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Observation of the 18Ne ground state in the pion double-charge-exchange reaction 18O(π+, π−18Ne
Volume 69B, number 3
PHYSICS LETTERS
15 August 1977
OBSERVATION OF THE 18Ne GROUND STATE IN THE PION DOUBLE-CHARGE-EXCHANGE R E A C T I O N 18 O(Tr + ' 7r- )18 N e C. PERRIN, J.-P. ALBANi~SE lnstitut des Sciences Nucldaires, Universitd de Grenoble, BP 257, Centre de Tri, F-38044 Grenoble-Cedex, France
R. CORFU, J.-P. EGGER, P. GRETILLAT, C. LUNKE, J. P I F F A R E T T I , E. SCHWARZ Institut de Physique, b~ffversitd de Neuchdtel +, rue A.-L. Breguet, CH-2000 Neuch8tel, Switzerland
J. JANSEN SIN, CIt-5243 Villingen, Switzerland
and B.M. PREEDOM University o f South Carolina ÷ and SIN, CH-5234 Villingen, Switzerland
Received 2 June 1977 The 18Ne ground state was observed in the pion double-charge-exchange reaction 180(7r+, ~r -)18 Ne. The experiment was done at a scattering angle of 18° and pion energies of 148 MeV and 187 MeV. Values of da/dS2 = 0.30 +0.10 ~zb/sr at 148 MeV and 0.21 +- 0.08 ub/sr at 187 MeV were obtained for the ~ T z = 2 isobaric transition to the 18Ne ground state.
Pion double charge exchange A x ( n +-, rrZ-~Z_.2x ' has long been regarded as a process of interest both for nuclear structure research and for the dynamics involved. The reaction is usually assumed to take place in two steps, a charge exchange on one nucleon and then on another [5]. However, other mechanisms via delta production [7] or scattering from a virtual pion [8] could also be competitive. We report here the first high-resolution measurement of the pion double-charge-exchange (DCE) reaction 180(n+, 7r-)l 8Ne and the observation of the 18Ne ground state which is a A T Z = 2 isobaric analog state of the 1 8 0 ground state. Numerous experiments [1] were performed over the last 13 years in an attempt to identify specific final states without showing any clear evidence for such states. Recently a group from Los Alamos [2] reported a clear peak at the location of the 18Ne ground state in the 180(7r +, 7r-)18Ne DCE reaction at 139 MeV and 0 °. Since their resoluWork supported in part through the Swiss National Science Foundation. * Supported by the US National Science Foundation.
tion was 4 MeV FWHM and the excitation of the first 18Ne excited state (JP = 2 +, 1.89 MeV) is probably negligibly weak with respect to the ground state at 0 °, they attributed the entire peak to the 18Ne ground state. With a resolution of typically 1 MeV FWHM, we are able in principle to identify the 1.89 MeV state, depending only on its cross section. In addition, we determine a value for the differential cross section, integrated over the DCE continuum and the excited states of 18Ne up to 20 MeV excitation energy. The experiment was done at the SIN 7rM1 beam and pion spectrometer [3]. The channel was tuned for a 7r+ beam and the spectrometer had the opposite polarity. Measurements were done at 148 MeV and 187 MeV. For the 148 MeV data, a 5 m m liquid H2180(98%) target was used. Because of very low count rates, the 187 MeV data was taken with the above target plus a 10 mm liquid D2180(95%) target. Resolution was 0.7 MeV and 1.1 MeV FWHM, respectively. The spectrometer was set at a fixed 18 ° scattering angle in order to avoid the direct beam. This is 301
Volume 69B, n u m b e r 3
PttYSICS LETTERS
Ill
C1
N (events) 18Ne break up 180 (rr,+ 7t-)lONe
III
1.89 15 104
TO = 148 MeV
!l o.,.
C/* ~ / T - "
~
s3 cs
Energy ,n (channels)
, n~
?'.,
10
v2
P
$4
20
(p" C1 ' C2" S1)* (C3" $2" $3)* anti(V1 or V2). Chambers C2, C3 and C4 were used for projection onto the scattering target and for scattering angle calculations. Thus events which did not pass through the target were rejected; C3 was also useful for rejection of 3,-rays from ~r0 decay. The histograms of fig. 2 passed the following tests:a) TOF in the 7rM1 channel, b) target position, c) scattering angle, d) optics in the spectrometer, e) TOF in the spectrometer. The field in the spectrometer magnets was adjusted to fill only about 75% (Ap/p) of the focal plane with DCE events. Thus the remaining 25% provided a background measurement and a good test for the rejection
40
IN (events) + l 18O(~r, Tr- ) 18Ne 15J
smallest angle compatible with low background. During the experiment, the incident pion flux was about 3 X 106 7r+/s corresponding to a primary proton beam of 40/aA. The detection apparatus is shown in fig. 1. A fast MWPC C1 measured the momentum of the incoming particles. Protons in the beam were removed with an electrostatic separator, muons and electrons by time of flight (TOF) between counter S1 and the accelarator RF 50 MHz signal (p). Two fast MWPC's C1 and C2 and the scintillator S1 monitored the beam. Muons and erratic particles in the spectrometer were rejected through ion optics and TOF between counter $3 and the 50 MHz signal. The TOF resolution was 1 ns FWHM for the channel and 1.5 ns for the spectrometer which resulted in unambiguous electron rejection. The DCE candidates were identified by the logic trigger requirement.
30
50
60
70
Fig. 2a. lSo(Tr+, nO]8Ne spectrum obtained at an incident pion energy of 148 MeV and a spectrometer angle of 18 ° for events surviving the tests described in the text. The DCE cont i n u u m was not corrected for spectrometer transmission.
Fig. 1. Schematic layout of the SIN ~rM1 channel and pion spectrometer. C 1 - C 6 are MWPC's; C1 and C2 have a fast digital readout and in C3 to C6 delay lines are used; S 1 - $ 4 are scintillators and V 1 - V 2 veto counters.
302
15 August 1977
!
-
-
-
0-I~
10
I
118. •
I Nesg
-
To= 187 MeV
54 /
18~Ne breok up I
I
]
llllhl
o r~J -t,
20
,
-u ,
30
Inn LP - -
,
~
40
,
I 18
' '*
Energy
--LLF] r]J-Ln (chonnels) ,
50
I
'
,
60
'
,
,
70
Fig. 2b. lSO0r÷, lr")18Ne spetrum obtained at a 187 MeV pion energy with the same conditions as for fig. 2a.
conditions mentioned above. For calibration purposes and extraction of an absolute cross section, elastic scattering spectra on 18 O and 12 C were taken at 18 ° and a polyethylene [(CH2)n] target was used at 45 °. In fig. 2 we present our two DCE spectra. There is no background and the 1 8 N e g.s. is clearly separated from the DCE continuum which starts at about 4.5 MeV excitation energy. The full spectrometer acceptance of 8 ° FWHM was used and no angular binning was done. Using the value of 293 mb/sr for the lab differential pion-carbon elextic cross section at 18 ° and 148 MeV (303 mb/sr at 18 ° and 187 MeV) obtained from our carbon data [3], we normalized our 12 C elastic scattering data. This normalization then contains the solid angle acceptance of the spetrometer, the transmission through the spectrometer, the focal plane acceptance, etc. It was then used to normalize the DCE spectra. We get a differential cross section for the 180(/1- + , / i . - ) 1 8 N e ground state of do/dgZla b (18 °, 148 MeV) = 0.30 +- 0.10/ab/sr, do/df21a b (18 °, 187 MeV) = 0.21 +- 0.08/ab/sr. The quoted error is obviously dominated by statistical uncertainties. A cross check of our [(CH2)n] spectra
Volume 69B, number 3
PHYSICS LETTERS
against the known hydrogen cross section [4] gives consistent results. For the transition to the first excited state o f 18Ne ' due to the lack of statistics, only an upper limit can be presented for the differential cross section at 148 MeV, dcr/df21a b (18 °, 148 MeV) < 70 nb/sr. At 187 MeV the cross section was estimated as da/d~21a b (18 °, 187 MeV) ~ 120 nb/sr. The important DCE contribution for strongly excited and unbound states is easily observed because of the large momentum acceptance of the spectrometer. Integrating over the DCE continuum and the excited states of 18Ne up to 20 MeV excitation energy, yields a lab differential cross section of
Exj=2o
MeV d2 °
(18°)dEx = 3.8 + 7 / l b / s r
and
Ex=4.5 MeV dg2dEx 3.0 + 0.5/Jb/sr , at E,~ = 148 and 187 MeV, respectively. The quoted error includes statistics and uncertainties due to spectrometer transmission. The comparison of our results with the value of 1.78 -+ 0.30/ab/sr at 0 ° and 139 MeV given in ref. [2] probably shows a quite important angular dependence of the cross section. Recently, several theoretical calculations [5] have been performed, based on multiple scattering theory. It is interesting to note that the prediction of Liu and Franco, for example, is in qualitative agreement with b o t h our results and those o f ref. [2]. Now that the pion DCE reaction to a specific final
15 August 1977
state has been seen, it is clear that it can be used as a spectroscopic tool for nuclear structure research as suggested in 1961 by Drell et al. [6]. We wish to thank the crew o f the SIN machine for their cooperation and reliable operation of the accelerator. We are very grateful to Dr. B~irtschi and Prof. Backenstoss for providing the 18 0 targets.
References [1 ] L. Gilly et al., Phys. Lett. 11 (1964) 244 and 19 (1965) 335; P.E. Boynton, T.J. Devlin, J. Solomon and V. PerezMendez, Phys. Rev. 174 (1968) 1083. C.J. Cook, M.E. Nordberg and R.L. Burman, Phys. Rev. 174 (1968) 1374; J. Sperinde et al., Phys. Lett. 32B (1970) 185. [2] T. Marks er al., Phys. Rev. Lett. 38 (1977) 149. [3] J. Piffaretti et al., Plays. Lett. 67B (1977) 289. [4] J. Ashkin, J.-P. Blaser, F. Feiner and M.O. Stern, Phys. Rev. 101 (1956) 1149; P.J. Bussy et al., Nucl. Phys. B58 (1973) 363. [5] L.C. Liu and V. Franco, Phys. Rev. C l l (1975) 760; W.B. Kaufman, J.C. Jackson and W.R. Gibbs, Phys. Rev. C9 (1974) 1340; G.A. Miller and J.E. Spencer, Phys. Lett. 53B (1974) 329 and Ann. of Phys. 100 (1976) 562. [6] Quoted in R.G. Parsons, J.S. Trefil and S.D. Drell, Phys. Rev. 138 (1965) B847. [7] O.D. Dal'karov and I.S. Shapiro, Phys. Lett. 26B (1968) 706. [8] J.-F. Germond and C. Wilkin, Nuovo Cimento Lett. 13 (1975) 605.
303
PHYSICS LETTERS
15 August 1977
OBSERVATION OF THE 18Ne GROUND STATE IN THE PION DOUBLE-CHARGE-EXCHANGE R E A C T I O N 18 O(Tr + ' 7r- )18 N e C. PERRIN, J.-P. ALBANi~SE lnstitut des Sciences Nucldaires, Universitd de Grenoble, BP 257, Centre de Tri, F-38044 Grenoble-Cedex, France
R. CORFU, J.-P. EGGER, P. GRETILLAT, C. LUNKE, J. P I F F A R E T T I , E. SCHWARZ Institut de Physique, b~ffversitd de Neuchdtel +, rue A.-L. Breguet, CH-2000 Neuch8tel, Switzerland
J. JANSEN SIN, CIt-5243 Villingen, Switzerland
and B.M. PREEDOM University o f South Carolina ÷ and SIN, CH-5234 Villingen, Switzerland
Received 2 June 1977 The 18Ne ground state was observed in the pion double-charge-exchange reaction 180(7r+, ~r -)18 Ne. The experiment was done at a scattering angle of 18° and pion energies of 148 MeV and 187 MeV. Values of da/dS2 = 0.30 +0.10 ~zb/sr at 148 MeV and 0.21 +- 0.08 ub/sr at 187 MeV were obtained for the ~ T z = 2 isobaric transition to the 18Ne ground state.
Pion double charge exchange A x ( n +-, rrZ-~Z_.2x ' has long been regarded as a process of interest both for nuclear structure research and for the dynamics involved. The reaction is usually assumed to take place in two steps, a charge exchange on one nucleon and then on another [5]. However, other mechanisms via delta production [7] or scattering from a virtual pion [8] could also be competitive. We report here the first high-resolution measurement of the pion double-charge-exchange (DCE) reaction 180(n+, 7r-)l 8Ne and the observation of the 18Ne ground state which is a A T Z = 2 isobaric analog state of the 1 8 0 ground state. Numerous experiments [1] were performed over the last 13 years in an attempt to identify specific final states without showing any clear evidence for such states. Recently a group from Los Alamos [2] reported a clear peak at the location of the 18Ne ground state in the 180(7r +, 7r-)18Ne DCE reaction at 139 MeV and 0 °. Since their resoluWork supported in part through the Swiss National Science Foundation. * Supported by the US National Science Foundation.
tion was 4 MeV FWHM and the excitation of the first 18Ne excited state (JP = 2 +, 1.89 MeV) is probably negligibly weak with respect to the ground state at 0 °, they attributed the entire peak to the 18Ne ground state. With a resolution of typically 1 MeV FWHM, we are able in principle to identify the 1.89 MeV state, depending only on its cross section. In addition, we determine a value for the differential cross section, integrated over the DCE continuum and the excited states of 18Ne up to 20 MeV excitation energy. The experiment was done at the SIN 7rM1 beam and pion spectrometer [3]. The channel was tuned for a 7r+ beam and the spectrometer had the opposite polarity. Measurements were done at 148 MeV and 187 MeV. For the 148 MeV data, a 5 m m liquid H2180(98%) target was used. Because of very low count rates, the 187 MeV data was taken with the above target plus a 10 mm liquid D2180(95%) target. Resolution was 0.7 MeV and 1.1 MeV FWHM, respectively. The spectrometer was set at a fixed 18 ° scattering angle in order to avoid the direct beam. This is 301
Volume 69B, n u m b e r 3
PttYSICS LETTERS
Ill
C1
N (events) 18Ne break up 180 (rr,+ 7t-)lONe
III
1.89 15 104
TO = 148 MeV
!l o.,.
C/* ~ / T - "
~
s3 cs
Energy ,n (channels)
, n~
?'.,
10
v2
P
$4
20
(p" C1 ' C2" S1)* (C3" $2" $3)* anti(V1 or V2). Chambers C2, C3 and C4 were used for projection onto the scattering target and for scattering angle calculations. Thus events which did not pass through the target were rejected; C3 was also useful for rejection of 3,-rays from ~r0 decay. The histograms of fig. 2 passed the following tests:a) TOF in the 7rM1 channel, b) target position, c) scattering angle, d) optics in the spectrometer, e) TOF in the spectrometer. The field in the spectrometer magnets was adjusted to fill only about 75% (Ap/p) of the focal plane with DCE events. Thus the remaining 25% provided a background measurement and a good test for the rejection
40
IN (events) + l 18O(~r, Tr- ) 18Ne 15J
smallest angle compatible with low background. During the experiment, the incident pion flux was about 3 X 106 7r+/s corresponding to a primary proton beam of 40/aA. The detection apparatus is shown in fig. 1. A fast MWPC C1 measured the momentum of the incoming particles. Protons in the beam were removed with an electrostatic separator, muons and electrons by time of flight (TOF) between counter S1 and the accelarator RF 50 MHz signal (p). Two fast MWPC's C1 and C2 and the scintillator S1 monitored the beam. Muons and erratic particles in the spectrometer were rejected through ion optics and TOF between counter $3 and the 50 MHz signal. The TOF resolution was 1 ns FWHM for the channel and 1.5 ns for the spectrometer which resulted in unambiguous electron rejection. The DCE candidates were identified by the logic trigger requirement.
30
50
60
70
Fig. 2a. lSo(Tr+, nO]8Ne spectrum obtained at an incident pion energy of 148 MeV and a spectrometer angle of 18 ° for events surviving the tests described in the text. The DCE cont i n u u m was not corrected for spectrometer transmission.
Fig. 1. Schematic layout of the SIN ~rM1 channel and pion spectrometer. C 1 - C 6 are MWPC's; C1 and C2 have a fast digital readout and in C3 to C6 delay lines are used; S 1 - $ 4 are scintillators and V 1 - V 2 veto counters.
302
15 August 1977
!
-
-
-
0-I~
10
I
118. •
I Nesg
-
To= 187 MeV
54 /
18~Ne breok up I
I
]
llllhl
o r~J -t,
20
,
-u ,
30
Inn LP - -
,
~
40
,
I 18
' '*
Energy
--LLF] r]J-Ln (chonnels) ,
50
I
'
,
60
'
,
,
70
Fig. 2b. lSO0r÷, lr")18Ne spetrum obtained at a 187 MeV pion energy with the same conditions as for fig. 2a.
conditions mentioned above. For calibration purposes and extraction of an absolute cross section, elastic scattering spectra on 18 O and 12 C were taken at 18 ° and a polyethylene [(CH2)n] target was used at 45 °. In fig. 2 we present our two DCE spectra. There is no background and the 1 8 N e g.s. is clearly separated from the DCE continuum which starts at about 4.5 MeV excitation energy. The full spectrometer acceptance of 8 ° FWHM was used and no angular binning was done. Using the value of 293 mb/sr for the lab differential pion-carbon elextic cross section at 18 ° and 148 MeV (303 mb/sr at 18 ° and 187 MeV) obtained from our carbon data [3], we normalized our 12 C elastic scattering data. This normalization then contains the solid angle acceptance of the spetrometer, the transmission through the spectrometer, the focal plane acceptance, etc. It was then used to normalize the DCE spectra. We get a differential cross section for the 180(/1- + , / i . - ) 1 8 N e ground state of do/dgZla b (18 °, 148 MeV) = 0.30 +- 0.10/ab/sr, do/df21a b (18 °, 187 MeV) = 0.21 +- 0.08/ab/sr. The quoted error is obviously dominated by statistical uncertainties. A cross check of our [(CH2)n] spectra
Volume 69B, number 3
PHYSICS LETTERS
against the known hydrogen cross section [4] gives consistent results. For the transition to the first excited state o f 18Ne ' due to the lack of statistics, only an upper limit can be presented for the differential cross section at 148 MeV, dcr/df21a b (18 °, 148 MeV) < 70 nb/sr. At 187 MeV the cross section was estimated as da/d~21a b (18 °, 187 MeV) ~ 120 nb/sr. The important DCE contribution for strongly excited and unbound states is easily observed because of the large momentum acceptance of the spectrometer. Integrating over the DCE continuum and the excited states of 18Ne up to 20 MeV excitation energy, yields a lab differential cross section of
Exj=2o
MeV d2 °
(18°)dEx = 3.8 + 7 / l b / s r
and
Ex=4.5 MeV dg2dEx 3.0 + 0.5/Jb/sr , at E,~ = 148 and 187 MeV, respectively. The quoted error includes statistics and uncertainties due to spectrometer transmission. The comparison of our results with the value of 1.78 -+ 0.30/ab/sr at 0 ° and 139 MeV given in ref. [2] probably shows a quite important angular dependence of the cross section. Recently, several theoretical calculations [5] have been performed, based on multiple scattering theory. It is interesting to note that the prediction of Liu and Franco, for example, is in qualitative agreement with b o t h our results and those o f ref. [2]. Now that the pion DCE reaction to a specific final
15 August 1977
state has been seen, it is clear that it can be used as a spectroscopic tool for nuclear structure research as suggested in 1961 by Drell et al. [6]. We wish to thank the crew o f the SIN machine for their cooperation and reliable operation of the accelerator. We are very grateful to Dr. B~irtschi and Prof. Backenstoss for providing the 18 0 targets.
References [1 ] L. Gilly et al., Phys. Lett. 11 (1964) 244 and 19 (1965) 335; P.E. Boynton, T.J. Devlin, J. Solomon and V. PerezMendez, Phys. Rev. 174 (1968) 1083. C.J. Cook, M.E. Nordberg and R.L. Burman, Phys. Rev. 174 (1968) 1374; J. Sperinde et al., Phys. Lett. 32B (1970) 185. [2] T. Marks er al., Phys. Rev. Lett. 38 (1977) 149. [3] J. Piffaretti et al., Plays. Lett. 67B (1977) 289. [4] J. Ashkin, J.-P. Blaser, F. Feiner and M.O. Stern, Phys. Rev. 101 (1956) 1149; P.J. Bussy et al., Nucl. Phys. B58 (1973) 363. [5] L.C. Liu and V. Franco, Phys. Rev. C l l (1975) 760; W.B. Kaufman, J.C. Jackson and W.R. Gibbs, Phys. Rev. C9 (1974) 1340; G.A. Miller and J.E. Spencer, Phys. Lett. 53B (1974) 329 and Ann. of Phys. 100 (1976) 562. [6] Quoted in R.G. Parsons, J.S. Trefil and S.D. Drell, Phys. Rev. 138 (1965) B847. [7] O.D. Dal'karov and I.S. Shapiro, Phys. Lett. 26B (1968) 706. [8] J.-F. Germond and C. Wilkin, Nuovo Cimento Lett. 13 (1975) 605.
303