- Email: [email protected]
Neutron-proton bremsstrahlung at 14.4 MeV
Nuclear Physics Al56 (1970) 105- 112; @ North-Holland Not to be
Publishing
Co., Amsterdam
reproduced by photoprint or micro6lm without written permission from the publisher
NEUTRON-PROTON
BREMSSTRAHLUNG
AT 14.4 MeV
M. FURIC, V. VALKOVIC, D. MILJANIC, P. TOMAS and B. ANTOLKOVIC Institute
“Ruder
BoSkovic”,
Zagreb,
Yugoslavia
Received 19 June 1970 Abstract: A kinematically complete experiment on the neutron-proton bremsstrahlung at a neutron energy of 14.4 MeV was performed. Protons and neutrons were detected on opposite sides of the neutron beam. Protons were identified and their energy measured. The associated particle method and the neutron-proton time-of-flight difference were used to reduce the background. An upper limit of 400 fib * srm2was found for the neutron-proton bremsstrahlung differential cross section at the detector setting f& = 8, = 30”. E
NUCLEAR
REACTIONS
‘H(n, np)y, E = 14.4 MeV; measured a&,
&, 0,).
1. Introduction Off-shell nucleon-nucleon t-matrix elements play an important role in finding the exact solution of the nuclear three-body problem ‘) in nuclear structure calculations, and they may serve as a possible test for various nucleon-nucleon potentials “). Nucleon-nucleon bremsstrahlung is the simplest process that yields information on the behaviour of the nucleon-nucleon t-matrix off the energy shell. A number of experiments has been performed on proton-proton bremsstrahlung3 - “). Neutron-proton bremsstrahlung, npy, has so far received little attention due to considerable experimental difficulties, such as the nonexistence of the neutron target, low intensity of the neutron beam, particularly at higher neutron energies, and large background usually present in experiments with neutron beams. Reported data on the npy are scarce. Koehler et al. “) extracted the npy cross section at a proton bombarding energy of 197 MeV from the quasifree process on the deuterium target. Such a procedure is doubtful, since large uncertainties are present in estimating the conversion factor to a real process. Brady et al. lo) performed measurements with a neutron beam of a mean energy of 208 MeV with a FWHM of 45 MeV. So far, no measurements at lower neutron bombarding energy have been reported. Recent developments of experimental techniques 11) have made it possible to perform measurements of the npy process at a bombarding energy of 14 MeV. Both the angular momentum analysis of radiation 12) and the OPE model for the nucleon-nucleon bremsstrahlung process l”) indicate that the neutron-proton bremsstrahlung is far more important than the corresponding proton-proton process. This is due to the El transition which cancels out in the ppy process but is an allowed 105
106
M. FUR16 et al.
transition in the npy process. The charged pion exchange, which is much more important for the nucleon-nucleon interaction at low energies than the neutral pion exchange, is also possible only in the npy case. NEUTRON DETECTOR
.-
DETECTOR
Fig. 1. The experimental setup. Positions of the CHZ target, neutron detector and counter telescope (consisting of AE~, AE, and E-detectors) are shown with respect to the neutron beam. The neutron beam was defined by a slit in front of the associated alpha-particle detector.
,O
$,:60’
6
Ep=36MeV
Z
En= ?XMeV
----- _t_
I-
z
!--I
-’
-$ = 2
Lo*
f
lo- t+ = 8.6MeV ’
-
=
Ep=58MeV _ _
_
_z,
,‘-
=
-
3, -mm v-m----_a_ _..------mm.----==
-
_
,==
- fip= 30’ ” Ep= ln7Mdl SEn= 37MeV -_
50
= --_-_..
c 60
-=-
-= 70
____ -_-_ -_-_---s-m
_-_ ------
NiilBER
90
1
1c10
T-CHANNEL Fig. 2. Neutron-proton time-of-flight difference, T, measured in neutron-proton elastic scattering. The position of the T-peak was investigated for different neutron and proton energies by varying the positions of the detectors and a smooth variation was observed. In this way the position of the T-peak for bremsstrahlung events was determined.
II-I,
107
BREMSSTRAHLUNG
This paper presents our first attempt to reach a reasonable upper limit for the npy process. Sect. 2 is devoted to the experimental technique, its calibration and use. Sect. 3 contains the discussion of the results obtained and suggestions for future improveof the measurement. 10 .
. . .
.
.
20 . .
BACKGROUND
RUN
.
~-
30
60
.
. 70
.
. .
. .
60 .
.
.
.
.
. .
.
.
. . . . .
10
. .
. . . .
. . . . .
.
100
. .
. .
.
20
. .
.
. . .
. .
. .
. . .
30
. . . .
.
. .
LO T -
.
. .
. . . . . . . .
.
.
. l .
.
.
.
. . .
. .
. . . . . .
. .
.
. . . .
.
.
0 90
.
. .
.
.
. . . . .
. . .
.
. .
. . . .
.
. .
.
.
. .
.
.
.
.
50
CHANNEL
.*.. . . . .
.
60
.
. .
. . . .
. . .
. . .
.
. .
.
a 70
60
NUMBER
Fig. 3. Part of the total E-T distribution. Bremsstrahlung events have to be in the restricted region depicted as “run”. The “background” region is obtained by shifting the brernsstrahlung region along the T-axis. (The magnitude of the point reflects the number of events at that position: 0 number of counts L 5, l 5 > number of counts 2 2, * number of counts = 1.)
2. Experimental setup A neutron beam was obtained Walton accelerator as a deuteron
from the 3H(d, n)4He reaction using a Cockcroftsource. The associated alpha particles were detected
108
M. FUR16
et al.
in order to “collimate” neutrons (fig. 1). “Collimation” was performed by the fast coincidence requirement between the E-detector and the alpha detector and between the E-detector and the neutron detector. The whole CH2 target (2.2 cm in diameter, lOmg*cm-’ thick) was placed inside the beam. Protons were identified in a counter telescope by measuring the energy loss AE and the energy E. A NE 218 liquid scintillator (7.6 cm in diameter and 7.6 cm thick, coupled to an Amperex 56 AVP photo5.5 . 5.4 y---w 5.3
/
/
.N ‘.
\
5.2-
‘\
5.1 -
;
5.0 -
t -
a-
n+p-n+,p+r 0”*30” Q&)=30” E,=14.4 MeV
\
‘\
\
El
\
i \
‘\ \
CJ lx 46-
\ \
\ \ \
\ \
z
4.6 -
\
0 E 45-
31
\
\
38
39
1D
41
42
4.3
LA
\
\
\
\
4.5
4.6
II
PROTON
49
49
50
5.1
5.2
5.3
5.4
5.5
56
53
5.8
!
ENERGY (MeV)
Fig. 4. Kinematically allowed locus for the neutron-proton bremsstrahlung at 0, = 30”, 0, = 30” and the neutron energy E, = 14.4 MeV.
multiplier) was used to detect neutrons. Fast pulses from the E- and the neutron detectors were fed to a time-to-amplitude converter. The time difference T between proton and neutron pulses was registered without performing the time-of-flight analysis. The T-data were recorded for background reduction, as will be shown below. A set of three pulses T, AE, E was fed into a 100 x 100 x 100~channel analyser, and the output was recorded by a printer. The energy calibration of the E-detector, the data normalisation and the check of the stability of the set-up were performed by measuring neutron-proton coincidences in neutron-proton elastic scattering. True neutron-proton events were concentrated in a sharp peak in the T-spectrum. Accidental coincidences caused by protons which move along the telescope and coincide with neutrons, were uniformly distributed along the T-axis and easily identified as background events. The position of the T-peak was investigated for different neutron and proton energies (fig. 2). A smooth variation
n-p
was found and the position from the npy process was studied for various neutron In order to measure the detectors were fixed at polar
-
109
BREMSSTRAHLUNG
of the T-peak corresponding
to the energies of the particles
determined. The efficiency of the neutron detector was energies. npy differential cross section, the proton and neutron angles 8, = en = 30” on opposite sides of the beam. The
0 P
Q=30’
P
I 20
I 30
I 40
I 50
I 100
I 60
I 200
: 0
El, (MeVl Fig. 5. Comparison of the existing ppy data and the measured upper limit to the npy cross section which was divided by the ratio R. 0 ref. 3), 77 ref. “), + ref. 5), x ref. 6), n ref. ‘), _ ref. *), 0 ref. sa), 7 present result divided by R. angular openings of the detectors were ASL, = 3.5 x lo-’ sr and AR, = 5 x 10T2 sr. The data were collected during a one-month run with a flux of 5 10’ neutrons/set in 47~ sr. The AE-E part was plotted for each T, AE, E event, and only events from the proton hyperbola were considered further. Part of the E-T distribution for these events is shown in fig. 3. Bremsstrahlung events have to be in the restricted region depicted as “run” in the same figure. The allowed interval in the T variable was chosen as explained in the discussion of fig. 2. The E interval was determined from the kinematics of the npy process (fig. 4) taking into account the smearing due to angular and energy
110
M. FUR16
et al.
resolution. The number of background events was found in the region determined shifting the bremsstrahlung region along the T-axis.
01
0
I
I
40
80
120
160
by
I
200
EINC(MN Fig. 6. Various
theoretical calculations for neutron-proton bremsstrahlung OBE - ret IA), 0 and O., - ref. Is). separable - ref. 16).
at 6$, = 0, = 30”.
3. Results and discussion During the measurement, 3 counts in the bremsstrahlung region and 5 counts in the background region were collected. Including one standard deviation, the upper limit of bremsstrahlung events was found to be 2.8 counts. The corresponding upper limit to the bremsstrahlung cross section was calculated by normalisation to n-p elastic scattering from the formula
Here N1 and Ni2 are the numbers
of elastic and bremsstrahlung
events, respectively,
n-p
BREMSSTRAHLUNG
111
obtained on the same number of target atoms and with the same number of impinging neutrons. The symbol A&!, denotes the angular opening of the neutron detector. The quantity f, is the overlap factor between two detectors and the target for the n-p elastic scattering geometry, and fi2 is the same factor for the br~msstrahlung geometry. The influence of the neutron detector efficiency was eliminated by measuring n-p elastic scattering at a neutron angle at which the neutron energy from the elastic scattering was equal to the mean neutron energy from the bremsstrahlung process. The ‘H(n, np)n reaction on the deuterium impurity in natural hydrogen and (n, np) reactions on telescope materials might also contribute to the “run” region. This contribution was estimated to be a few percent of the total number of events in this region, and therefore was neglected. In this way we obtained da/da, d52, 6 400 ,ub * sr -’ for the differential cross section at 0, = 8, = 30”. Theoretical caiculations for the npy process are scarce. Using the OPE model Baier et al. 13) estimated the ratio R -
DPW
a PPV
z
360
between the cross sections for npy and ppy at vanishing momentum transfer. Taking this ratio to be approximately correct we compared the measured upper limit to the npy cross section with the existing experimental results on the ppy process at the same angles (fig. 5). This gave the result that the measured upper limit for the npy cross section is in reasonable agreement with the ppy data. Recently Baier et aE. 14) calculated the npy process from the OBE model in a relativisti~ally and gauge invariant way. Their result at this energy is much lower than the limit established in the present paper (curve OBE in fig. 6). McGuire ’ ‘) calculated the npy cross section in a way similar to that used by Sobtl and Cromer “) for the ppy process. He performed two calculations: First, the offshell extension (curve 0 in fig. 6) was taken into account and second, it was replaced by the on-shell values (curve O,, in the same figure). These calculations give a much higher low-energy npy cross section than the OBE predictions and seem to follow the trend obtained by Pearce et al. 1“) for low neutron energies. However, the measured upper limit to the npy cross section is higher than the calculated values. The main cause of the background events in our experiment was the high counting rate in the neutron detector line. In order to eliminate this shortcoming, the following improvements should be made: (i) Two photomultipliers coupled in coincidence should be used and placed so as to view the liquid scintillator. (ii) A ny discriminator should be employed in the neutron detector line. (iii) The number of neutrons reflected from the walls should be significantly reduced. We conclude that more theoretical calculations with various potentials for the npy
112
M. PIJRIC et al.
process in the low-energy region are needed, since experiments on the npy process in this region have become feasible. We are much indebted to Prof. I. Slaus for many helpful discussions. References 1) L. D. Faddeev, JETP (Sov. Phys.) 39 (1960) 1459; C. Lovelace, Phys. Rev. 135 (1964) B1225 M. I. Sobel and A. M. Cromer, Phys. Rev. 132 (1963) 2698 B. Gottschalk, W. J. Shlaer and K. H. Wang, Nucl. Phys. A94 (1967) 491 K. W. Rothe, P. F. M. Koehler and E. H. Thordike, Phys. Rev. 157 (1967) 1247 I. Slaus, J. W. Verba, J. R. Richardson, R. F. Carlson, W. T. H. Van Oers and L. S. August, Phys. Rev. Lett. 17 (1966) 536 6) R. E. Warner, Can. J. Phys. 44 (1966) 1225 7) M. L. Halbert, D. L. Mason and L. C. Northcliffe, Phys. Rev. 168 (1968) 1130 8) A. Niiler, C. Joseph, V. ValkoviC, R. Spiger, T. Canada, S. T. Emerson, J. Sandler and G. C. Phillips, Phys. Rev. 178 (1969) 1621 8a)G. M. Crawley, D. L. Powell and B. V. Narashima Rao, Phys. Lett. 26B (1968) 576 9) P. F. M. Koehler, K. W. Rothe, E. H. Thordike, Phys. Rev. Lett. 18 (1967) 933 10) F. P. Brady, J. C. Young and C. Badinabhau, Phys. Rev. Lett. 20 (1968) 750 11) V. Valkovic, D. Miljanic, P. Tomas, B. Antolkovic and M. FuriC, Nucl. Instr. 76 (1969) 29 12) C. Dullemond and J. J. de Swart, Physica 26 (1960) 664 13) R. Baier, H. Ktihnelt, P. Urban and F. Widder, Acta Phys. Austriaca 28 (1968) 13 14) R. Baier, H. Kiihnelt and P. Urban, Nucl. Phys. Bll (1969) 675 15) J. H. McGuire, Phys. Rev. Cl (1970) 16) W. A. Pearce, W. A. Gale and I. M. Duck, Nucl. Phys. B3 (1967) 241 2) 3) 4) 5)
Publishing
Co., Amsterdam
reproduced by photoprint or micro6lm without written permission from the publisher
NEUTRON-PROTON
BREMSSTRAHLUNG
AT 14.4 MeV
M. FURIC, V. VALKOVIC, D. MILJANIC, P. TOMAS and B. ANTOLKOVIC Institute
“Ruder
BoSkovic”,
Zagreb,
Yugoslavia
Received 19 June 1970 Abstract: A kinematically complete experiment on the neutron-proton bremsstrahlung at a neutron energy of 14.4 MeV was performed. Protons and neutrons were detected on opposite sides of the neutron beam. Protons were identified and their energy measured. The associated particle method and the neutron-proton time-of-flight difference were used to reduce the background. An upper limit of 400 fib * srm2was found for the neutron-proton bremsstrahlung differential cross section at the detector setting f& = 8, = 30”. E
NUCLEAR
REACTIONS
‘H(n, np)y, E = 14.4 MeV; measured a&,
&, 0,).
1. Introduction Off-shell nucleon-nucleon t-matrix elements play an important role in finding the exact solution of the nuclear three-body problem ‘) in nuclear structure calculations, and they may serve as a possible test for various nucleon-nucleon potentials “). Nucleon-nucleon bremsstrahlung is the simplest process that yields information on the behaviour of the nucleon-nucleon t-matrix off the energy shell. A number of experiments has been performed on proton-proton bremsstrahlung3 - “). Neutron-proton bremsstrahlung, npy, has so far received little attention due to considerable experimental difficulties, such as the nonexistence of the neutron target, low intensity of the neutron beam, particularly at higher neutron energies, and large background usually present in experiments with neutron beams. Reported data on the npy are scarce. Koehler et al. “) extracted the npy cross section at a proton bombarding energy of 197 MeV from the quasifree process on the deuterium target. Such a procedure is doubtful, since large uncertainties are present in estimating the conversion factor to a real process. Brady et al. lo) performed measurements with a neutron beam of a mean energy of 208 MeV with a FWHM of 45 MeV. So far, no measurements at lower neutron bombarding energy have been reported. Recent developments of experimental techniques 11) have made it possible to perform measurements of the npy process at a bombarding energy of 14 MeV. Both the angular momentum analysis of radiation 12) and the OPE model for the nucleon-nucleon bremsstrahlung process l”) indicate that the neutron-proton bremsstrahlung is far more important than the corresponding proton-proton process. This is due to the El transition which cancels out in the ppy process but is an allowed 105
106
M. FUR16 et al.
transition in the npy process. The charged pion exchange, which is much more important for the nucleon-nucleon interaction at low energies than the neutral pion exchange, is also possible only in the npy case. NEUTRON DETECTOR
.-
DETECTOR
Fig. 1. The experimental setup. Positions of the CHZ target, neutron detector and counter telescope (consisting of AE~, AE, and E-detectors) are shown with respect to the neutron beam. The neutron beam was defined by a slit in front of the associated alpha-particle detector.
,O
$,:60’
6
Ep=36MeV
Z
En= ?XMeV
----- _t_
I-
z
!--I
-’
-$ = 2
Lo*
f
lo- t+ = 8.6MeV ’
-
=
Ep=58MeV _ _
_
_z,
,‘-
=
-
3, -mm v-m----_a_ _..------mm.----==
-
_
,==
- fip= 30’ ” Ep= ln7Mdl SEn= 37MeV -_
50
= --_-_..
c 60
-=-
-= 70
____ -_-_ -_-_---s-m
_-_ ------
NiilBER
90
1
1c10
T-CHANNEL Fig. 2. Neutron-proton time-of-flight difference, T, measured in neutron-proton elastic scattering. The position of the T-peak was investigated for different neutron and proton energies by varying the positions of the detectors and a smooth variation was observed. In this way the position of the T-peak for bremsstrahlung events was determined.
II-I,
107
BREMSSTRAHLUNG
This paper presents our first attempt to reach a reasonable upper limit for the npy process. Sect. 2 is devoted to the experimental technique, its calibration and use. Sect. 3 contains the discussion of the results obtained and suggestions for future improveof the measurement. 10 .
. . .
.
.
20 . .
BACKGROUND
RUN
.
~-
30
60
.
. 70
.
. .
. .
60 .
.
.
.
.
. .
.
.
. . . . .
10
. .
. . . .
. . . . .
.
100
. .
. .
.
20
. .
.
. . .
. .
. .
. . .
30
. . . .
.
. .
LO T -
.
. .
. . . . . . . .
.
.
. l .
.
.
.
. . .
. .
. . . . . .
. .
.
. . . .
.
.
0 90
.
. .
.
.
. . . . .
. . .
.
. .
. . . .
.
. .
.
.
. .
.
.
.
.
50
CHANNEL
.*.. . . . .
.
60
.
. .
. . . .
. . .
. . .
.
. .
.
a 70
60
NUMBER
Fig. 3. Part of the total E-T distribution. Bremsstrahlung events have to be in the restricted region depicted as “run”. The “background” region is obtained by shifting the brernsstrahlung region along the T-axis. (The magnitude of the point reflects the number of events at that position: 0 number of counts L 5, l 5 > number of counts 2 2, * number of counts = 1.)
2. Experimental setup A neutron beam was obtained Walton accelerator as a deuteron
from the 3H(d, n)4He reaction using a Cockcroftsource. The associated alpha particles were detected
108
M. FUR16
et al.
in order to “collimate” neutrons (fig. 1). “Collimation” was performed by the fast coincidence requirement between the E-detector and the alpha detector and between the E-detector and the neutron detector. The whole CH2 target (2.2 cm in diameter, lOmg*cm-’ thick) was placed inside the beam. Protons were identified in a counter telescope by measuring the energy loss AE and the energy E. A NE 218 liquid scintillator (7.6 cm in diameter and 7.6 cm thick, coupled to an Amperex 56 AVP photo5.5 . 5.4 y---w 5.3
/
/
.N ‘.
\
5.2-
‘\
5.1 -
;
5.0 -
t -
a-
n+p-n+,p+r 0”*30” Q&)=30” E,=14.4 MeV
\
‘\
\
El
\
i \
‘\ \
CJ lx 46-
\ \
\ \ \
\ \
z
4.6 -
\
0 E 45-
31
\
\
38
39
1D
41
42
4.3
LA
\
\
\
\
4.5
4.6
II
PROTON
49
49
50
5.1
5.2
5.3
5.4
5.5
56
53
5.8
!
ENERGY (MeV)
Fig. 4. Kinematically allowed locus for the neutron-proton bremsstrahlung at 0, = 30”, 0, = 30” and the neutron energy E, = 14.4 MeV.
multiplier) was used to detect neutrons. Fast pulses from the E- and the neutron detectors were fed to a time-to-amplitude converter. The time difference T between proton and neutron pulses was registered without performing the time-of-flight analysis. The T-data were recorded for background reduction, as will be shown below. A set of three pulses T, AE, E was fed into a 100 x 100 x 100~channel analyser, and the output was recorded by a printer. The energy calibration of the E-detector, the data normalisation and the check of the stability of the set-up were performed by measuring neutron-proton coincidences in neutron-proton elastic scattering. True neutron-proton events were concentrated in a sharp peak in the T-spectrum. Accidental coincidences caused by protons which move along the telescope and coincide with neutrons, were uniformly distributed along the T-axis and easily identified as background events. The position of the T-peak was investigated for different neutron and proton energies (fig. 2). A smooth variation
n-p
was found and the position from the npy process was studied for various neutron In order to measure the detectors were fixed at polar
-
109
BREMSSTRAHLUNG
of the T-peak corresponding
to the energies of the particles
determined. The efficiency of the neutron detector was energies. npy differential cross section, the proton and neutron angles 8, = en = 30” on opposite sides of the beam. The
0 P
Q=30’
P
I 20
I 30
I 40
I 50
I 100
I 60
I 200
: 0
El, (MeVl Fig. 5. Comparison of the existing ppy data and the measured upper limit to the npy cross section which was divided by the ratio R. 0 ref. 3), 77 ref. “), + ref. 5), x ref. 6), n ref. ‘), _ ref. *), 0 ref. sa), 7 present result divided by R. angular openings of the detectors were ASL, = 3.5 x lo-’ sr and AR, = 5 x 10T2 sr. The data were collected during a one-month run with a flux of 5 10’ neutrons/set in 47~ sr. The AE-E part was plotted for each T, AE, E event, and only events from the proton hyperbola were considered further. Part of the E-T distribution for these events is shown in fig. 3. Bremsstrahlung events have to be in the restricted region depicted as “run” in the same figure. The allowed interval in the T variable was chosen as explained in the discussion of fig. 2. The E interval was determined from the kinematics of the npy process (fig. 4) taking into account the smearing due to angular and energy
110
M. FUR16
et al.
resolution. The number of background events was found in the region determined shifting the bremsstrahlung region along the T-axis.
01
0
I
I
40
80
120
160
by
I
200
EINC(MN Fig. 6. Various
theoretical calculations for neutron-proton bremsstrahlung OBE - ret IA), 0 and O., - ref. Is). separable - ref. 16).
at 6$, = 0, = 30”.
3. Results and discussion During the measurement, 3 counts in the bremsstrahlung region and 5 counts in the background region were collected. Including one standard deviation, the upper limit of bremsstrahlung events was found to be 2.8 counts. The corresponding upper limit to the bremsstrahlung cross section was calculated by normalisation to n-p elastic scattering from the formula
Here N1 and Ni2 are the numbers
of elastic and bremsstrahlung
events, respectively,
n-p
BREMSSTRAHLUNG
111
obtained on the same number of target atoms and with the same number of impinging neutrons. The symbol A&!, denotes the angular opening of the neutron detector. The quantity f, is the overlap factor between two detectors and the target for the n-p elastic scattering geometry, and fi2 is the same factor for the br~msstrahlung geometry. The influence of the neutron detector efficiency was eliminated by measuring n-p elastic scattering at a neutron angle at which the neutron energy from the elastic scattering was equal to the mean neutron energy from the bremsstrahlung process. The ‘H(n, np)n reaction on the deuterium impurity in natural hydrogen and (n, np) reactions on telescope materials might also contribute to the “run” region. This contribution was estimated to be a few percent of the total number of events in this region, and therefore was neglected. In this way we obtained da/da, d52, 6 400 ,ub * sr -’ for the differential cross section at 0, = 8, = 30”. Theoretical caiculations for the npy process are scarce. Using the OPE model Baier et al. 13) estimated the ratio R -
DPW
a PPV
z
360
between the cross sections for npy and ppy at vanishing momentum transfer. Taking this ratio to be approximately correct we compared the measured upper limit to the npy cross section with the existing experimental results on the ppy process at the same angles (fig. 5). This gave the result that the measured upper limit for the npy cross section is in reasonable agreement with the ppy data. Recently Baier et aE. 14) calculated the npy process from the OBE model in a relativisti~ally and gauge invariant way. Their result at this energy is much lower than the limit established in the present paper (curve OBE in fig. 6). McGuire ’ ‘) calculated the npy cross section in a way similar to that used by Sobtl and Cromer “) for the ppy process. He performed two calculations: First, the offshell extension (curve 0 in fig. 6) was taken into account and second, it was replaced by the on-shell values (curve O,, in the same figure). These calculations give a much higher low-energy npy cross section than the OBE predictions and seem to follow the trend obtained by Pearce et al. 1“) for low neutron energies. However, the measured upper limit to the npy cross section is higher than the calculated values. The main cause of the background events in our experiment was the high counting rate in the neutron detector line. In order to eliminate this shortcoming, the following improvements should be made: (i) Two photomultipliers coupled in coincidence should be used and placed so as to view the liquid scintillator. (ii) A ny discriminator should be employed in the neutron detector line. (iii) The number of neutrons reflected from the walls should be significantly reduced. We conclude that more theoretical calculations with various potentials for the npy
112
M. PIJRIC et al.
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