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Absolute cross sections for pion reactions on light nuclei at the (3,3) resonance
Volume 58B, number 4
PHYSICS LE'I~ERS
29 September 1975
A B S O L U T E C R O S S S E C T I O N S F O R P I O N R E A C T I O N S ON L I G H T N U C L E I A T T H E (3,3) R E S O N A N C E ~r P.J. KAROL, M.V. YESTER*, R.L. KLOBUCHAR** and A.A. CARETTO Jr. Department of Chemistry, Carnegie.Mellon University, Pittsburgh, Pennsylvania 15213, USA Received 18 August 1975 Absolute cross sections for pion-induced neutron knockout reactions and some more complex reactions on 14N, 160, 19F and 31p were determined by activation at 190 MeV. Results are consistent with existing spallation models as modified by the recent final-state interaction theory of Sternheim and Silbar. During the past several years, investigations of neutron knockout cross sections on light nuclei induced by pions at the (3,3) resonance have provided a variety of seemingly incongruous results [1-7]. However, the recent report of Dropesky et al. [8] presented extensive new data on the 12C(ff~, rr~n)llc excitation function which supports the SternheimSilbar reaction model [9], namely a one-step quasifree nucleon knockout in which the final-state interaction plays a crucial role. We present here briefly some of our stacked foil activation results which further support the Sternheim-Silbar description and which disagree with similar experiments published by others. The experimental method has been described earlier [7]. It is based on the stacked foil technique in which two or more thin, overlapping targets of identical geometry are exposed normal to the beam axis and the induced radioactivity in each subsequently determined. One target serves as beam monitor through a reaction of known cross section, the other is the target under investigation. This procedure effectively eliminates a separate uncertainty contribution from beam flux determination. In addition, the radioactivity, if positron decay, can also be determined on a relative basis by measuring the 511 keV annihilation quanta emission rates from monitor and Work supported by the US Energy Research and Development Administration. * Present address: Medical Physics Division, Department of Radiology, West Virginia University Medical Center, Morgantown, W.Va. 26506 USA. ** Present address: Brookhaven National Laboratory, Upton, L.I., New York, 11973 USA.
sample under identical counting geometries using NaI(T1) detectors. In this manner, cross sections relative to the 12C(/r~:, rr±n) 11C monitor reaction are determined with excellent precision and several sources of systematic error are substantially avoided. Irradiations were performed with the 190 MeV pion beam at the Space Radiation Effects Laboratory [10]. The pion contribution to the beam was 85-90% for rr+ and 50% for rr-, the remainder being muons and electrons (protons < 1%) as determined by (~erenkov spectrometry [ 11 ]. The beam flux was monitored using polyvinyltoluene NE-102 scintillators as carbonaceous radioactivation targets. The nitrogen and fluorine targets were discussed previously [7] as was the activation measurement technique. Oxygen targets were used in the form of fused B20 3. The phosphorus target consisted of red phosphorus packed in a thin aluminum cannister. All targets and monitors were identically 76 mm diameter discs 6.4 mm thick. Table 1 lists the cross sections of all 13+-emitting products measured relative to 11C from the 12C monitor. These data are reported as absolute cross sections based on the 11C production rates of Dropesky et al. [8]. Also included in table 1 are yields and, if known, uncertainties reported for some of the same reactions by Chivers et al. [1], Hogstrom et al. [3], and Plendl et al. [4, 5], the latter as originally reported and also as "corrected" for the new monitor cross sections. Several of these other data are in conspicuous disagreement with the present results [ 12]. The absolute cross sections for neutron knockout from fluorine as reported here have a lr- : rr+ resonance cross section ratio R = 1.5 + 0.2 in very satisfactory agreement with the Sternheim-Silbar model which takes into account the considerable probability 489
Volume 58B, number 4
PHYSICS LETTERS
29 September 1975
Table 1 Absolute pion reaction cross sections at 190 MeV. Cross section in mb Target
Projectile
Product
]2 C
n+
11C
11"-
llc
7r+
13N
10.6 -+ 0.8 d
]]C
-+ 2 -+ 3 -+ 4
14N
This worka
75 -* 7 [1], 52 + 3 b [3], 50 ± 4 [6], 45 _+3 [81 73.5 ± 7 [1], 68 -+ 6 [2], 70 +- 5 [81
"rr-
11C
24 21
160
7r+
150
39
19F
7r÷
18F 13N
29.3 ± 0.5 d 4.8 -+ 1.6
n_
l]c lSF
8.4 -+ 0.8 43 -+ I d
lr+ *r-
3Op 30p
40
31p
Others
±3
56 -+ 6 [11
41±411] 37 [51, 22 c [51
39 -+ 5 [4] 37 [5], 22c [5] 60 ± 11 [4]
a Adopting monitor reactions in ref. [8]. b Interpolated. e Corrected from reported cross sections using monitor reactions in ref. [81. d Corrected from this laboratory's relative cross sections reported in ref. [7] using monitor reactions in ref. [8].
for charge-exchange of the struck nucleon with the spectator residual nucleus. One should also note that the 7r- : zr+ cross section ratio for more complicated reactions, as exemplified by the production of 11C from 14N, is nearly unity as expected on the basis of Monte Carlo calculations [13, 14] for pion-induced spallation reactions. The authors would like to thank Dr. Robert T. Siegel and the Space Radiation Effects Laboratory synchrocyclotron staff for their hospitality and invaluable assistance. Our appreciation is also extended to Professor John Kane of the College of William and Mary for the ~erenkov spectrometry determinations.
References [ 11 D.J~ Chivers et al., Nucl. Phys. A 126 (1969) 129.
490
[2] P.L. Reeder and S.S. Markovitz, Phys. Rev. 133 (1964) B639. [3] K.R. Hogstrom et al., Nucl. Phys. A215 (1973) 598. [4] H.R. Plendl et al., Nucl. Phys. B44 (1972) 413. [5] H.R. Plendl et al., Proc. Pion-nucleus interactions, Strasbourg, France 1971, to be published. [6] M.V. Yester et al., Phys. Lett. 45B (1973) 327. [7] P.J. Karol et al., Phys. Lett. 44B (1973) 459. [8] B.J. Dropesky et al., Phys. Rev. Lett. 34 (1975) 821. [9] M.M. Sternheim and R.R. Silbar, Phys. Rev. Lett. 34 .(1975) 824. [101 The Space Radiation Effects Laboratory is supported by the National Science Foundation, the National Aeronautics and Space Administration and the Commonwealth of Virginia. [111 J. Kane, private communication. [121 N.P. Jacob Jr. and S.S. Mark~vitz, unpublished results in satisfactory agreement with this work for 14N, 160 and 19F, 170th ACS National Meeting, Chicago, 1975. [131 G.D. Harp et al., Phys. Rev. C8 (1973) 581, and G,D. Harp, Los Alamos Scientific Laboratory, unpublished. [141 H.W. Bertini,.Phys. Rev. C6 (1972) 631.
PHYSICS LE'I~ERS
29 September 1975
A B S O L U T E C R O S S S E C T I O N S F O R P I O N R E A C T I O N S ON L I G H T N U C L E I A T T H E (3,3) R E S O N A N C E ~r P.J. KAROL, M.V. YESTER*, R.L. KLOBUCHAR** and A.A. CARETTO Jr. Department of Chemistry, Carnegie.Mellon University, Pittsburgh, Pennsylvania 15213, USA Received 18 August 1975 Absolute cross sections for pion-induced neutron knockout reactions and some more complex reactions on 14N, 160, 19F and 31p were determined by activation at 190 MeV. Results are consistent with existing spallation models as modified by the recent final-state interaction theory of Sternheim and Silbar. During the past several years, investigations of neutron knockout cross sections on light nuclei induced by pions at the (3,3) resonance have provided a variety of seemingly incongruous results [1-7]. However, the recent report of Dropesky et al. [8] presented extensive new data on the 12C(ff~, rr~n)llc excitation function which supports the SternheimSilbar reaction model [9], namely a one-step quasifree nucleon knockout in which the final-state interaction plays a crucial role. We present here briefly some of our stacked foil activation results which further support the Sternheim-Silbar description and which disagree with similar experiments published by others. The experimental method has been described earlier [7]. It is based on the stacked foil technique in which two or more thin, overlapping targets of identical geometry are exposed normal to the beam axis and the induced radioactivity in each subsequently determined. One target serves as beam monitor through a reaction of known cross section, the other is the target under investigation. This procedure effectively eliminates a separate uncertainty contribution from beam flux determination. In addition, the radioactivity, if positron decay, can also be determined on a relative basis by measuring the 511 keV annihilation quanta emission rates from monitor and Work supported by the US Energy Research and Development Administration. * Present address: Medical Physics Division, Department of Radiology, West Virginia University Medical Center, Morgantown, W.Va. 26506 USA. ** Present address: Brookhaven National Laboratory, Upton, L.I., New York, 11973 USA.
sample under identical counting geometries using NaI(T1) detectors. In this manner, cross sections relative to the 12C(/r~:, rr±n) 11C monitor reaction are determined with excellent precision and several sources of systematic error are substantially avoided. Irradiations were performed with the 190 MeV pion beam at the Space Radiation Effects Laboratory [10]. The pion contribution to the beam was 85-90% for rr+ and 50% for rr-, the remainder being muons and electrons (protons < 1%) as determined by (~erenkov spectrometry [ 11 ]. The beam flux was monitored using polyvinyltoluene NE-102 scintillators as carbonaceous radioactivation targets. The nitrogen and fluorine targets were discussed previously [7] as was the activation measurement technique. Oxygen targets were used in the form of fused B20 3. The phosphorus target consisted of red phosphorus packed in a thin aluminum cannister. All targets and monitors were identically 76 mm diameter discs 6.4 mm thick. Table 1 lists the cross sections of all 13+-emitting products measured relative to 11C from the 12C monitor. These data are reported as absolute cross sections based on the 11C production rates of Dropesky et al. [8]. Also included in table 1 are yields and, if known, uncertainties reported for some of the same reactions by Chivers et al. [1], Hogstrom et al. [3], and Plendl et al. [4, 5], the latter as originally reported and also as "corrected" for the new monitor cross sections. Several of these other data are in conspicuous disagreement with the present results [ 12]. The absolute cross sections for neutron knockout from fluorine as reported here have a lr- : rr+ resonance cross section ratio R = 1.5 + 0.2 in very satisfactory agreement with the Sternheim-Silbar model which takes into account the considerable probability 489
Volume 58B, number 4
PHYSICS LETTERS
29 September 1975
Table 1 Absolute pion reaction cross sections at 190 MeV. Cross section in mb Target
Projectile
Product
]2 C
n+
11C
11"-
llc
7r+
13N
10.6 -+ 0.8 d
]]C
-+ 2 -+ 3 -+ 4
14N
This worka
75 -* 7 [1], 52 + 3 b [3], 50 ± 4 [6], 45 _+3 [81 73.5 ± 7 [1], 68 -+ 6 [2], 70 +- 5 [81
"rr-
11C
24 21
160
7r+
150
39
19F
7r÷
18F 13N
29.3 ± 0.5 d 4.8 -+ 1.6
n_
l]c lSF
8.4 -+ 0.8 43 -+ I d
lr+ *r-
3Op 30p
40
31p
Others
±3
56 -+ 6 [11
41±411] 37 [51, 22 c [51
39 -+ 5 [4] 37 [5], 22c [5] 60 ± 11 [4]
a Adopting monitor reactions in ref. [8]. b Interpolated. e Corrected from reported cross sections using monitor reactions in ref. [81. d Corrected from this laboratory's relative cross sections reported in ref. [7] using monitor reactions in ref. [8].
for charge-exchange of the struck nucleon with the spectator residual nucleus. One should also note that the 7r- : zr+ cross section ratio for more complicated reactions, as exemplified by the production of 11C from 14N, is nearly unity as expected on the basis of Monte Carlo calculations [13, 14] for pion-induced spallation reactions. The authors would like to thank Dr. Robert T. Siegel and the Space Radiation Effects Laboratory synchrocyclotron staff for their hospitality and invaluable assistance. Our appreciation is also extended to Professor John Kane of the College of William and Mary for the ~erenkov spectrometry determinations.
References [ 11 D.J~ Chivers et al., Nucl. Phys. A 126 (1969) 129.
490
[2] P.L. Reeder and S.S. Markovitz, Phys. Rev. 133 (1964) B639. [3] K.R. Hogstrom et al., Nucl. Phys. A215 (1973) 598. [4] H.R. Plendl et al., Nucl. Phys. B44 (1972) 413. [5] H.R. Plendl et al., Proc. Pion-nucleus interactions, Strasbourg, France 1971, to be published. [6] M.V. Yester et al., Phys. Lett. 45B (1973) 327. [7] P.J. Karol et al., Phys. Lett. 44B (1973) 459. [8] B.J. Dropesky et al., Phys. Rev. Lett. 34 (1975) 821. [9] M.M. Sternheim and R.R. Silbar, Phys. Rev. Lett. 34 .(1975) 824. [101 The Space Radiation Effects Laboratory is supported by the National Science Foundation, the National Aeronautics and Space Administration and the Commonwealth of Virginia. [111 J. Kane, private communication. [121 N.P. Jacob Jr. and S.S. Mark~vitz, unpublished results in satisfactory agreement with this work for 14N, 160 and 19F, 170th ACS National Meeting, Chicago, 1975. [131 G.D. Harp et al., Phys. Rev. C8 (1973) 581, and G,D. Harp, Los Alamos Scientific Laboratory, unpublished. [141 H.W. Bertini,.Phys. Rev. C6 (1972) 631.