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Angular distribution of the 19F(n, α)16N reaction at En=4.7 MeV
Nuclear Physics 83 (1966) 407---412; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprlnt or microfilmwithout written permissionfrom the publisher
A N G U L A R D I S T R I B U T I O N O F T H E 19F(n, ~t)t6N R E A C T I O N A T En = 4.7 M e V s. M. BHARATHI, U. T. RAHEJA and. E. KONDAIAH ? Tam Institute of Fundamental Research, Colaba, Bombay 5 Received 14 December 1965
Abstract: The energy and angular distribution of alpha particles arising from the lSF(n, 0~)lSN reaction at En = 4.7~0.05 MeV were studied using a gas proportional counter telescope. The angular distribution fits well with a Legendre polynomial ao÷aiPt(cos 0), with ao = 1.0±0.1 and al = 0.4±0.15, showing interference between resonances occurring in the neighbourhood. of En = 4.7 MeV used in this study. The cross section for this reaction is obtained as 175 i 18 mb by comparing the yield of this reaction at 0 ° with the yield of recoil protons from an alkathene target. E / L
NUCLEAR REACTION 19F(n, ~), E = 4.7 MeV; measured t~(E, 0)
1. Introduction T h e o c c u r r e n c e o f resonances in the 19F(n, ~)16N r e a c t i o n was first o b s e r v e d by W i l h e l m y 1). His energy s p e c t r a showed p e a k s which can be a t t r i b u t e d to r e s o n a n c e s in the cross section at n e u t r o n energies o f 4.4 a n d 4.9 MeV. M e a s u r e m e n t s o f the excitation f u n c t i o n a n d a b s o l u t e cross sections for the 19F(n, e) r e a c t i o n b y M a r i o n a n d Brugger 2) also gave r e s o n a n c e s at these energies. A t 4.7 M e V n e u t r o n energy, one may, therefore, expect a n interference o f resonances between n e i g h b o u r i n g levels in the c o m p o u n d nucleus 2°F. I n such a case, the a n g u l a r d i s t r i b u t i o n c o u l d be fitted with a series L e g e n d r e p o l y n o m i a l e x p a n s i o n in which b o t h even a n d o d d terms are t a k e n into account. T h e a n g u l a r d i s t r i b u t i o n results p r e s e n t e d in this p a p e r s h o w t h a t such an interference does t a k e place.
2. The Experimental Method and Results T h e c o u n t e r telescope a n d the a s s o c i a t e d electronics were r e p o r t e d earlier b y K o n d a i a h a n d R a h e j a a), I t essentially consists o f two p r o p o r t i o n a l counters filled with 95 % o f k r y p t o n a n d 5 ~ o f CO2 at a pressure o f 2 c m o f Hg. The gas pressure a n d hence the gas m u l t i p l i c a t i o n was such that a l p h a s f r o m fluorine c o u l d give l a r g e r pulses t h a n p r o t o n s . G r a p h i t e lining on the inner walls o f the cathodes, was necess a r y to p r e v e n t c h a r g e d particles t h a t m a y arise f r o m the m a t e r i a l o f the c a t h o d e due to n e u t r o n b o m b a r d m e n t f r o m entering the sensitive v o l u m e o f the counter. A t i t a n i u m - t r i t i u m target b o m b a r d e d with 5.5 M e V p r o t o n s f r o m the V a n de G r a a f f t Present address: Professor and Head of the Department, Laboratories for Nuclear Studies, Andhra University, Waltair 407
408
s. hi. BHARATHIet al.
accelerator at T r o m b a y served as the source of neutrons; the neutron beam at 0 ° to the incident proton beam has been used for this experiment. Neutrons were monitored by a glycyryl borate plastic plus Zns scintillator-photomultiplier assembly, which is insensitive to g a m m a rays. A teflon ribbon of 20 mg/cm 2 thickness was used as the target of fluorine. By alternately exposing teflon and graphite to the neutron beam, the contribution to fluorine as well as background could be determined. Graphite was chosen as the background target because the neutron energy used in this experiment is below the threshold energy for any reaction with graphite. It should also be noted 15
10
>bz LIJ Z
5
8
12 CHANNEL
16
20
24
NUMBER
Fig. 1. The dE/dX spectrum of americium alphas.
that the cross section for the 19F(n,p)i90 reaction is negligible at E , = 4.7 MeV according to Marion and Brugger 2). The two proportional counters were used as d E / d X counters and were operated in coincidence in order to minimize the background arising from charged particle reactions in krypton gas filling due to neutron bombardment. The pulse-height spectrum of alphas from the counter near the source, gated by the coincidence pulse derived from both the counters, was recorded on a T M C 400-channel analyser. The spectrometer was calibrated using an 241Am alpha source. The energies of alpha particles giving rise to observed pulse-heights could be assessed by means of standard range
19F(n,
409
energy curves given by Livington and Bethe 4). A typical pulse-height spectrum of americium alphas is shown in fig. I. Fig. 2 gives the pulse-height distribution of fluorine reaction alphas taken at 0 ° to the neutron beam after taking into account the background. The peak of the spectrum shown in fig. 2 corresponds, roughly, to 3 MeV in agreement with the ground state Q-value of the 19F(n, ct)16N reaction. The close-lying states which occur within 300 Cu ( n , p)
F i g ( n , o(" )
PROTONS
ALPHAS
Eoc-3.0MeV I00(: >tz
Iod
z_
t t
z
Q. 50O
|
8
12
16 CHANNEL
20
24
z_
5O
I
|
28
|
I
J
32
NUMBER
Fig. 2. The dE/dX spectrum of ]gF(n, u)a~N alphas and copper protons at 0° to the incident neutron beam. keV above the ground state of 16N, could not be resolved. As the angle of observation increased, a gradual shift in the peak of the spectrum towards the low-energy side has been observed, which is in accordance with the decrease of alpha energy with increase of lab angle. The gas pressure and the operation of the counter were such that protons of energy less than 0.2 MeV could not give rise to any coincidences. However, protons of energy greater than 0.2 MeV do contribute below the 14th channel in the spectrum shown in fig. 2 and the contribution in the region up to the 14th channel is thus attributed to protons arising from the Cu(n, p) reaction because copper has been used as the window of the counter. These protons from copper were completely cut off by the graphite (,~ 160 mg/cm 2) target used for background meas-
410
s.M.
BHARATHI
et aL
75(
50C
Z Z
2~
I0
20
30
40
SO
CHANNEL F i g . 3. T h e
dE/dX s p e c t r u m
of recoil protons
60
70
80
NUMBER
from
an alkathene
target
at 0 ° to neutron
beam.
40
~
20
o LI
i
0
I
40
i
I
80
~
I
120
i
I
160
e C . M . { DEGREES) Fig. 4. Angular distribution o f ZSF(n, ~¢)x6N alphas. The curve shown is the best fit curve,
ae+azPx(cos O), w i t h a0 = 1.0-4-0.1 a n d ax ----- 0 . 4 - 4 - 0 . 1 5 .
lOF(n, g)l°N REACTION
411
urements, whereas they were mainly reduced in energy by the teflon target (20 mg/ cm 2) and hence are present in fig. 2 at pulse-heights below the 14th channel. The pulse-height spectrum of alkathene protons at 0 ° is shown in fig. 3. The energy loss in the gas of the first counter, for protons of 4.7 MeV arising from alkathene, is roughly 10 times less than that for 241Am alphas of energy 5.5 MeV. Hence it was necessary to increase the amplification by a factor of 10 in order to observe the pulseheight spectrum of alkathene protons. The contribution of Cu(n, p) protons in fig. 3 turned out to be ~ 1 ~ of the alkathene yield and has been taken into account in the cross-section calculation. The 0 ° yield of alphas from fluorine has been compared with the 0 ° recoil proton yield from alkathene for which the differential cross section is reported by Fowler and Brolley 5) as 530 mb/sr. With a knowledge of the effective thicknesses of the alkathene and teflon targets and hence the number of hydrogen and fluorine atoms, a(0 °) for fluorine alphas has been computed to be 25+ 5 mb/sr. The error in the cross section comprises the counting statistics, the finite angular aperture of the telescope and the large uncertainty in the effective fluorine target thickness particularly at low alpha energies. The total cross section for the 19F(n, ~)16N reaction was evaluated as 175_+ 18 mb after taking these errors into consideration. Least-squares fits of the measured angular distribution with a Legendre polynomial expansion in which both even and odd order terms occur have been tried. The best fit curve is shown in fig. 4. It is a first-order Legendre polynomial ao + alPl(COS 0) with ao = 1_+0.1 and at = 0.40_+0.15. The geometry of the telescope was such that the uncertainty in the scattering angle 0 between the incident neutron direction and the direction of alpha particles scattered from any point of the target into the aperture of the telescope and the true setting 0 o of the spectrometer axis varied from a maximum of 16 ° at 0o = 0 ° to 4 ° at 0o = 180°; this happened because the telescope had to be kept at a larger distance at backward angles than at forward angles. Because of the finite angular aperture of the telescope, one might expect the measured angular distribution to be different from the true angular distribution. In order to fit the theoretical angular distributions to the measured curve, one could smear the theoretical distribution, as has been done by Ribe and Seagrave 6) and then compare it with the measured distribution. However, it has been shown by Ribe and Seagrave 6) that the finite angular aperture of a spectrometer having similar window and aperture, has negligible effect even on stripping type angular distributions. On account o f the smooth behaviour of the Legendre polynomial expansion, which fairly fitted the measured curve in this particular case, the angular aperture will not affect the results o f this experiment. 3. Conclusions
The following points give support to the conclusion that this reaction is a compound nucleus reaction.
412
s.M. BHARATHIet
al.
(i) The 19F nucleus is not heavy and therefore the Coulomb barrier does not inhibit alpha particle emission appreciably. (ii) Resonances in the 19F(n, ~x) cross section have been reported close to the neutron energy of 4.7 MeV and these can interfere and give rise to the Pl(cos 0) term if they have opposite parities. (iii) This study shows that the angular distribution fits well with 1 + a l P ~ ( c o s 0) and it does not need any higher order coefficients that might imply direct interactions. (iv) The interference term in this angular distribution can be attributed to interference between opposite parity levels in the compound nucleus 2°F, as all the unresolved levels in the residual nucleus 16N up to 3 MeV excitation involved in this study are known to have negative parities. The measured cross section 175+_18 m b agrees within the limits of error with the cross section 150 mb_+40 ~ reported by Marion and Brugger 2). Our thanks are due to Professor R. Ramanna, Head of the Nuclear Physics Division. We are grateful to the Van de Graaff maintenance and operation group for running the accelerator for this experiment. Technical help rendered by S. G. Multani, P. J. Bhalerao, V. S. Ambekar and M. Y. Vaze during the course of this experiment is gratefully acknowledged. Our acknowledgements are also due to M. L. Jhingan for valuable discussions.
References 1) 2) 3) 4) 5) 6)
E. Wilhelmy, Z. Phys. 107 (1937) 769 J. B. Marion and R. M. Brugger, Phys. Rev. 100 (1955) 69 E. Kondaiah and U. T. Raheja, Nucl. Instr. 33 (1965) 241 M. S. Livingston and H. A. Bethe, Revs. Mod. Phys. 9 (1937) 245 J. L. Fowler and J. E. Brolley, Revs. Mod. Phys. 28 (1956) 103 F. L. Ribe and J. D. Seagrave, Phys. Rev. 94 (1954) 934
A N G U L A R D I S T R I B U T I O N O F T H E 19F(n, ~t)t6N R E A C T I O N A T En = 4.7 M e V s. M. BHARATHI, U. T. RAHEJA and. E. KONDAIAH ? Tam Institute of Fundamental Research, Colaba, Bombay 5 Received 14 December 1965
Abstract: The energy and angular distribution of alpha particles arising from the lSF(n, 0~)lSN reaction at En = 4.7~0.05 MeV were studied using a gas proportional counter telescope. The angular distribution fits well with a Legendre polynomial ao÷aiPt(cos 0), with ao = 1.0±0.1 and al = 0.4±0.15, showing interference between resonances occurring in the neighbourhood. of En = 4.7 MeV used in this study. The cross section for this reaction is obtained as 175 i 18 mb by comparing the yield of this reaction at 0 ° with the yield of recoil protons from an alkathene target. E / L
NUCLEAR REACTION 19F(n, ~), E = 4.7 MeV; measured t~(E, 0)
1. Introduction T h e o c c u r r e n c e o f resonances in the 19F(n, ~)16N r e a c t i o n was first o b s e r v e d by W i l h e l m y 1). His energy s p e c t r a showed p e a k s which can be a t t r i b u t e d to r e s o n a n c e s in the cross section at n e u t r o n energies o f 4.4 a n d 4.9 MeV. M e a s u r e m e n t s o f the excitation f u n c t i o n a n d a b s o l u t e cross sections for the 19F(n, e) r e a c t i o n b y M a r i o n a n d Brugger 2) also gave r e s o n a n c e s at these energies. A t 4.7 M e V n e u t r o n energy, one may, therefore, expect a n interference o f resonances between n e i g h b o u r i n g levels in the c o m p o u n d nucleus 2°F. I n such a case, the a n g u l a r d i s t r i b u t i o n c o u l d be fitted with a series L e g e n d r e p o l y n o m i a l e x p a n s i o n in which b o t h even a n d o d d terms are t a k e n into account. T h e a n g u l a r d i s t r i b u t i o n results p r e s e n t e d in this p a p e r s h o w t h a t such an interference does t a k e place.
2. The Experimental Method and Results T h e c o u n t e r telescope a n d the a s s o c i a t e d electronics were r e p o r t e d earlier b y K o n d a i a h a n d R a h e j a a), I t essentially consists o f two p r o p o r t i o n a l counters filled with 95 % o f k r y p t o n a n d 5 ~ o f CO2 at a pressure o f 2 c m o f Hg. The gas pressure a n d hence the gas m u l t i p l i c a t i o n was such that a l p h a s f r o m fluorine c o u l d give l a r g e r pulses t h a n p r o t o n s . G r a p h i t e lining on the inner walls o f the cathodes, was necess a r y to p r e v e n t c h a r g e d particles t h a t m a y arise f r o m the m a t e r i a l o f the c a t h o d e due to n e u t r o n b o m b a r d m e n t f r o m entering the sensitive v o l u m e o f the counter. A t i t a n i u m - t r i t i u m target b o m b a r d e d with 5.5 M e V p r o t o n s f r o m the V a n de G r a a f f t Present address: Professor and Head of the Department, Laboratories for Nuclear Studies, Andhra University, Waltair 407
408
s. hi. BHARATHIet al.
accelerator at T r o m b a y served as the source of neutrons; the neutron beam at 0 ° to the incident proton beam has been used for this experiment. Neutrons were monitored by a glycyryl borate plastic plus Zns scintillator-photomultiplier assembly, which is insensitive to g a m m a rays. A teflon ribbon of 20 mg/cm 2 thickness was used as the target of fluorine. By alternately exposing teflon and graphite to the neutron beam, the contribution to fluorine as well as background could be determined. Graphite was chosen as the background target because the neutron energy used in this experiment is below the threshold energy for any reaction with graphite. It should also be noted 15
10
>bz LIJ Z
5
8
12 CHANNEL
16
20
24
NUMBER
Fig. 1. The dE/dX spectrum of americium alphas.
that the cross section for the 19F(n,p)i90 reaction is negligible at E , = 4.7 MeV according to Marion and Brugger 2). The two proportional counters were used as d E / d X counters and were operated in coincidence in order to minimize the background arising from charged particle reactions in krypton gas filling due to neutron bombardment. The pulse-height spectrum of alphas from the counter near the source, gated by the coincidence pulse derived from both the counters, was recorded on a T M C 400-channel analyser. The spectrometer was calibrated using an 241Am alpha source. The energies of alpha particles giving rise to observed pulse-heights could be assessed by means of standard range
19F(n,
409
energy curves given by Livington and Bethe 4). A typical pulse-height spectrum of americium alphas is shown in fig. I. Fig. 2 gives the pulse-height distribution of fluorine reaction alphas taken at 0 ° to the neutron beam after taking into account the background. The peak of the spectrum shown in fig. 2 corresponds, roughly, to 3 MeV in agreement with the ground state Q-value of the 19F(n, ct)16N reaction. The close-lying states which occur within 300 Cu ( n , p)
F i g ( n , o(" )
PROTONS
ALPHAS
Eoc-3.0MeV I00(: >tz
Iod
z_
t t
z
Q. 50O
|
8
12
16 CHANNEL
20
24
z_
5O
I
|
28
|
I
J
32
NUMBER
Fig. 2. The dE/dX spectrum of ]gF(n, u)a~N alphas and copper protons at 0° to the incident neutron beam. keV above the ground state of 16N, could not be resolved. As the angle of observation increased, a gradual shift in the peak of the spectrum towards the low-energy side has been observed, which is in accordance with the decrease of alpha energy with increase of lab angle. The gas pressure and the operation of the counter were such that protons of energy less than 0.2 MeV could not give rise to any coincidences. However, protons of energy greater than 0.2 MeV do contribute below the 14th channel in the spectrum shown in fig. 2 and the contribution in the region up to the 14th channel is thus attributed to protons arising from the Cu(n, p) reaction because copper has been used as the window of the counter. These protons from copper were completely cut off by the graphite (,~ 160 mg/cm 2) target used for background meas-
410
s.M.
BHARATHI
et aL
75(
50C
Z Z
2~
I0
20
30
40
SO
CHANNEL F i g . 3. T h e
dE/dX s p e c t r u m
of recoil protons
60
70
80
NUMBER
from
an alkathene
target
at 0 ° to neutron
beam.
40
~
20
o LI
i
0
I
40
i
I
80
~
I
120
i
I
160
e C . M . { DEGREES) Fig. 4. Angular distribution o f ZSF(n, ~¢)x6N alphas. The curve shown is the best fit curve,
ae+azPx(cos O), w i t h a0 = 1.0-4-0.1 a n d ax ----- 0 . 4 - 4 - 0 . 1 5 .
lOF(n, g)l°N REACTION
411
urements, whereas they were mainly reduced in energy by the teflon target (20 mg/ cm 2) and hence are present in fig. 2 at pulse-heights below the 14th channel. The pulse-height spectrum of alkathene protons at 0 ° is shown in fig. 3. The energy loss in the gas of the first counter, for protons of 4.7 MeV arising from alkathene, is roughly 10 times less than that for 241Am alphas of energy 5.5 MeV. Hence it was necessary to increase the amplification by a factor of 10 in order to observe the pulseheight spectrum of alkathene protons. The contribution of Cu(n, p) protons in fig. 3 turned out to be ~ 1 ~ of the alkathene yield and has been taken into account in the cross-section calculation. The 0 ° yield of alphas from fluorine has been compared with the 0 ° recoil proton yield from alkathene for which the differential cross section is reported by Fowler and Brolley 5) as 530 mb/sr. With a knowledge of the effective thicknesses of the alkathene and teflon targets and hence the number of hydrogen and fluorine atoms, a(0 °) for fluorine alphas has been computed to be 25+ 5 mb/sr. The error in the cross section comprises the counting statistics, the finite angular aperture of the telescope and the large uncertainty in the effective fluorine target thickness particularly at low alpha energies. The total cross section for the 19F(n, ~)16N reaction was evaluated as 175_+ 18 mb after taking these errors into consideration. Least-squares fits of the measured angular distribution with a Legendre polynomial expansion in which both even and odd order terms occur have been tried. The best fit curve is shown in fig. 4. It is a first-order Legendre polynomial ao + alPl(COS 0) with ao = 1_+0.1 and at = 0.40_+0.15. The geometry of the telescope was such that the uncertainty in the scattering angle 0 between the incident neutron direction and the direction of alpha particles scattered from any point of the target into the aperture of the telescope and the true setting 0 o of the spectrometer axis varied from a maximum of 16 ° at 0o = 0 ° to 4 ° at 0o = 180°; this happened because the telescope had to be kept at a larger distance at backward angles than at forward angles. Because of the finite angular aperture of the telescope, one might expect the measured angular distribution to be different from the true angular distribution. In order to fit the theoretical angular distributions to the measured curve, one could smear the theoretical distribution, as has been done by Ribe and Seagrave 6) and then compare it with the measured distribution. However, it has been shown by Ribe and Seagrave 6) that the finite angular aperture of a spectrometer having similar window and aperture, has negligible effect even on stripping type angular distributions. On account o f the smooth behaviour of the Legendre polynomial expansion, which fairly fitted the measured curve in this particular case, the angular aperture will not affect the results o f this experiment. 3. Conclusions
The following points give support to the conclusion that this reaction is a compound nucleus reaction.
412
s.M. BHARATHIet
al.
(i) The 19F nucleus is not heavy and therefore the Coulomb barrier does not inhibit alpha particle emission appreciably. (ii) Resonances in the 19F(n, ~x) cross section have been reported close to the neutron energy of 4.7 MeV and these can interfere and give rise to the Pl(cos 0) term if they have opposite parities. (iii) This study shows that the angular distribution fits well with 1 + a l P ~ ( c o s 0) and it does not need any higher order coefficients that might imply direct interactions. (iv) The interference term in this angular distribution can be attributed to interference between opposite parity levels in the compound nucleus 2°F, as all the unresolved levels in the residual nucleus 16N up to 3 MeV excitation involved in this study are known to have negative parities. The measured cross section 175+_18 m b agrees within the limits of error with the cross section 150 mb_+40 ~ reported by Marion and Brugger 2). Our thanks are due to Professor R. Ramanna, Head of the Nuclear Physics Division. We are grateful to the Van de Graaff maintenance and operation group for running the accelerator for this experiment. Technical help rendered by S. G. Multani, P. J. Bhalerao, V. S. Ambekar and M. Y. Vaze during the course of this experiment is gratefully acknowledged. Our acknowledgements are also due to M. L. Jhingan for valuable discussions.
References 1) 2) 3) 4) 5) 6)
E. Wilhelmy, Z. Phys. 107 (1937) 769 J. B. Marion and R. M. Brugger, Phys. Rev. 100 (1955) 69 E. Kondaiah and U. T. Raheja, Nucl. Instr. 33 (1965) 241 M. S. Livingston and H. A. Bethe, Revs. Mod. Phys. 9 (1937) 245 J. L. Fowler and J. E. Brolley, Revs. Mod. Phys. 28 (1956) 103 F. L. Ribe and J. D. Seagrave, Phys. Rev. 94 (1954) 934