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Reactions of nuclei with pions at the 3 - 3 resonance
Volume 26B, number
9
REACTIONS
PHYSICS
OF
D. T. CHIVERS,
NUCLEI
WITH
LETTERS
PIONS
1 April
AT
THE
3-3
1968
RESONANCE
J. J. DOMINGO, E. M. RIMMER *, R. C. WITCOMB **, B. W. ALLARDYCE and N. W. TANNER Nuclear Physics Laboratory, Oxford Received
6 March
1968
Activation cross-sections of reactions of pions with light nuclei have been measured over the energy range 80 to 280 MeV. Most of the reactions appear to be of single-particle character, but quasi-free scattering does not show the expected ratio of 3 for 8-/n+. The reactions of various light nuclei with pions at 180 MeV and elsewhere in the energy range 80 to 280 MeV have been studied by detecting the residual radio-activity after bombardment. Pions of both signs, nf and II-, were used, and were generated from polythene by the 600 MeV external proton beam of the CERN synchrocyclotron. The pion beam was magnetically deflected and focussed onto a target, which after bombardment for about one half-life of the activity of interest, was transferred pneumatically to a NaI /3/y counting arrangement. Energy spectra and decay curves were recorded from the NaI counters. As the comparison of IT+and in- was of particular interest, considerable care was taken with the analysis of the two beams which were rather different in character: about a factor of ten in intensity, but with a similar focus. The momentum distribution, measured by magnetic analysis, had a width of about 20% for n- and 10% for v+ at 180 MeV. Two methods were used to determine the pion fraction in the beam: (a) total absorption by hydrogen (polythene minus carbon), and (b) the spectrum from a thick-liquid Cerenkov counter. The two methods agreed very well and yielded around 7570 pions and 25% muons and electrons depending on momentum. There were also up to 10% protons in the 7r+beams which ‘were rejected by pulse height in the beam counters. The particle flux was determined by direct counting or by sampling in the case of very intense beams (up to 106 s-l). From all targets the strongest activity was that due to the removal of one nucleon from a nucleus e.g. 12C 2 IIc, presumably the reac-
* Present ** Present Redhill,
address: address: Surrey.
CERN, Geneva, Switzerland. Mullard Research Laboaratories,
Fig. 1. Excitation
function for the reaction
12C +r-11~
tion l2(~, nn)IlC. Decay curves were analysed by least-squares fit and cross sections calculated from the beam measurements and the target composition. In some cases it was necessary to correct for proton induced activity. The form of the excitation function shown in the figure is fairly typical of all the reactions observed i.e. the 3-3 pion-nucleon resonance at 180 MeV, perhaps shifted a little, and broadened by the Fermi momentum distribution. The full curve which is normalized at 180 MeV to the experimental data is the result of a plane wave calculation for quasi-free scattering using experimental values for pion-nucleon scattering. A p-shell harmonic oscillator wavefunction was used for the nucleon. In form and order of mag573
Volume
26B, number
9
Cross Target
and
activity
1OB TL 1oc llB
c
llc
13C “2 13N 14 N Trr 14C 18C rr 18P 18C ?if 18Ne
12C Tf 1lC
sections
1 April
PHYSICS
LETTERS
and ratios
Table 1 of pion reactions
Reaction
Cross
at 180 MeV.
section
or ratio
(?r+,nO) w+J”)
6.4 * 1.1 mb
(r+,“O)
4.1 + 1.3 mb
Plane wave, single-particle calculation
1.3 * 0.2 mb
w+, To)
5 0.05 mb
(a-+, no,
4.4 + 0.9 mb
w+, 71)
< 0.1 mb
(n+an;+n)
1968
90+5
mb
Ratio 13C/14N 18 : 1
w+,n OP) 14N 7if 13N
dito
6’7+7
mb
160 i?! 150
dito
49+5
mb
1lB ?‘f IOC
b+, ?TOn) and (7(+,8-P)
12C Q
1OC
(7r+,np) (7i+,s+2n) etc.
12&
1OC
@-, 8-2n)
12C E
1lC
12C z
1lC
14N nz 13N 14N n< 13N
0.83 * 0.3 mb
4.7 * 0.6 mb (
0.5 mb
1.03 * 0.09
1.05 * 0.09
0.33 for quasi-free scattering
0.98 + 0.09
nitude this result is in agreement with that of Kolybasov [l]. A summary of the most interesting 180 MeV data is given in the table. The n+/r- ratios do not depend on the activity counting equipment or target uncertainties; in fact the errors are almost wholly determined by the beam composition and flux measurements. The disturbing result is the x+/r- ratio for quasi-free scattering which is unity rather than 5 expected from the free nucleon-pion scattering at the 3-3 resonance. Note that the cross section for l2C 4 1lC is very larg e and substantially in agreement with the measurement of Reeder and Markowitz [2] and the single-particle calculation of Kolybasov [l]. It may be possible to contrive
574
a “final-state interaction” to obtain the observed ratio, but in the limit of statistical boil-off far too much 12C c 10~ will be predicted. Inelastic pion scattering exciting the giant dipole resonance (or other virtual states of pure isotopic spin) is possible but was observed by Meunier et al. [3] to have a cross section of only 1 or 2 mb. ThTre are two reac&ions in the table, viz. 12C 5 10~ and llB f l°C (and others not quoted), that necessarily involve interactions with two nucleons. If it is supposed that there is a literal pion/nucleon pair interaction (nNN), then apparently it must be weak in the isotopic spin T = 2 state, since 12C c 10~ was not observed, but not in the T = 1 state. Presumably the T = 0 part of (xNN) can be neglected as it does
Volume 26B, number 9
PHYSICS
LETTERS
not couple to N*N. Assuming a pure T = 1 (rNN) state decaying through N* + N, a very crude calculation of isoto ic spin coupling gives the ratio (1% d llC/(l Hc L - llC) = 1 6. cf. the singleparticle value of 5 and the exper;mental value of 1.0. The addition of a T = 1 pair interaction to the conventional single-particle interaction offers a qualitative explanation for most of the data of table 1. The embarrassment is the charge exchange (n’, no) which shows marked single-particle character: the cross sections are very nearly in the same ratio as observed for (p,n) (Valentin [4]) and in particular the cross section for 14N X? 140 is small. For a single-particle interaction it is easy to understand the 14N result in
terms
1 April 1968
of the famous inhibited /3 decay of 14C and
l40 to 14N. This explanation will not be valid if a pair interaction
contributes.
We should like to thank Dr. Max Huber for pointing out the possibility of a pair interaction. This work was supported by the Science Research Council.
References 1. V. M. Kolybasov, Yadernaya Fiz. 2 (1966) 101. 2. P.L.Reeder and S,S.Markowitz, Phys.Rev. 133B (1964) 639. 3. R. Meunier, private communication. 4. L. Valentin, Nucl. Phys. 62 (1965) 81.
*****
575
9
REACTIONS
PHYSICS
OF
D. T. CHIVERS,
NUCLEI
WITH
LETTERS
PIONS
1 April
AT
THE
3-3
1968
RESONANCE
J. J. DOMINGO, E. M. RIMMER *, R. C. WITCOMB **, B. W. ALLARDYCE and N. W. TANNER Nuclear Physics Laboratory, Oxford Received
6 March
1968
Activation cross-sections of reactions of pions with light nuclei have been measured over the energy range 80 to 280 MeV. Most of the reactions appear to be of single-particle character, but quasi-free scattering does not show the expected ratio of 3 for 8-/n+. The reactions of various light nuclei with pions at 180 MeV and elsewhere in the energy range 80 to 280 MeV have been studied by detecting the residual radio-activity after bombardment. Pions of both signs, nf and II-, were used, and were generated from polythene by the 600 MeV external proton beam of the CERN synchrocyclotron. The pion beam was magnetically deflected and focussed onto a target, which after bombardment for about one half-life of the activity of interest, was transferred pneumatically to a NaI /3/y counting arrangement. Energy spectra and decay curves were recorded from the NaI counters. As the comparison of IT+and in- was of particular interest, considerable care was taken with the analysis of the two beams which were rather different in character: about a factor of ten in intensity, but with a similar focus. The momentum distribution, measured by magnetic analysis, had a width of about 20% for n- and 10% for v+ at 180 MeV. Two methods were used to determine the pion fraction in the beam: (a) total absorption by hydrogen (polythene minus carbon), and (b) the spectrum from a thick-liquid Cerenkov counter. The two methods agreed very well and yielded around 7570 pions and 25% muons and electrons depending on momentum. There were also up to 10% protons in the 7r+beams which ‘were rejected by pulse height in the beam counters. The particle flux was determined by direct counting or by sampling in the case of very intense beams (up to 106 s-l). From all targets the strongest activity was that due to the removal of one nucleon from a nucleus e.g. 12C 2 IIc, presumably the reac-
* Present ** Present Redhill,
address: address: Surrey.
CERN, Geneva, Switzerland. Mullard Research Laboaratories,
Fig. 1. Excitation
function for the reaction
12C +r-11~
tion l2(~, nn)IlC. Decay curves were analysed by least-squares fit and cross sections calculated from the beam measurements and the target composition. In some cases it was necessary to correct for proton induced activity. The form of the excitation function shown in the figure is fairly typical of all the reactions observed i.e. the 3-3 pion-nucleon resonance at 180 MeV, perhaps shifted a little, and broadened by the Fermi momentum distribution. The full curve which is normalized at 180 MeV to the experimental data is the result of a plane wave calculation for quasi-free scattering using experimental values for pion-nucleon scattering. A p-shell harmonic oscillator wavefunction was used for the nucleon. In form and order of mag573
Volume
26B, number
9
Cross Target
and
activity
1OB TL 1oc llB
c
llc
13C “2 13N 14 N Trr 14C 18C rr 18P 18C ?if 18Ne
12C Tf 1lC
sections
1 April
PHYSICS
LETTERS
and ratios
Table 1 of pion reactions
Reaction
Cross
at 180 MeV.
section
or ratio
(?r+,nO) w+J”)
6.4 * 1.1 mb
(r+,“O)
4.1 + 1.3 mb
Plane wave, single-particle calculation
1.3 * 0.2 mb
w+, To)
5 0.05 mb
(a-+, no,
4.4 + 0.9 mb
w+, 71)
< 0.1 mb
(n+an;+n)
1968
90+5
mb
Ratio 13C/14N 18 : 1
w+,n OP) 14N 7if 13N
dito
6’7+7
mb
160 i?! 150
dito
49+5
mb
1lB ?‘f IOC
b+, ?TOn) and (7(+,8-P)
12C Q
1OC
(7r+,np) (7i+,s+2n) etc.
12&
1OC
@-, 8-2n)
12C E
1lC
12C z
1lC
14N nz 13N 14N n< 13N
0.83 * 0.3 mb
4.7 * 0.6 mb (
0.5 mb
1.03 * 0.09
1.05 * 0.09
0.33 for quasi-free scattering
0.98 + 0.09
nitude this result is in agreement with that of Kolybasov [l]. A summary of the most interesting 180 MeV data is given in the table. The n+/r- ratios do not depend on the activity counting equipment or target uncertainties; in fact the errors are almost wholly determined by the beam composition and flux measurements. The disturbing result is the x+/r- ratio for quasi-free scattering which is unity rather than 5 expected from the free nucleon-pion scattering at the 3-3 resonance. Note that the cross section for l2C 4 1lC is very larg e and substantially in agreement with the measurement of Reeder and Markowitz [2] and the single-particle calculation of Kolybasov [l]. It may be possible to contrive
574
a “final-state interaction” to obtain the observed ratio, but in the limit of statistical boil-off far too much 12C c 10~ will be predicted. Inelastic pion scattering exciting the giant dipole resonance (or other virtual states of pure isotopic spin) is possible but was observed by Meunier et al. [3] to have a cross section of only 1 or 2 mb. ThTre are two reac&ions in the table, viz. 12C 5 10~ and llB f l°C (and others not quoted), that necessarily involve interactions with two nucleons. If it is supposed that there is a literal pion/nucleon pair interaction (nNN), then apparently it must be weak in the isotopic spin T = 2 state, since 12C c 10~ was not observed, but not in the T = 1 state. Presumably the T = 0 part of (xNN) can be neglected as it does
Volume 26B, number 9
PHYSICS
LETTERS
not couple to N*N. Assuming a pure T = 1 (rNN) state decaying through N* + N, a very crude calculation of isoto ic spin coupling gives the ratio (1% d llC/(l Hc L - llC) = 1 6. cf. the singleparticle value of 5 and the exper;mental value of 1.0. The addition of a T = 1 pair interaction to the conventional single-particle interaction offers a qualitative explanation for most of the data of table 1. The embarrassment is the charge exchange (n’, no) which shows marked single-particle character: the cross sections are very nearly in the same ratio as observed for (p,n) (Valentin [4]) and in particular the cross section for 14N X? 140 is small. For a single-particle interaction it is easy to understand the 14N result in
terms
1 April 1968
of the famous inhibited /3 decay of 14C and
l40 to 14N. This explanation will not be valid if a pair interaction
contributes.
We should like to thank Dr. Max Huber for pointing out the possibility of a pair interaction. This work was supported by the Science Research Council.
References 1. V. M. Kolybasov, Yadernaya Fiz. 2 (1966) 101. 2. P.L.Reeder and S,S.Markowitz, Phys.Rev. 133B (1964) 639. 3. R. Meunier, private communication. 4. L. Valentin, Nucl. Phys. 62 (1965) 81.
*****
575