to spin assignments of ½- for the 11.195 MeV level, and 9- or ~for the level at 12.544 MeV.
The l o w e s t - l y i n g T = ~ e n e r g y l e v e l s in light nuclei with A = 4n+ 1 a r e bound with r e s p e c t to i s o s p i n - c o n s e r v i n g p a r t i c l e d e c a y s , and hence a r e expected to be quite n a r r o w in spite of t h e i r high excitation e n e r g i e s . As p a r t of a s y s t e m a tic study of t h es e higher isospin l e v e l s , we s e a r c h e d for the T = ~ l e v e l s in 17F with the r e a c t i o n 15N(3He,n)17F. N a r r o w states in 17F at excitation e n e r g i e s of 12.522 and 13.037 MeV have been p r e v i o u s l y r e p o r t e d [1] in the e l a s t i c s c a t t e r i n g of protons by 160. It was p r o p o s e d that t he s e s t a t e s w e r e possibly the analogs of the f i r s t , and second (or third) e x c i t e d states of 17N. A gas cell containing 15N gas with 99.9% nominal isotopic purity was b o m b a r d e d at 16 di ff e r en t 3He e n e r g i e s between 7.72 and 11.88 MeV, and n e u t r o n s w e r e detected on the t i m e - o f flight s p e c t r o m e t e r a s s o c i a t e d with the ONRCIT tandem a c c e l e r a t o r . The e n e r g y l o s s of p r o tons in the 5000/~ e n t r a n c e foil was d e t e r m i n e d by m e a s u r i n g the d i s p l a c e m e n t of the 872 keV 19F(p,ot~) r e s o n a n c e when the foil was placed ahead of a thin c a l c i u m fluoride t a r g e t . This proton e n e r g y l o s s was then s c a l e d for 3He ions using the t a b l e s of D e m i r l i o g l u and Whaling [2]. A t i m e s p e c t r u m taken at 10.5 MeV incident e n e r g y is p r e s e n t e d in fig. la . N a r r o w neutron groups appear at e x c i t a t i o n s of 11.195 + 0.007, 12.544 + 0.010, and 13.055 + 0.009 MeV. No other sharp s t r u c t u r e was seen at e x c i t a t i o n s up to 14.3 MeV. T h e s e l e v e l s cannot be a t t r i b u t e d to contaminants in the t a r g e t gas, and empty t a r g e t runs showed no pronounced s t r u c t u r e . The e x c i t a t i o n s have been m e a s u r e d using the 15N(~,n)ISF(0.0) and 14N(~,n)17F(0.0) r e a c t i o n s to c a l i b r a t e the time scale. The incident e n e r * Supported in part by the Office of Naval Research [Nonr-220(47)]. 474
g i e s w e r e adjusted so that the neutron e n e r g i e s f r o m the l e v e l under study and f r o m the c a l i b r a tion r e a c t i o n were a l m o s t identical. With this method, e r r o r s in the c a l i b r a t i o n constant of the an al y si n g m a g n e t and foil t h i ck n ess v e r y n e a r l y c a n c e l in the d e r i v e d Q v a l u e s . A typical c a l i b r a tion s p e c t r u m is shown in fig. lb. An upper l i m i t on the width of the 11.195 MeV l e v e l of 20 keV was obtained by c o m p a r i n g the shape of the neutron group f r o m 15N(3He,n) with that f r o m the p a r t i c l e - s t a b l e 3.36 MeV l e v e l in 18Ne, populated by 160(3He,n). It was n e c e s s a r y to change the beam e n e r g y by only 5% in o r d e r to produce n e u t r o n s of the s a m e e n e r g y f r o m these two r e a c t i o n s . Since the 11.195 MeV state was not seen in the work of r e f . 1, its width is p r o bably l e s s than a few keV. F r o m the p r e s e n t work an upper l i m i t of 25 keV is found f o r the widths of the 12.544 and 13.055 MeV st at es, but if these s t a t e s a r e a s s o c i a t e d with those seen in ref. 1, the widths a r e l e s s than 5 keV. Angular d i s t r i b u t i o n s (see fig. 2) have been taken at E(3He) = 11.88 MeV w h er e all t h r e e l e v e l s a r e strongly populated. F o r c o m p a r i s o n , angular d i s t r i b u t i o n s f r o m known L = 0 and L = 2 t r a n s i t i o n s in 160(3He,n)18Ne at 10.5 MeV i n cident e n e r g y a r e a l s o shown. The 17F and 18Ne d i s t r i b u t i o n s a r e s t r i k i n g l y s i m i l a r , as one would expect. The e r r o r b a r s include c o n t r i b u tions f r o m counting s t a t i s t i c s , u n cer t ai n t y in the choice of background, and a 5% contribution f r o m the d e t e c t o r efficiency and beam c u r r e n t i n t e g r a t i o n . In addition to the e r r o r s denoted by the f l ag s t h e r e is an e s t i m a t e d uncertainty i~ the absolute n o r m a l i z a t i o n of 10%. The smooth c u r v e s a r e DWBA fits using the z e r o - r a n g e optical model code TSALLY. The c u r v e s w e r e computed using published potentials d e r i v e d f r o m e l a s t i c s c a t t e r i n g , identical
Vohune 23, n u m b e r 8
PHYSICS
2000
RUN 23
7/26/66 ._ . N'o[H.e~njn)L . .
LETTERS
E~.~='10498 .
21 November 1966
8 ~
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Fig. 1. a) A t i m e - o f - f l i g h t s p e c t r u m of the 15N(3He,n)17F reaction at an incident energy of 10.498 MeV. The peak at the right of the figure is due to the prompt ~ radiation emitted by the target, b) A t i m e - o f - f l i g h t s p e c t r u m of the 15N(Ot, n)18F reaction at an incident energy of 8.915 MeV. This s p e c t r u m was used to c a l i b r a t e the time scale.
to those used in fitting 160(t,p) angular distributions [3]. The striking similarity with the known transitions to 18Ne, together with the DWBAfits, leads to unambiguous L-value assignments for the 11.195 and 12.544 MeV levels in 17F. The 13.055 MeV level is unbound with respect to diproton emission and hence cannot be handled by conventional stripping theory. The spins determined from the L values are ½- for the 11,195
M e V l e v e l , a n d ~ - o r ~" f o r t h e 12.544 M e V s t a t e . T h e T = ~2 a s s i g n m e n t of t h e s e l e v e l s i s b a s e d upon t h e e x c i t a t i o n e n e r g i e s , n a r r o w w i d t h s , s p i n s , a n d i n t e n s i t i e s . C a l c u l a t i o n s by M a r g o l i s a n d De T a k a c s y [4] s h o w t h a t t h e l o w - l y i n g T = l e v e l s in m a s s 17 h a v e c o n s i d e r a b l e 2 p a r t i c l e - 1 h o l e s t r e n g t h a n d s h o u l d t h e r e f o r e be s t r o n g l y p o p u l a t e d b y t h e (3He,n) r e a c t i o n on 15N. T h e s e r e s u l t s a n d p r e v i o u s w o r k [5] i n d i c a t e t h a t t h e 475
Volume 23, number 8
I00
I0
PHYSICS
I
I
---
§ F~7(ii.i95) ~ Nele(O.O)
I
I
"~, \
F
I
LETTERS
21 November 1966
(3He,n) r e a c t i o n i s a v a l u a b l e s p e c t r o s c o p i c t o o l for studying higher isotopic spin states. A summ a r y of t h e a b o v e r e s u l t s , t o g e t h e r w i t h e x c i t a t i o n e n e r g i e s of T = ~ p r a t e s f o u n d in 1 7 0 by H e n s l e y h a s a l r e a d y b e e n p r e s e n t e d [6].
I
L: 0
The authors are grateful for helpful discuss i o n s w i t h D r . N . De T a k a c s y , a n d to D r s . B a s s e l , Drisko, and Satchler who kindly provided their c o m p u t e r c o d e T S A L L Y ; A. B . M c D o n a l d p r o vided valuable help.
\ "~ 1.0 b
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0 8OM
r
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160
120
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I
I
~
L
1
I
-- ~ F%2.544)
--- ~ Net8(I.88) I0
L=2
~z~
1. G. Hardie, R . L . Dangle and L. D. Oppliger, Phys. Rev. 129 (1963) 353. 2. D. Demirlioglu and W. Whaling (1962) unpublished. 3. R . N . G l o v e r and A . D . W . Jones, Nucl. Phys. 81 (1966) 277. 4. B . M a r g o l i s and N. De Takacsy, Can. J. Phys. 44 (1966) 1431. 5. F . S . D i e t r i c h , Nucl. Phys. 69 (1965)49; E . A d e l b e r g e r and C.A. B a r n e s , Bull. Am. Phys. Soc. 10 (1965) 1195. 6. C.A. B a r n e s , E. G , A d e l b e r g e r , D.C. Hensley and A.B.McDonald, Intern. Nuclear Physics Conf., Gatlinburg, 1966.
\ "~ 1.0 b
0.1
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80
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120
160
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I
l
I
1
I
FI7(15.055) 1.0
I 0.1 b
.01
4;0
476
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I
1
120
I
160
Fig. 2. Neutron angular distributions f r o m the r e a c t i o n 15N(3He,n) at an incident energy of 11.88 MeV. T h e known L = 0 and L = 2 t r a n s i t i o n s in the reaction 160(3He,n)180 a r e shown for comparison. The smooth curves a r e z e r o - r a n g e DWBA fits using the s a m e potentials as those used in fitting the r e a c t i o n 160(t,p)180 in ref. 3.
PHYSICS LETTERS
T
= ~ LEVELS
IN
21 November 1966
17F*
E. G. ADE L B E R G E R and C. A. BARNES
California Institute of Technology. Pasadena, California Received 24 October 1966
T = ~ levels in 17F have been seen at excitation energies of 11.195, 12.544 and 13.055 MeV in the reaction 15N(3He,n). Angular distributions lead to spin assignments of ½- for the 11.195 MeV level, and 9- or ~for the level at 12.544 MeV.
The l o w e s t - l y i n g T = ~ e n e r g y l e v e l s in light nuclei with A = 4n+ 1 a r e bound with r e s p e c t to i s o s p i n - c o n s e r v i n g p a r t i c l e d e c a y s , and hence a r e expected to be quite n a r r o w in spite of t h e i r high excitation e n e r g i e s . As p a r t of a s y s t e m a tic study of t h es e higher isospin l e v e l s , we s e a r c h e d for the T = ~ l e v e l s in 17F with the r e a c t i o n 15N(3He,n)17F. N a r r o w states in 17F at excitation e n e r g i e s of 12.522 and 13.037 MeV have been p r e v i o u s l y r e p o r t e d [1] in the e l a s t i c s c a t t e r i n g of protons by 160. It was p r o p o s e d that t he s e s t a t e s w e r e possibly the analogs of the f i r s t , and second (or third) e x c i t e d states of 17N. A gas cell containing 15N gas with 99.9% nominal isotopic purity was b o m b a r d e d at 16 di ff e r en t 3He e n e r g i e s between 7.72 and 11.88 MeV, and n e u t r o n s w e r e detected on the t i m e - o f flight s p e c t r o m e t e r a s s o c i a t e d with the ONRCIT tandem a c c e l e r a t o r . The e n e r g y l o s s of p r o tons in the 5000/~ e n t r a n c e foil was d e t e r m i n e d by m e a s u r i n g the d i s p l a c e m e n t of the 872 keV 19F(p,ot~) r e s o n a n c e when the foil was placed ahead of a thin c a l c i u m fluoride t a r g e t . This proton e n e r g y l o s s was then s c a l e d for 3He ions using the t a b l e s of D e m i r l i o g l u and Whaling [2]. A t i m e s p e c t r u m taken at 10.5 MeV incident e n e r g y is p r e s e n t e d in fig. la . N a r r o w neutron groups appear at e x c i t a t i o n s of 11.195 + 0.007, 12.544 + 0.010, and 13.055 + 0.009 MeV. No other sharp s t r u c t u r e was seen at e x c i t a t i o n s up to 14.3 MeV. T h e s e l e v e l s cannot be a t t r i b u t e d to contaminants in the t a r g e t gas, and empty t a r g e t runs showed no pronounced s t r u c t u r e . The e x c i t a t i o n s have been m e a s u r e d using the 15N(~,n)ISF(0.0) and 14N(~,n)17F(0.0) r e a c t i o n s to c a l i b r a t e the time scale. The incident e n e r * Supported in part by the Office of Naval Research [Nonr-220(47)]. 474
g i e s w e r e adjusted so that the neutron e n e r g i e s f r o m the l e v e l under study and f r o m the c a l i b r a tion r e a c t i o n were a l m o s t identical. With this method, e r r o r s in the c a l i b r a t i o n constant of the an al y si n g m a g n e t and foil t h i ck n ess v e r y n e a r l y c a n c e l in the d e r i v e d Q v a l u e s . A typical c a l i b r a tion s p e c t r u m is shown in fig. lb. An upper l i m i t on the width of the 11.195 MeV l e v e l of 20 keV was obtained by c o m p a r i n g the shape of the neutron group f r o m 15N(3He,n) with that f r o m the p a r t i c l e - s t a b l e 3.36 MeV l e v e l in 18Ne, populated by 160(3He,n). It was n e c e s s a r y to change the beam e n e r g y by only 5% in o r d e r to produce n e u t r o n s of the s a m e e n e r g y f r o m these two r e a c t i o n s . Since the 11.195 MeV state was not seen in the work of r e f . 1, its width is p r o bably l e s s than a few keV. F r o m the p r e s e n t work an upper l i m i t of 25 keV is found f o r the widths of the 12.544 and 13.055 MeV st at es, but if these s t a t e s a r e a s s o c i a t e d with those seen in ref. 1, the widths a r e l e s s than 5 keV. Angular d i s t r i b u t i o n s (see fig. 2) have been taken at E(3He) = 11.88 MeV w h er e all t h r e e l e v e l s a r e strongly populated. F o r c o m p a r i s o n , angular d i s t r i b u t i o n s f r o m known L = 0 and L = 2 t r a n s i t i o n s in 160(3He,n)18Ne at 10.5 MeV i n cident e n e r g y a r e a l s o shown. The 17F and 18Ne d i s t r i b u t i o n s a r e s t r i k i n g l y s i m i l a r , as one would expect. The e r r o r b a r s include c o n t r i b u tions f r o m counting s t a t i s t i c s , u n cer t ai n t y in the choice of background, and a 5% contribution f r o m the d e t e c t o r efficiency and beam c u r r e n t i n t e g r a t i o n . In addition to the e r r o r s denoted by the f l ag s t h e r e is an e s t i m a t e d uncertainty i~ the absolute n o r m a l i z a t i o n of 10%. The smooth c u r v e s a r e DWBA fits using the z e r o - r a n g e optical model code TSALLY. The c u r v e s w e r e computed using published potentials d e r i v e d f r o m e l a s t i c s c a t t e r i n g , identical
Vohune 23, n u m b e r 8
PHYSICS
2000
RUN 23
7/26/66 ._ . N'o[H.e~njn)L . .
LETTERS
E~.~='10498 .
21 November 1966
8 ~
FLIGHT I~Tfl • 2.60m.
I1.t95--
0
~
iO00
z
EXCITATION IN FIT(MEV) i
13,0
,o
12'.0
~o
'l'.O
~
,~o
I00
80
,20
6.04,0 0.0
,~o
,~o
,~o
200
CHANNEL
7/26/G6
RUN 12
Nl~{4,n) E¢.algt5 MeV
8L.O"
FLIGHT PATH • 2.60 m
400
b
z
200
•
,% •, • "
,
•
•
°
•
~
•
~
•
°
.,~
°•
••
. .:. .......-:., :. _ ....:,:.,.-.:4,-:.:.:
f
4'0
610
8=0
I I leO 120 CHANNEL
I 140
I 160
i
180
200
Fig. 1. a) A t i m e - o f - f l i g h t s p e c t r u m of the 15N(3He,n)17F reaction at an incident energy of 10.498 MeV. The peak at the right of the figure is due to the prompt ~ radiation emitted by the target, b) A t i m e - o f - f l i g h t s p e c t r u m of the 15N(Ot, n)18F reaction at an incident energy of 8.915 MeV. This s p e c t r u m was used to c a l i b r a t e the time scale.
to those used in fitting 160(t,p) angular distributions [3]. The striking similarity with the known transitions to 18Ne, together with the DWBAfits, leads to unambiguous L-value assignments for the 11.195 and 12.544 MeV levels in 17F. The 13.055 MeV level is unbound with respect to diproton emission and hence cannot be handled by conventional stripping theory. The spins determined from the L values are ½- for the 11,195
M e V l e v e l , a n d ~ - o r ~" f o r t h e 12.544 M e V s t a t e . T h e T = ~2 a s s i g n m e n t of t h e s e l e v e l s i s b a s e d upon t h e e x c i t a t i o n e n e r g i e s , n a r r o w w i d t h s , s p i n s , a n d i n t e n s i t i e s . C a l c u l a t i o n s by M a r g o l i s a n d De T a k a c s y [4] s h o w t h a t t h e l o w - l y i n g T = l e v e l s in m a s s 17 h a v e c o n s i d e r a b l e 2 p a r t i c l e - 1 h o l e s t r e n g t h a n d s h o u l d t h e r e f o r e be s t r o n g l y p o p u l a t e d b y t h e (3He,n) r e a c t i o n on 15N. T h e s e r e s u l t s a n d p r e v i o u s w o r k [5] i n d i c a t e t h a t t h e 475
Volume 23, number 8
I00
I0
PHYSICS
I
I
---
§ F~7(ii.i95) ~ Nele(O.O)
I
I
"~, \
F
I
LETTERS
21 November 1966
(3He,n) r e a c t i o n i s a v a l u a b l e s p e c t r o s c o p i c t o o l for studying higher isotopic spin states. A summ a r y of t h e a b o v e r e s u l t s , t o g e t h e r w i t h e x c i t a t i o n e n e r g i e s of T = ~ p r a t e s f o u n d in 1 7 0 by H e n s l e y h a s a l r e a d y b e e n p r e s e n t e d [6].
I
L: 0
The authors are grateful for helpful discuss i o n s w i t h D r . N . De T a k a c s y , a n d to D r s . B a s s e l , Drisko, and Satchler who kindly provided their c o m p u t e r c o d e T S A L L Y ; A. B . M c D o n a l d p r o vided valuable help.
\ "~ 1.0 b
0.I
References .01 0
I00
0
I
0 8OM
r
I
I
160
120
I
I
I
~
L
1
I
-- ~ F%2.544)
--- ~ Net8(I.88) I0
L=2
~z~
1. G. Hardie, R . L . Dangle and L. D. Oppliger, Phys. Rev. 129 (1963) 353. 2. D. Demirlioglu and W. Whaling (1962) unpublished. 3. R . N . G l o v e r and A . D . W . Jones, Nucl. Phys. 81 (1966) 277. 4. B . M a r g o l i s and N. De Takacsy, Can. J. Phys. 44 (1966) 1431. 5. F . S . D i e t r i c h , Nucl. Phys. 69 (1965)49; E . A d e l b e r g e r and C.A. B a r n e s , Bull. Am. Phys. Soc. 10 (1965) 1195. 6. C.A. B a r n e s , E. G , A d e l b e r g e r , D.C. Hensley and A.B.McDonald, Intern. Nuclear Physics Conf., Gatlinburg, 1966.
\ "~ 1.0 b
0.1
.01
r
4JO
l
I
80
l
120
160
OcM
I
l
I
1
I
FI7(15.055) 1.0
I 0.1 b
.01
4;0
476
'
i0
I
1
120
I
160
Fig. 2. Neutron angular distributions f r o m the r e a c t i o n 15N(3He,n) at an incident energy of 11.88 MeV. T h e known L = 0 and L = 2 t r a n s i t i o n s in the reaction 160(3He,n)180 a r e shown for comparison. The smooth curves a r e z e r o - r a n g e DWBA fits using the s a m e potentials as those used in fitting the r e a c t i o n 160(t,p)180 in ref. 3.