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Low-energy (p,n) reaction on Mn55 and the excited states of Fe55
Volume 5, number 2
LOW-ENERGY AND THE
PHYSICS
LETTERS
(p,n) EXCITED
REACTION STATES
15 June 1963
O N M n 55 O F F e 55
H. J. K I M Oak Ridge National Laboratory, Oak Ridge, Tennessee
Received 20 May 1963
It has been well demonstrated I) that neutron emission in the major decay m o d e of the compound system formed by proton capture if the proton energy is well below the Coulomb barrier but still a few tenths of a M e V above the (p,n) threshold. Ericson 2,3) explains fluctuations in the excitation functions of nuclear reactions induced by lowenergy projectiles in terms of a "coherence energy ~' A E and width r (fluctuation has width A E ~ r), and discusses statistical compound reaction. Fluctuations of the above type have been observed in the reactions Si28(n,a)Mg 25 (ref. 4) and C 12(O16, ez) M g 25 (ref. 5) induced by 10 - 15 M e V projectiles, and in the A127(d,a)Mg25 (ref. 6) reaction. It is expected that the low-energy (p,n) reaction on m e dium A nuclei m a y reveal simpler fluctuations than those observed with 10 - 15 M e V projectiles since the contribution from direct reaction should be small. In view of the above considerations along with the availability of good resolution time-offlight neutron detection scheme, the low-energy (p ,n) reaction is a very attractive way to examine reaction mechanisms and low-lying excited states. Neutron groups corresponding to the low-lying excited states of Fe 55 as well as the ground state have been previously observed 7,8) by the (p,n) reaction on Mn55. Since Fe55 is an unstable isotope, the (p,n) reaction on Mn55 and the (d,p) reaction on Fe54 are convenient means of investigating l o w - l y i n g s t a t e s of Fe55. A thin Mn55 target (less than 5 keV thick for 2.4 MeV protons) evaporated on Pt backing was used for this investigation using the ORNL 3-MeV pulsed Van de Graaff. A neutron time-of-flight spectrum is shown in fig. 1. In addition to previously reported neutron groups corresponding to the ground, 413 keV and 933 keV states of Fe55, two weakly excited, previously unreported, neutron groups are observed. These are identified to be (p, n) neutrons corresponding to excited states of Fe55 so far unreported. Observation of de-excitation gamma rays from Mn55(p,n) Fe55. reaction firmly establishes the identification. The de-excitation gamma-ray spectrum of Fe55 is shown in fig. 2. The energies of the two states are 510 ± 10 keV and 680 + 10 keV. Gamma rays of 138
,o'
i
i
I
Mn55 ( p , n) Fe55 5
Ep -= 2.4 MaV FLIGHT PATH ~ 2 meters 8 = 0=
940k
GROUND STATE - -
680
NO 4t0
v
. .
CHANNEL NUMBER
Fig. 1. NeuRon t i m e - o f - f l i g h t spectrum. Energies in keV designate the excited states of Fe 55 (flight time
increases from right to left:). 4'104 L
~]
~ ~ 5 2
330410
510
680
11
1
1
l,rl
. . . .
.P"4-. t
.'..
~
• ' : ~.,t',,4..1: Mn55 (p, n )4 Fe55 '
950keV
~
! ". :
I 1
"l
"*,,
/p _~2.4 MeV
*',
iO2
-- -
]
,
_
CHANNEL NUMBER
Fig. 2. De-exmtatmn gamma-ray spectrum of Fe 55 formed by the reaction Mn5S(p, n) Fe 55.. Intense 330 keV gamma rays are due to Coulomb excitation of Pt backing.
505 k e V and 650 k e V w e r e a l s o o b s e r v e d b y M a r k e t a l . 9) v i a M n 5 5 ( p , n ~ ) F e 5 5 . E x c i t a t i o n f u n c t i o n s of the n e u t r o n g r o u p s f e e d ing the l o w - l y i n g s t a t e s of F e 5 5 w e r e t a k e n with 2 ~ 3 k e V s t e p s and t h e y a r e shown in fig. 3. I n t e n s i t i e s of t h e two w e a k l y e x c i t e d s t a t e s w e r e not s t r o n g enough to y i e l d s t a t i s t i c a l l y s i g n i f i c a n t e x c i -
Volume 5, number 2
PHYSICS
LETTERS
15 June 1963
8 ? 6 05
q4
GROON0\ /
• A
\,
l,k
8
GROUND STATE E ' = 9 4 0 keV
i
• .E = = 440 keV
I
£ . = 2 A 0 3 MeV
s
STATE..~=,,-'v
I
4
ul
F--
I E*' =940 keY' - ~ , . . /
'~1
0 t0
I
I
/E_ i = ;>,388 MeV 8 f E = 4 t 0 keY/ I 2.560
l
~G 2.580 2.400 Z.421 PROTON ENERGY (MeV)
2.44t
Fig. 3. F o r w a r d angle (6n = 0% excitation functions of the (p, n) reaction neutrons. The arrows indicate the proton energies where the angular distributions ar e taken. Neutron intensities are in the same a r bitrary unit, and the standard deviations are less than the size of the points.
t a t i o n f u n c t i o n s . T h e f l u c t u a t i o n s of th e n a t u r e e l a b o r a t e d in r e f s . 2 , 3 ) a r e c l e a r l y s e e n and the " c o h e r e n c e e n e r g y " AE i s about 10 keV. It i s q u i t e s i g n i f i c a n t and s t r i k i n g to n o t e the c o r r e l a t i o n in the e x c i t a t i o n f u n c t i o n s c o r r e s p o n d i n g to th e d i f f e r e n t s t a t e s of F e 5 5 ; d i f f e r e n t n e u t r o n g r o u p s f l u c t u a t e t o g e t h e r . In o r d e r to shed s o m e l i g h t on t he r e a c t i o n m e c h a n i s m , c r u d e a n g u l a r d i s t r i b u t i o n s of the n e u t r o n g r o u p s w e r e t a k e n . T h e y a r e shown in fig. 4. Although c r u d e , t h e y show that t he a n g u l a r d i s t r i b u t i o n s a r e not s y m m e t r i c ab o u t 90 o. T h e a b o v e a s y m m e t r y and the c o r r e l R t i o n of the f l u c t u a t i o n s in the e x c i t a t i o n f u n c t i o n s i m p l y n o n - v a l i d i t y of the s t a t i s t i c a l c o m p o u n d r e a c t i o n assumptions for this particular investigation. A m o r e c o m p l e t e i n v e s t i g a t i o n of the f l u c t u a t i o n s i s in p r o g r e s s . I) C.H. Johnson, A. Galonsky and J. P. Ulrich, Phys. Rev. 109 (1958) 1243. 2) T.Ericson, Phys. Rev. Letters 5 (1960) 430. 3) T. Ericson, l>roc, of the Int. Conf. on Nuclear Structure ed. D.A. Bromley and E.W.Vogt (University of Toronto Press, Toronto, Canada, 1960) p. 697.
= o
2
J
0 8 6
I Ep = 2,372
I
MeV
I
0
o
30
4 2 0
60
90
420
15o
18o
8. (deg}
Fig. 4. Angular distributions of three neutron groups taken for the proton energies indicated. Energies in keV r e f e r to the excited states of Fe55. Relative e r r o r s for E* = 410 keV are about 7% and the e r r o r s for E* = 940 keV and g.s. are less than the size of the points.
4) L. CoUi et al., Physics Letters 1 (1962) 120. 5) F . E . Durham, M.L. Halbert, C.D. Moak and A. Zucker, to be published. 6) Y.Cassagnou et al., Physics Letters 2 (1962) 93. 7) L. Cranberg et al., Nuclear Instr. and Meth. 12 (1961) 335 8) A . J . Elwyn et al., Phys. Rev. 112 (1958) 1200. 9) H. Mark, C. McClelland and C. Goodman, Phys. Rev. 98 (1955) 1245.
139
LOW-ENERGY AND THE
PHYSICS
LETTERS
(p,n) EXCITED
REACTION STATES
15 June 1963
O N M n 55 O F F e 55
H. J. K I M Oak Ridge National Laboratory, Oak Ridge, Tennessee
Received 20 May 1963
It has been well demonstrated I) that neutron emission in the major decay m o d e of the compound system formed by proton capture if the proton energy is well below the Coulomb barrier but still a few tenths of a M e V above the (p,n) threshold. Ericson 2,3) explains fluctuations in the excitation functions of nuclear reactions induced by lowenergy projectiles in terms of a "coherence energy ~' A E and width r (fluctuation has width A E ~ r), and discusses statistical compound reaction. Fluctuations of the above type have been observed in the reactions Si28(n,a)Mg 25 (ref. 4) and C 12(O16, ez) M g 25 (ref. 5) induced by 10 - 15 M e V projectiles, and in the A127(d,a)Mg25 (ref. 6) reaction. It is expected that the low-energy (p,n) reaction on m e dium A nuclei m a y reveal simpler fluctuations than those observed with 10 - 15 M e V projectiles since the contribution from direct reaction should be small. In view of the above considerations along with the availability of good resolution time-offlight neutron detection scheme, the low-energy (p ,n) reaction is a very attractive way to examine reaction mechanisms and low-lying excited states. Neutron groups corresponding to the low-lying excited states of Fe 55 as well as the ground state have been previously observed 7,8) by the (p,n) reaction on Mn55. Since Fe55 is an unstable isotope, the (p,n) reaction on Mn55 and the (d,p) reaction on Fe54 are convenient means of investigating l o w - l y i n g s t a t e s of Fe55. A thin Mn55 target (less than 5 keV thick for 2.4 MeV protons) evaporated on Pt backing was used for this investigation using the ORNL 3-MeV pulsed Van de Graaff. A neutron time-of-flight spectrum is shown in fig. 1. In addition to previously reported neutron groups corresponding to the ground, 413 keV and 933 keV states of Fe55, two weakly excited, previously unreported, neutron groups are observed. These are identified to be (p, n) neutrons corresponding to excited states of Fe55 so far unreported. Observation of de-excitation gamma rays from Mn55(p,n) Fe55. reaction firmly establishes the identification. The de-excitation gamma-ray spectrum of Fe55 is shown in fig. 2. The energies of the two states are 510 ± 10 keV and 680 + 10 keV. Gamma rays of 138
,o'
i
i
I
Mn55 ( p , n) Fe55 5
Ep -= 2.4 MaV FLIGHT PATH ~ 2 meters 8 = 0=
940k
GROUND STATE - -
680
NO 4t0
v
. .
CHANNEL NUMBER
Fig. 1. NeuRon t i m e - o f - f l i g h t spectrum. Energies in keV designate the excited states of Fe 55 (flight time
increases from right to left:). 4'104 L
~]
~ ~ 5 2
330410
510
680
11
1
1
l,rl
. . . .
.P"4-. t
.'..
~
• ' : ~.,t',,4..1: Mn55 (p, n )4 Fe55 '
950keV
~
! ". :
I 1
"l
"*,,
/p _~2.4 MeV
*',
iO2
-- -
]
,
_
CHANNEL NUMBER
Fig. 2. De-exmtatmn gamma-ray spectrum of Fe 55 formed by the reaction Mn5S(p, n) Fe 55.. Intense 330 keV gamma rays are due to Coulomb excitation of Pt backing.
505 k e V and 650 k e V w e r e a l s o o b s e r v e d b y M a r k e t a l . 9) v i a M n 5 5 ( p , n ~ ) F e 5 5 . E x c i t a t i o n f u n c t i o n s of the n e u t r o n g r o u p s f e e d ing the l o w - l y i n g s t a t e s of F e 5 5 w e r e t a k e n with 2 ~ 3 k e V s t e p s and t h e y a r e shown in fig. 3. I n t e n s i t i e s of t h e two w e a k l y e x c i t e d s t a t e s w e r e not s t r o n g enough to y i e l d s t a t i s t i c a l l y s i g n i f i c a n t e x c i -
Volume 5, number 2
PHYSICS
LETTERS
15 June 1963
8 ? 6 05
q4
GROON0\ /
• A
\,
l,k
8
GROUND STATE E ' = 9 4 0 keV
i
• .E = = 440 keV
I
£ . = 2 A 0 3 MeV
s
STATE..~=,,-'v
I
4
ul
F--
I E*' =940 keY' - ~ , . . /
'~1
0 t0
I
I
/E_ i = ;>,388 MeV 8 f E = 4 t 0 keY/ I 2.560
l
~G 2.580 2.400 Z.421 PROTON ENERGY (MeV)
2.44t
Fig. 3. F o r w a r d angle (6n = 0% excitation functions of the (p, n) reaction neutrons. The arrows indicate the proton energies where the angular distributions ar e taken. Neutron intensities are in the same a r bitrary unit, and the standard deviations are less than the size of the points.
t a t i o n f u n c t i o n s . T h e f l u c t u a t i o n s of th e n a t u r e e l a b o r a t e d in r e f s . 2 , 3 ) a r e c l e a r l y s e e n and the " c o h e r e n c e e n e r g y " AE i s about 10 keV. It i s q u i t e s i g n i f i c a n t and s t r i k i n g to n o t e the c o r r e l a t i o n in the e x c i t a t i o n f u n c t i o n s c o r r e s p o n d i n g to th e d i f f e r e n t s t a t e s of F e 5 5 ; d i f f e r e n t n e u t r o n g r o u p s f l u c t u a t e t o g e t h e r . In o r d e r to shed s o m e l i g h t on t he r e a c t i o n m e c h a n i s m , c r u d e a n g u l a r d i s t r i b u t i o n s of the n e u t r o n g r o u p s w e r e t a k e n . T h e y a r e shown in fig. 4. Although c r u d e , t h e y show that t he a n g u l a r d i s t r i b u t i o n s a r e not s y m m e t r i c ab o u t 90 o. T h e a b o v e a s y m m e t r y and the c o r r e l R t i o n of the f l u c t u a t i o n s in the e x c i t a t i o n f u n c t i o n s i m p l y n o n - v a l i d i t y of the s t a t i s t i c a l c o m p o u n d r e a c t i o n assumptions for this particular investigation. A m o r e c o m p l e t e i n v e s t i g a t i o n of the f l u c t u a t i o n s i s in p r o g r e s s . I) C.H. Johnson, A. Galonsky and J. P. Ulrich, Phys. Rev. 109 (1958) 1243. 2) T.Ericson, Phys. Rev. Letters 5 (1960) 430. 3) T. Ericson, l>roc, of the Int. Conf. on Nuclear Structure ed. D.A. Bromley and E.W.Vogt (University of Toronto Press, Toronto, Canada, 1960) p. 697.
= o
2
J
0 8 6
I Ep = 2,372
I
MeV
I
0
o
30
4 2 0
60
90
420
15o
18o
8. (deg}
Fig. 4. Angular distributions of three neutron groups taken for the proton energies indicated. Energies in keV r e f e r to the excited states of Fe55. Relative e r r o r s for E* = 410 keV are about 7% and the e r r o r s for E* = 940 keV and g.s. are less than the size of the points.
4) L. CoUi et al., Physics Letters 1 (1962) 120. 5) F . E . Durham, M.L. Halbert, C.D. Moak and A. Zucker, to be published. 6) Y.Cassagnou et al., Physics Letters 2 (1962) 93. 7) L. Cranberg et al., Nuclear Instr. and Meth. 12 (1961) 335 8) A . J . Elwyn et al., Phys. Rev. 112 (1958) 1200. 9) H. Mark, C. McClelland and C. Goodman, Phys. Rev. 98 (1955) 1245.
139