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Neutron spectra at 0° from p—p and p—d collisions at 647 and 800 MeV incident energies
Volume 63B, number 1
PHYSICS LETTERS
5 July 1976
NEUTRON SPECTRA AT 0 ° FROM p-p AND p-d COLLISIONS AT 647 AND 800 MeV INCIDENT ENERGIES e
C.W. BJORK and P.J. RILEY University o f Texas, Austin, Texas 78712, USA
B.E. BONNER, J.E. SIMMONS and K.D. WlLLIAMSON Jr. Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87545, USA
M.L. EVANS, G. GLASS, J.C. HIEBERT, M. JAIN, R.A. KENEFICK and L.C. NORTHCLIFFE Texas A & M University, College Station, Texas 77843, USA
C.G. CASSAPAKIS, H.C. BRYANT, B.D. DIETERLE, C.P. LEAVITT and D.M. WOLFE University o f New Mexico, Albuquerque, New Mexico 87131, USA
and D.W. WERREN University o f Geneva, Geneva, Switzerland
Received 25 May 1976 The p-d spectra show sharp hi_gh-e'nergypeaks (~- 15 MeV FWHM) with integrated yields of 30 and 31 mb/sr, respectively and with lower-energybumps associated with pion production. The p-p spectra reflect the influence of the A(1232) resonance.
In a theoretical study of the nucleon-nucleon interaction, Gluckstern and Bethe [1 ] suggested that the reaction p + d ~ n + 2p would be a potentially useful neutron source at 0 ° emission angle. Their calculations, for energies near 90 MeV, showed that the neutron spectrum was enhanced and narrowed at its upper limit by the strong final-state two-proton interaction. This early proposal has now received its full realization at the Clinton P. Anderson Meson Physics Facility (LAMPF). Measday [2] and Langsford et al. [3] made measurements on the neutron spectrum from the p - d process at 0 ° for incident energies in the range 90 to 150 MeV. The last named authors concluded that at 96 MeV the inherent width of the peak was less than 2 MeV (FWHM). At LAMPF energies detailed measurements had not been made of the neutron spectrum in Work performed under the auspices of the U.S. Energy Research and Development Administration.
the p - d process; however, Larsen [4] produced and used a neutron beam of 710 MeV using a liquid deuterium target and 7.1 o emission angle. By measurements on the emitted protons he deduced that the peak neutron energy distribution had a standard deviation of 8 MeV. Neutron production in p - p collisions at medium energy is a manifestation of the lowest apion-nucleon resonance, the A(1232) with I and J = ~. Very few experiments exist at or below 1 GeV energy. The chief reference is that of Bugg et al. [5] who measured approximately 1600 interactions of the type p + p n + p + lr+ in a bubble chamber experiment. The n+p mass distributions were found to be consistent with A decay, and the peripheral nature of the process showed itself in the neutron angular distribution, which was peaked fore and aft in the c.m. system. In this paper we report new measurements on the 0 ° spectra of neutrons produced at 0 ° in p - p and p - d collisions at 647 and 800 MeV incident energy. 31
Volume 63B, number 1
PHYSICS LETTERS
Fig. 1. Experimental layout at the Nucleon Physics Lab at LAMPF. Fig. 1 shows a plan view of the experimental setup in the Nucleon Physics Lab. at LAMPF. The cryogenic target system [6] consisted of a forced convection loop containing either liquid hydrogen (LH2) or liquid deuterium (LD2) as required. The loop was cooled via a helium refrigerator with counter-current heat exchange between the cold helium gas and the fluid in the loop. Pressure and temperature of the loop fluid were measured. The production target thickness was 0.77 g/cm 2 for LH 2 and 1.80 g/cm 2 for LD 2. The proton beam intensity was measured by integration of the output of a toroid beam-current monitor. This current integrator was checked against nuclear activation measurements to the +10% level of accuracy. The neutron beam was formed by a steel collimator of 25.4 mm diameter placed at 0 ° with solid angle of 9.1 X 10 -6 sr. A neutron beam monitor (NMon) was placed at the collimator exit; a scintillation telescope also monitored the interaction rate at the LH 2 target (H-Mon). Protons from elastic and inelastic interactions in the LH 2 radiator (0.93 g/cm 2) were detected in a multiwire proportional chamber spectrometer. The spectrometer consisted of a magnet (M2), four x - y pairs of MWPC planes (Wl-W4) and two scintillation counters S1 and $2 which triggered the system and provided velocity information. System live time was determined by scaling N-Mon in relation to the inhibit signals of the spectrometer system. Some further details of the apparatus have recently been published [7] and a detailed report has been prepared [8]. A fraction of the lower-momentum protons originated from inelastic n - p interactions. The contribution of such inelastic processes was determined in a set of runs in which an aluminum target was used to 32
5 July 1976
generate a broad distribution of energies, using time of flight to determine incident neutron momenta. At 800 MeV these inelastic corrections were substantial. For example, in the p - d reaction about half of the proton recoils below 500 MeV/c momentum were inelastic; the p - p corrections at 800 MeV were somewhat smaller. At 647 MeV the corrections were much smaller. The elastic proton recoil distributions were converted to neutron spectra by knowledge of the n - p charge exchange cross section ( n - p CEX) as discussed in ref. [9]. The final absolute double differential cross sections were normalized with respect to incident proton beam intensity, the collimator solid angle, and the areal densities of the production and scattering targets. The major source of systematic error arose from long term drift of the proton beam current integration. This was estimated to be a 10% effect. Other sources of systematic error include: collimator solid angle +3%; areal density of production and radiator targets, -+2% from each; knowledge of the n - p CEX cross section, -+8%. Combining these effects in quadrature, the overall systematic error of the resultant cross sections is estimated to be -+13%. The results are plotted in fig. 2. The representative error bars are mainly statistical. Each p - d spectrum shows an intense and narrow ( ~ 15 MeV FWHM) highenergy charge exchange scattering peak ( p - d CEX) well separated from events at lower energy associated with pion production. The differential cross sections integrated over the peak region and the pion production bump are given in table 1. From a comparison of the peak characteristics in the processes p + d ~ n and n + d ~ p, using the same apparatus, we have obtained a crude estimate of the intrinsic peak width which turns out to be ~ 10 MeV, FWHM. This is substantially larger than that measured [3] at 96 MeV. The comparison also shows that the centroid of the peak in the process p + d ~ n lies approximately 7 MeV below the incident proton energy. Although the accuracy of these determinations is limited by spectrometer resolution (~ 10 MeV) the results are in fair agreement with calculations by Ward [10]. The energies at which we are quoting these results were the nominal accelerator energies. Our estimates of beam energy may differ by 1.5% from the accelerator energy depending on the run. The source of such discrepancy is not clear; in any case it contributes negligible error to these cross sections.
Volume 63B, number 1
PHYSICS LETTERS
T, 100
200 I
300
Table 1 Values of I d20/(da dp)hp at 0”.
(MeV) 400
500
600
700 600 / c
i!
-07 -0.6
P,
5 July 1976
(MeV/cl
Fig. 2. Neutron spectra (d’a/dndp) from the reactions p + d + n (circles) and p + p + n + p + tr+ (triangles) at nominal incident energies of 647 and 800 MeV.
From the integrated peak cross sections given in table 1 and from published [7] and unpublished results of our group, we can form the ratios (p-d CEX)/ (nip CEX) for 0” (lab) proton emission angle. The results for the ratios are 0.60 + 0.08 and 0.66 + 0.08 at 647 and 800 MeV, respectively. The reduction from unit ratio occurs because the Pauli principle inhibits transitions which do not result in a spin singlet state for the residual two protons. We may compare our results with those of Larsen [4] at 710 MeV, ignoring the energy difference in the process. He obtained the value 0.45 & 0.08 for the ratio as defined above. In view of the stated errors these results may be considered to be compatible. However, his value of the p-d CEX cross section is about half of ours; the source of this discrepancy is not known. It is interesting to note that Dzhelepov et al. [ 1 l] using a neutron beam of 380 MeV and the charge symmetric reaction measured a value (n-d CEX)/(n-p CEX) = 0.20 f 0.04 at 0’. This low value may indicate an anomalous behavior in the spin flip probability at this energy. The ratio of the integrated cross section in the pd CEX peak to that in an equal momentum interval in the adjacent valley is about 30 to 1. This marked isola-
Reaction
2eV)
CEXa Peak (mb/sr)
P-P P-P p-d p-d
647 800 647 800
30 31
Pionb
29 33 50 67
Region
W/sr)
a At 647 (800) MeV the range-is p > 1100 (1290) MeV/c. b At 647 (800) MeV the range is 380
tion of the peak neutrons from those at lower energy makes the p + d + n reaction an excellent source of monoenergetic neutrons at medium energy. The peak neutron beam intensity is approximately 8.7 X lo5 neutrons per microcoulomb through the 0” collimator. The neutron spectra from p-p collisions shown in fig. 2 are produced by the reaction p + p + n + p + rr+. Other processes, including double pion production, are negligible by comparison. The spectrum near 800 MeV is fairly well fitted by theoretical calculations [ 121 which assume that the dominant process is rr+ exchange resulting in an intermediate state of a high-energy neutron and the A( 1232) which subsequently decays. Not visible in the data as shown at 800 MeV is an indication of an n-p final-state interaction near 800 MeV/c. The effect of the n-p FSI is evident in the 647 MeV spectrum where it produces enhancement at 700 MeV/c. The similarity of the p-p and p-d spectra in the low momentum region leads to the speculation that the same mechanism is operating in both cases. It appears likely that in the p-d collisions the neutrons in the broad bump are associated with pion production in quasi-free p-p or p-n collisions which proceed through the A(1232) channel with the third nucleon as a spectator. This assumption would lead to a tintensity ratio between the p-d and p-p spectra. It neglects pion production which proceeds through n-n isospin 3 states in p-n collisions and contributions from p-d breakup without pion production. Although our normalization errors do not permit us to draw firm conclusions, our data are reasonably consistent with this simple picture.
33
Volume 63B, number 1
PHYSICS LETTERS
We would like to acknowledge the very important assistance in the LD 2 target development by J. Fretwell, J. Novak, and valuable support from the MP-Division. R. Michaud, B. Royball and F. Newcom provided engineering design and drafting help. The efforts of J. Boissevain were indispensable to our success; We thank A. Niethammer for her contribution to the data acquisition program and thank P. Grant for the nuclear chemistry calibration.
References [1] R.L.Gluckstern and H.A. Bethe, Phys. Rev. 81 (1951) 761. [2] D.F. Measday, Nucl. Instr. 40 (1966) 213.
34
[3] [4] [5] [6]
5 July 1976
A. Langsford et al., Nucl. Phys. A99 (1967) 246. R.R. Larsen, I! Nuovo Cimento, Ser. X 18 (1960) 1039. D.V. Bugg et al., Phys. Rev. 133 (1964) B1017. The prototype tacget loop is described in K.D. WiUiamsonet al., Advances Cryogenic Engineering 19 (1974) 241 ; the refrigerator is described in R.O. Voth et al., Advances Cryogenic Engineering 11 (1966) 126. [7] M.L. Evans et al., Phys. Rev. Lett. 36 (1976) 497. [8] C.W. Bjork, Ph.D. thesis, University Texas (1975), LASL report LA-6192-T. [9] C.G.'Cassapakis et al., phys Lett. 63B (1976) 35 [10] D.S. Ward, Ph.D. thesis, University Texas (1971). [ 11 ] V.P. Dzhelepov et al., I1 Nuovo Cimento, Suppl. to Ser. X 3 (1956) 61. [12] G.J. Stephenson in: Int. Conf. High energy physics and nuclear structure, Santa Fe and Los Alamos 1975, eds. D.E. Nagle et al. (A.I.P. Conf. Proc. No. 26, Am. Inst. Phys. 1975) 55.
PHYSICS LETTERS
5 July 1976
NEUTRON SPECTRA AT 0 ° FROM p-p AND p-d COLLISIONS AT 647 AND 800 MeV INCIDENT ENERGIES e
C.W. BJORK and P.J. RILEY University o f Texas, Austin, Texas 78712, USA
B.E. BONNER, J.E. SIMMONS and K.D. WlLLIAMSON Jr. Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico 87545, USA
M.L. EVANS, G. GLASS, J.C. HIEBERT, M. JAIN, R.A. KENEFICK and L.C. NORTHCLIFFE Texas A & M University, College Station, Texas 77843, USA
C.G. CASSAPAKIS, H.C. BRYANT, B.D. DIETERLE, C.P. LEAVITT and D.M. WOLFE University o f New Mexico, Albuquerque, New Mexico 87131, USA
and D.W. WERREN University o f Geneva, Geneva, Switzerland
Received 25 May 1976 The p-d spectra show sharp hi_gh-e'nergypeaks (~- 15 MeV FWHM) with integrated yields of 30 and 31 mb/sr, respectively and with lower-energybumps associated with pion production. The p-p spectra reflect the influence of the A(1232) resonance.
In a theoretical study of the nucleon-nucleon interaction, Gluckstern and Bethe [1 ] suggested that the reaction p + d ~ n + 2p would be a potentially useful neutron source at 0 ° emission angle. Their calculations, for energies near 90 MeV, showed that the neutron spectrum was enhanced and narrowed at its upper limit by the strong final-state two-proton interaction. This early proposal has now received its full realization at the Clinton P. Anderson Meson Physics Facility (LAMPF). Measday [2] and Langsford et al. [3] made measurements on the neutron spectrum from the p - d process at 0 ° for incident energies in the range 90 to 150 MeV. The last named authors concluded that at 96 MeV the inherent width of the peak was less than 2 MeV (FWHM). At LAMPF energies detailed measurements had not been made of the neutron spectrum in Work performed under the auspices of the U.S. Energy Research and Development Administration.
the p - d process; however, Larsen [4] produced and used a neutron beam of 710 MeV using a liquid deuterium target and 7.1 o emission angle. By measurements on the emitted protons he deduced that the peak neutron energy distribution had a standard deviation of 8 MeV. Neutron production in p - p collisions at medium energy is a manifestation of the lowest apion-nucleon resonance, the A(1232) with I and J = ~. Very few experiments exist at or below 1 GeV energy. The chief reference is that of Bugg et al. [5] who measured approximately 1600 interactions of the type p + p n + p + lr+ in a bubble chamber experiment. The n+p mass distributions were found to be consistent with A decay, and the peripheral nature of the process showed itself in the neutron angular distribution, which was peaked fore and aft in the c.m. system. In this paper we report new measurements on the 0 ° spectra of neutrons produced at 0 ° in p - p and p - d collisions at 647 and 800 MeV incident energy. 31
Volume 63B, number 1
PHYSICS LETTERS
Fig. 1. Experimental layout at the Nucleon Physics Lab at LAMPF. Fig. 1 shows a plan view of the experimental setup in the Nucleon Physics Lab. at LAMPF. The cryogenic target system [6] consisted of a forced convection loop containing either liquid hydrogen (LH2) or liquid deuterium (LD2) as required. The loop was cooled via a helium refrigerator with counter-current heat exchange between the cold helium gas and the fluid in the loop. Pressure and temperature of the loop fluid were measured. The production target thickness was 0.77 g/cm 2 for LH 2 and 1.80 g/cm 2 for LD 2. The proton beam intensity was measured by integration of the output of a toroid beam-current monitor. This current integrator was checked against nuclear activation measurements to the +10% level of accuracy. The neutron beam was formed by a steel collimator of 25.4 mm diameter placed at 0 ° with solid angle of 9.1 X 10 -6 sr. A neutron beam monitor (NMon) was placed at the collimator exit; a scintillation telescope also monitored the interaction rate at the LH 2 target (H-Mon). Protons from elastic and inelastic interactions in the LH 2 radiator (0.93 g/cm 2) were detected in a multiwire proportional chamber spectrometer. The spectrometer consisted of a magnet (M2), four x - y pairs of MWPC planes (Wl-W4) and two scintillation counters S1 and $2 which triggered the system and provided velocity information. System live time was determined by scaling N-Mon in relation to the inhibit signals of the spectrometer system. Some further details of the apparatus have recently been published [7] and a detailed report has been prepared [8]. A fraction of the lower-momentum protons originated from inelastic n - p interactions. The contribution of such inelastic processes was determined in a set of runs in which an aluminum target was used to 32
5 July 1976
generate a broad distribution of energies, using time of flight to determine incident neutron momenta. At 800 MeV these inelastic corrections were substantial. For example, in the p - d reaction about half of the proton recoils below 500 MeV/c momentum were inelastic; the p - p corrections at 800 MeV were somewhat smaller. At 647 MeV the corrections were much smaller. The elastic proton recoil distributions were converted to neutron spectra by knowledge of the n - p charge exchange cross section ( n - p CEX) as discussed in ref. [9]. The final absolute double differential cross sections were normalized with respect to incident proton beam intensity, the collimator solid angle, and the areal densities of the production and scattering targets. The major source of systematic error arose from long term drift of the proton beam current integration. This was estimated to be a 10% effect. Other sources of systematic error include: collimator solid angle +3%; areal density of production and radiator targets, -+2% from each; knowledge of the n - p CEX cross section, -+8%. Combining these effects in quadrature, the overall systematic error of the resultant cross sections is estimated to be -+13%. The results are plotted in fig. 2. The representative error bars are mainly statistical. Each p - d spectrum shows an intense and narrow ( ~ 15 MeV FWHM) highenergy charge exchange scattering peak ( p - d CEX) well separated from events at lower energy associated with pion production. The differential cross sections integrated over the peak region and the pion production bump are given in table 1. From a comparison of the peak characteristics in the processes p + d ~ n and n + d ~ p, using the same apparatus, we have obtained a crude estimate of the intrinsic peak width which turns out to be ~ 10 MeV, FWHM. This is substantially larger than that measured [3] at 96 MeV. The comparison also shows that the centroid of the peak in the process p + d ~ n lies approximately 7 MeV below the incident proton energy. Although the accuracy of these determinations is limited by spectrometer resolution (~ 10 MeV) the results are in fair agreement with calculations by Ward [10]. The energies at which we are quoting these results were the nominal accelerator energies. Our estimates of beam energy may differ by 1.5% from the accelerator energy depending on the run. The source of such discrepancy is not clear; in any case it contributes negligible error to these cross sections.
Volume 63B, number 1
PHYSICS LETTERS
T, 100
200 I
300
Table 1 Values of I d20/(da dp)hp at 0”.
(MeV) 400
500
600
700 600 / c
i!
-07 -0.6
P,
5 July 1976
(MeV/cl
Fig. 2. Neutron spectra (d’a/dndp) from the reactions p + d + n (circles) and p + p + n + p + tr+ (triangles) at nominal incident energies of 647 and 800 MeV.
From the integrated peak cross sections given in table 1 and from published [7] and unpublished results of our group, we can form the ratios (p-d CEX)/ (nip CEX) for 0” (lab) proton emission angle. The results for the ratios are 0.60 + 0.08 and 0.66 + 0.08 at 647 and 800 MeV, respectively. The reduction from unit ratio occurs because the Pauli principle inhibits transitions which do not result in a spin singlet state for the residual two protons. We may compare our results with those of Larsen [4] at 710 MeV, ignoring the energy difference in the process. He obtained the value 0.45 & 0.08 for the ratio as defined above. In view of the stated errors these results may be considered to be compatible. However, his value of the p-d CEX cross section is about half of ours; the source of this discrepancy is not known. It is interesting to note that Dzhelepov et al. [ 1 l] using a neutron beam of 380 MeV and the charge symmetric reaction measured a value (n-d CEX)/(n-p CEX) = 0.20 f 0.04 at 0’. This low value may indicate an anomalous behavior in the spin flip probability at this energy. The ratio of the integrated cross section in the pd CEX peak to that in an equal momentum interval in the adjacent valley is about 30 to 1. This marked isola-
Reaction
2eV)
CEXa Peak (mb/sr)
P-P P-P p-d p-d
647 800 647 800
30 31
Pionb
29 33 50 67
Region
W/sr)
a At 647 (800) MeV the range-is p > 1100 (1290) MeV/c. b At 647 (800) MeV the range is 380
tion of the peak neutrons from those at lower energy makes the p + d + n reaction an excellent source of monoenergetic neutrons at medium energy. The peak neutron beam intensity is approximately 8.7 X lo5 neutrons per microcoulomb through the 0” collimator. The neutron spectra from p-p collisions shown in fig. 2 are produced by the reaction p + p + n + p + rr+. Other processes, including double pion production, are negligible by comparison. The spectrum near 800 MeV is fairly well fitted by theoretical calculations [ 121 which assume that the dominant process is rr+ exchange resulting in an intermediate state of a high-energy neutron and the A( 1232) which subsequently decays. Not visible in the data as shown at 800 MeV is an indication of an n-p final-state interaction near 800 MeV/c. The effect of the n-p FSI is evident in the 647 MeV spectrum where it produces enhancement at 700 MeV/c. The similarity of the p-p and p-d spectra in the low momentum region leads to the speculation that the same mechanism is operating in both cases. It appears likely that in the p-d collisions the neutrons in the broad bump are associated with pion production in quasi-free p-p or p-n collisions which proceed through the A(1232) channel with the third nucleon as a spectator. This assumption would lead to a tintensity ratio between the p-d and p-p spectra. It neglects pion production which proceeds through n-n isospin 3 states in p-n collisions and contributions from p-d breakup without pion production. Although our normalization errors do not permit us to draw firm conclusions, our data are reasonably consistent with this simple picture.
33
Volume 63B, number 1
PHYSICS LETTERS
We would like to acknowledge the very important assistance in the LD 2 target development by J. Fretwell, J. Novak, and valuable support from the MP-Division. R. Michaud, B. Royball and F. Newcom provided engineering design and drafting help. The efforts of J. Boissevain were indispensable to our success; We thank A. Niethammer for her contribution to the data acquisition program and thank P. Grant for the nuclear chemistry calibration.
References [1] R.L.Gluckstern and H.A. Bethe, Phys. Rev. 81 (1951) 761. [2] D.F. Measday, Nucl. Instr. 40 (1966) 213.
34
[3] [4] [5] [6]
5 July 1976
A. Langsford et al., Nucl. Phys. A99 (1967) 246. R.R. Larsen, I! Nuovo Cimento, Ser. X 18 (1960) 1039. D.V. Bugg et al., Phys. Rev. 133 (1964) B1017. The prototype tacget loop is described in K.D. WiUiamsonet al., Advances Cryogenic Engineering 19 (1974) 241 ; the refrigerator is described in R.O. Voth et al., Advances Cryogenic Engineering 11 (1966) 126. [7] M.L. Evans et al., Phys. Rev. Lett. 36 (1976) 497. [8] C.W. Bjork, Ph.D. thesis, University Texas (1975), LASL report LA-6192-T. [9] C.G.'Cassapakis et al., phys Lett. 63B (1976) 35 [10] D.S. Ward, Ph.D. thesis, University Texas (1971). [ 11 ] V.P. Dzhelepov et al., I1 Nuovo Cimento, Suppl. to Ser. X 3 (1956) 61. [12] G.J. Stephenson in: Int. Conf. High energy physics and nuclear structure, Santa Fe and Los Alamos 1975, eds. D.E. Nagle et al. (A.I.P. Conf. Proc. No. 26, Am. Inst. Phys. 1975) 55.