- Email: [email protected]
Coincidence measurements of the reactions 12C(π±, π±d)10Band12C(π±, π±t)9B
Volume 88B, number 3,4
PHYSICS LETTERS
17 December 1979
COINCIDENCE MEASUREMENTS OF THE REACTIONS 12C(ff±, n±d)10B AND 12C(1r±,n±t)9B R.J. ELLIS, H.J. ZIOCK and K.O.H. ZIOCK Physics Department, University of Virginia, Charlottesville, VA 22901, USA
J. BOLGER and E. BOSCHITZ Kernforschungszentrum and Universitiit Karlsruhe, IEKP, D-7500 Karlsruhe, Germany
J. ARVIEUX lnstitut des Sciences Nucleaires, Universitd de Grenoble, F-38044 Grenoble-Cddex, France
and R. CORFU and J. PIFFARETTI Institut de Physique, Universitd de Neuchatel, CH-2000 Neuchatel, Switzerland
Received 1 October 1979
We have measured the reactions 12C(rr±, ~r±d)l°B and t2C(~r±, ~r±t)9B at an incident pion energy of 180 MeV, with two spectrometers in coincidence. The ratio a(Tr÷, 7r+d)/o0r-, 1r-d) agrees with the iso-spin prediction of unity. There is evidence for quasi-elastic lrd scattering. We obtain the surprising result that the ratio o0r, ~rd)/o(lr, ~rp), for excitation energies less than 10 MeV, is equal to the ratio (do/dS2) 0rd)/(do/d~2) (lrp) for elastic (lrd) and (np) scattering at all the points we have measured. The ratio o(~r-, ~r-t)/o(rr +, ~r÷t) is approximately one and there is no evidence for quasi-elastic 7rt scattering.
The quasi elastic knock out of deuterons by high energy protons was first observed by Azhgirei et al. [1 ]. This observation and similar ones with other incident and outgoing particles have led to the development of the cluster model, which presumes that multinucleon systems already exist as "clusters" inside the nucleus [1,2]. There has been considerable discussion on how to reconcile the coexistence of the "cluster" properties of nuclei with their well established single particle and collective properties [3]. At the present time the reaction mechanism has not been unambiguously identified because the data are inadequate. High resolution measurements of the excitation energy spectrum and the m o m e n t u m distribution of the recoil nucleus and other quantities, which are best measured in a two spectrometer coincidence experiment, should help to determine the underlying mechanism(s). During the course of studying the reactions 12C(7r± ,
7r± p)llB, with two high resolution spectrometers in coincidence [4], we have obtained data on the reactions 12C(lr±, 7r±d)10B andl2C(rr ±, 7r±t)9B. The only previous measurements of these reactions employed the emulsion technique [5], with its well known limitations. Our experiment was carried out in the SIN ,1 pion channel at an incident pion energy of 180 MeV. The magnetic spectrometer SUSI [6] was used in coincidence with a solid state spectrometer, consisting of one Si(Li) detector (1 mm thick, 35 m m diameter) followed by three ion-implanted intrinsic germanium detectors (each 12 m m thick, 35 m m diameter). The thickness of the 12C target was 202 mg/cm 2. The magnetic spectrometer determined the momentum of the scattered pions, its acceptance peaked for a pion m o m e n t u m of 169.5 MeV/c and dropped to 80% ~:1 Schweizerisches Institut fiir Nuklearforschung, ViUigen, Switzerland. 253
Volume 88B, number 3,4
PItYSICS LI'TTERS
of the central value at 146 and 192 MeV/c. The solid state spectrometer determined the energy of the protons, deuterons and tritons, which were cleanly identified by E, 2d?, cuts. Range effects limited the minimum energy of protons to 15 MeV, of deuterons to 20 MeV and of tritons to 24 MeV. The solid state spectrometer was able to stop all these particles up to their kinematic limit and thus imposed no upper cut-off. The observation angles were 30 ° for the deuterons and tritons and - l O 0 ° and - 1 1 0 ° for the pions. The acceptance o f both spectrometers was about 16 msr. The measurement of the angles and energies of the outgoing pion and deuteron together with the knowledge of the incident pion energy specifies the events completely and we can thus determine the n~omentum and mass (excitation) of the residual nucleus. Figs. ]a
a 10
12C~.* n.*.D)l° B
(/1 IZ
D 8 O (3
~6 hi
4
m Z
2
0
-10
7
0
- b
~
10 20 30 40 50 I°B EXCITATION ENERGY (MeV)
'
60
70
i
6
12C(rr" 7r-. D)IOB
==5
o
;
,
i
,
i
10
20
30
~0
so
6'0
70
I°B EXCITATION ENERGY (MeV) Fig. 1. ]°B excitation energy spectrum of the reaction 12C(n,
nd)l°B. Incident pion energy 180 MeV, Op = 30°. (a) Incident n ÷, On' = - 100 ° . (b) Incident n-, On' = -110 ° . 254
and b show the 10B excitation energy spectra obtained with positive and negative pions respectively. Table 1 summarizes the results. 0 n is the pion scattering angle in the laboratory. The index A refers to all events, L to those in the low energy region Eex c ~< 10 MeV (i.e. to particle stable states) and H to those in the region Eex c > 10 MeV. For a comparison with the elastic differential (rid) cross sections we have also listed the mean scattering angle O* in the c.m. of the outgoing 0rd) system and the mean incident pion kinetic energy T ' . The latter we have defined as the energy that would give, in free nd scattering, the same invariant mass for the outgoing (rid) system that we observe in our experinaent. In the analysis of (n, 7r+p) scattering we have observed these variables to give the best agreement when used in an impulse approximation calculation [4]. The ratio N H / N L is the ratio of the number of events in the high and low excitation energy regions. It is o f the order one for all runs. This means that about 50% of these interactions leave the 10B nucleus in the ground state or first few excited states, corresponding to the peaks in figs. la and lb. The deuterons in these events are probably knocked out quasi-elastically as assumed in the impulse approximation. Events that leave the nucleus highly excited are more likely to be due to final state interactions. It is important to note that the excitation energy spectrum o f the residual nucleus and the reaction mechanism are closely related so that single arm experiments which do not determine the former can shed little light on the latter. Similarly one cannot draw general conclusions about the full excitations energy range from experiments that are sensitive to only one particular final state [7]. The ratio R 1 is defined as: R l = o(12C(7r +, 7r+d)10B)/o(lZc(g - , 7r-d)10B).
1
-10
17 December 1979
(1)
It agrees well with the value unity expected from isospin conservation and does not show any significant dependence on the excitation energy. Also given in the table is the ratio R 2 = o(Tr, 7rd)/a(Tr, 7rp) for the various regimes o f the excitation energy. The proton data [4] have been cut at the same energy threshold as the deuteron data to make them directly comparable. The overall deuteron rate is 8.3% of the proton rate for positive pions and about 46% for negative pions. R L ~ 1 i.e. in the low excitation energy region negative pions knock-out as many deuterons as pro-
Volume 88B, number
3,4
PttYSICS
LETTERS
17 December
1979
t"l
+q
+~ +1
+1 oo C~
o
02 +1 4-1
+1
~
N
C~ +1
+1
t~ C~ C~
II
o
-lO
6
I KHtiii1111 HR 10
20
9B EXCITATION
'r,
+1
+1
II ,--1e.i
C~
+E
C~ +1
+1 +~
+1
II
tr~ C~ oo +1
+1
oc 0¢
+1 C~ oo
m +1
+1
C~ +1 Cr~
+1
+1
+1
d'211
• ~5~5 +1
+1 +1
t"q ,-~ ~ o ÷1 ÷1 tr~
? o
o'~
~.~d~
F i g . 2 . 9 B excitation spectrum of the reaction 1 2 C ( ~ r - , n - t ) 9 B . Incident pion energy 1 8 0 M e V , 0 p = 3 0 ° , 0~r' = - 1 1 0 ° .
~
e N ~
~ c ,~
60
~ oo
'~, .~
50
+b
+1
~
40
ENERGY(MeV)
C~
.2
,.. , .
3"0
tons! In the last column we have given the corresponding ratio o(nd)/o(np) for elastic pion scattering on free deuterons and protons under the same or nearly the same kinematic conditions. A comparison gives the surprising result that R~ is, within the statistical limits, equal to o(nd)/o(Trp) for each of the four runs. We have also observed a few (n, nt) events and although the statistical errors are large there are some significant results: fig. 2 shows the 9B excitation energy spectrum observed in this experiment for tritons knocked out by 7r- (run 3). The low excitation energy peak, which is seen in the deuteron data in fig. 1 above is absent• The other runs (not shown) confirm this for pions of both charges• We take this as evidence that the tritons are not the result of direct knock-out. This is corroborated by the fact that the ratioR 3 = o(lr-, zr-t)/a(n +, 7r+t) in table 1 is approximately unity and does not seem to depend significantly on the 9B excitation energy• This suggests that the triton events have their origin in neutron pick-up by deuterons since the probability that a deuteron picks up a neutron is independent of the sign of the pion which knocked out the deuteron• Furthermore the recoil 9B nucleus is likely to be in a more highly excited state as we observe here because two interactions have taken place within it. We gratefully acknowledge the hospitality extended to us by SIN and wish to thank Q. Ingram, F. Lenz R. Silbar, H.J. Weber and J. Zichy for interesting discussions. One of us (K.O.H.Z.) wishes to express his appreciation to the Alexander yon Humboldt Stiftung for the Senior American Scientist Award that made 255
Volume 88B, number 3,4
PHYSICS LETTERS
this collaboration possible. This w o r k was s u p p o r t e d in part by the U.S. D e p a r t m e n t o f Energy and the N.S.F.
References [1] L.S. Azhgirei et al., Zh. Eksp. Teor. Fiz. [Sov. Phys. JETP 6 (1958) 911]. [2] D.I. Blokhintsev, Zh. Eksp. Teor. Fiz. 33 (1957) 1295 [Soy. Phys. JETP 6 (1958) 995]; K. Wildermuth and Y.C. Tang, A unified theory of the nucleus (Vieweg, Braunschweig, 1977).
256
17 December 1979
[3] V.V. Balashov, AIP Conf. Proc. No. 47 (Winnipeg, Canada, 1978) p. 252; see also other contributions at this conference. [4] R.J. Ellis et al., Proc. 2nd Intern. Conf. on Meson-nuclear physics (Houston, March 1979); H.J. Ziock et al. Univ. of Virginia preprint. [5] Yu.R. Gismatulin et al., Yad. Fiz. 26 (1977) 243 [Sov. J. Nucl. Phys. 26 (1977) 126]. [6] J.P. Albanese et al., Nuct. Instrum. Methods 158 (1979) 363. [7] H.S. Plendl et al., Proc. 2nd Intern. Conf. on Nucl. reaction mechanisms (Varenna, Italy, 1979).
PHYSICS LETTERS
17 December 1979
COINCIDENCE MEASUREMENTS OF THE REACTIONS 12C(ff±, n±d)10B AND 12C(1r±,n±t)9B R.J. ELLIS, H.J. ZIOCK and K.O.H. ZIOCK Physics Department, University of Virginia, Charlottesville, VA 22901, USA
J. BOLGER and E. BOSCHITZ Kernforschungszentrum and Universitiit Karlsruhe, IEKP, D-7500 Karlsruhe, Germany
J. ARVIEUX lnstitut des Sciences Nucleaires, Universitd de Grenoble, F-38044 Grenoble-Cddex, France
and R. CORFU and J. PIFFARETTI Institut de Physique, Universitd de Neuchatel, CH-2000 Neuchatel, Switzerland
Received 1 October 1979
We have measured the reactions 12C(rr±, ~r±d)l°B and t2C(~r±, ~r±t)9B at an incident pion energy of 180 MeV, with two spectrometers in coincidence. The ratio a(Tr÷, 7r+d)/o0r-, 1r-d) agrees with the iso-spin prediction of unity. There is evidence for quasi-elastic lrd scattering. We obtain the surprising result that the ratio o0r, ~rd)/o(lr, ~rp), for excitation energies less than 10 MeV, is equal to the ratio (do/dS2) 0rd)/(do/d~2) (lrp) for elastic (lrd) and (np) scattering at all the points we have measured. The ratio o(~r-, ~r-t)/o(rr +, ~r÷t) is approximately one and there is no evidence for quasi-elastic 7rt scattering.
The quasi elastic knock out of deuterons by high energy protons was first observed by Azhgirei et al. [1 ]. This observation and similar ones with other incident and outgoing particles have led to the development of the cluster model, which presumes that multinucleon systems already exist as "clusters" inside the nucleus [1,2]. There has been considerable discussion on how to reconcile the coexistence of the "cluster" properties of nuclei with their well established single particle and collective properties [3]. At the present time the reaction mechanism has not been unambiguously identified because the data are inadequate. High resolution measurements of the excitation energy spectrum and the m o m e n t u m distribution of the recoil nucleus and other quantities, which are best measured in a two spectrometer coincidence experiment, should help to determine the underlying mechanism(s). During the course of studying the reactions 12C(7r± ,
7r± p)llB, with two high resolution spectrometers in coincidence [4], we have obtained data on the reactions 12C(lr±, 7r±d)10B andl2C(rr ±, 7r±t)9B. The only previous measurements of these reactions employed the emulsion technique [5], with its well known limitations. Our experiment was carried out in the SIN ,1 pion channel at an incident pion energy of 180 MeV. The magnetic spectrometer SUSI [6] was used in coincidence with a solid state spectrometer, consisting of one Si(Li) detector (1 mm thick, 35 m m diameter) followed by three ion-implanted intrinsic germanium detectors (each 12 m m thick, 35 m m diameter). The thickness of the 12C target was 202 mg/cm 2. The magnetic spectrometer determined the momentum of the scattered pions, its acceptance peaked for a pion m o m e n t u m of 169.5 MeV/c and dropped to 80% ~:1 Schweizerisches Institut fiir Nuklearforschung, ViUigen, Switzerland. 253
Volume 88B, number 3,4
PItYSICS LI'TTERS
of the central value at 146 and 192 MeV/c. The solid state spectrometer determined the energy of the protons, deuterons and tritons, which were cleanly identified by E, 2d?, cuts. Range effects limited the minimum energy of protons to 15 MeV, of deuterons to 20 MeV and of tritons to 24 MeV. The solid state spectrometer was able to stop all these particles up to their kinematic limit and thus imposed no upper cut-off. The observation angles were 30 ° for the deuterons and tritons and - l O 0 ° and - 1 1 0 ° for the pions. The acceptance o f both spectrometers was about 16 msr. The measurement of the angles and energies of the outgoing pion and deuteron together with the knowledge of the incident pion energy specifies the events completely and we can thus determine the n~omentum and mass (excitation) of the residual nucleus. Figs. ]a
a 10
12C~.* n.*.D)l° B
(/1 IZ
D 8 O (3
~6 hi
4
m Z
2
0
-10
7
0
- b
~
10 20 30 40 50 I°B EXCITATION ENERGY (MeV)
'
60
70
i
6
12C(rr" 7r-. D)IOB
==5
o
;
,
i
,
i
10
20
30
~0
so
6'0
70
I°B EXCITATION ENERGY (MeV) Fig. 1. ]°B excitation energy spectrum of the reaction 12C(n,
nd)l°B. Incident pion energy 180 MeV, Op = 30°. (a) Incident n ÷, On' = - 100 ° . (b) Incident n-, On' = -110 ° . 254
and b show the 10B excitation energy spectra obtained with positive and negative pions respectively. Table 1 summarizes the results. 0 n is the pion scattering angle in the laboratory. The index A refers to all events, L to those in the low energy region Eex c ~< 10 MeV (i.e. to particle stable states) and H to those in the region Eex c > 10 MeV. For a comparison with the elastic differential (rid) cross sections we have also listed the mean scattering angle O* in the c.m. of the outgoing 0rd) system and the mean incident pion kinetic energy T ' . The latter we have defined as the energy that would give, in free nd scattering, the same invariant mass for the outgoing (rid) system that we observe in our experinaent. In the analysis of (n, 7r+p) scattering we have observed these variables to give the best agreement when used in an impulse approximation calculation [4]. The ratio N H / N L is the ratio of the number of events in the high and low excitation energy regions. It is o f the order one for all runs. This means that about 50% of these interactions leave the 10B nucleus in the ground state or first few excited states, corresponding to the peaks in figs. la and lb. The deuterons in these events are probably knocked out quasi-elastically as assumed in the impulse approximation. Events that leave the nucleus highly excited are more likely to be due to final state interactions. It is important to note that the excitation energy spectrum o f the residual nucleus and the reaction mechanism are closely related so that single arm experiments which do not determine the former can shed little light on the latter. Similarly one cannot draw general conclusions about the full excitations energy range from experiments that are sensitive to only one particular final state [7]. The ratio R 1 is defined as: R l = o(12C(7r +, 7r+d)10B)/o(lZc(g - , 7r-d)10B).
1
-10
17 December 1979
(1)
It agrees well with the value unity expected from isospin conservation and does not show any significant dependence on the excitation energy. Also given in the table is the ratio R 2 = o(Tr, 7rd)/a(Tr, 7rp) for the various regimes o f the excitation energy. The proton data [4] have been cut at the same energy threshold as the deuteron data to make them directly comparable. The overall deuteron rate is 8.3% of the proton rate for positive pions and about 46% for negative pions. R L ~ 1 i.e. in the low excitation energy region negative pions knock-out as many deuterons as pro-
Volume 88B, number
3,4
PttYSICS
LETTERS
17 December
1979
t"l
+q
+~ +1
+1 oo C~
o
02 +1 4-1
+1
~
N
C~ +1
+1
t~ C~ C~
II
o
-lO
6
I KHtiii1111 HR 10
20
9B EXCITATION
'r,
+1
+1
II ,--1e.i
C~
+E
C~ +1
+1 +~
+1
II
tr~ C~ oo +1
+1
oc 0¢
+1 C~ oo
m +1
+1
C~ +1 Cr~
+1
+1
+1
d'211
• ~5~5 +1
+1 +1
t"q ,-~ ~ o ÷1 ÷1 tr~
? o
o'~
~.~d~
F i g . 2 . 9 B excitation spectrum of the reaction 1 2 C ( ~ r - , n - t ) 9 B . Incident pion energy 1 8 0 M e V , 0 p = 3 0 ° , 0~r' = - 1 1 0 ° .
~
e N ~
~ c ,~
60
~ oo
'~, .~
50
+b
+1
~
40
ENERGY(MeV)
C~
.2
,.. , .
3"0
tons! In the last column we have given the corresponding ratio o(nd)/o(np) for elastic pion scattering on free deuterons and protons under the same or nearly the same kinematic conditions. A comparison gives the surprising result that R~ is, within the statistical limits, equal to o(nd)/o(Trp) for each of the four runs. We have also observed a few (n, nt) events and although the statistical errors are large there are some significant results: fig. 2 shows the 9B excitation energy spectrum observed in this experiment for tritons knocked out by 7r- (run 3). The low excitation energy peak, which is seen in the deuteron data in fig. 1 above is absent• The other runs (not shown) confirm this for pions of both charges• We take this as evidence that the tritons are not the result of direct knock-out. This is corroborated by the fact that the ratioR 3 = o(lr-, zr-t)/a(n +, 7r+t) in table 1 is approximately unity and does not seem to depend significantly on the 9B excitation energy• This suggests that the triton events have their origin in neutron pick-up by deuterons since the probability that a deuteron picks up a neutron is independent of the sign of the pion which knocked out the deuteron• Furthermore the recoil 9B nucleus is likely to be in a more highly excited state as we observe here because two interactions have taken place within it. We gratefully acknowledge the hospitality extended to us by SIN and wish to thank Q. Ingram, F. Lenz R. Silbar, H.J. Weber and J. Zichy for interesting discussions. One of us (K.O.H.Z.) wishes to express his appreciation to the Alexander yon Humboldt Stiftung for the Senior American Scientist Award that made 255
Volume 88B, number 3,4
PHYSICS LETTERS
this collaboration possible. This w o r k was s u p p o r t e d in part by the U.S. D e p a r t m e n t o f Energy and the N.S.F.
References [1] L.S. Azhgirei et al., Zh. Eksp. Teor. Fiz. [Sov. Phys. JETP 6 (1958) 911]. [2] D.I. Blokhintsev, Zh. Eksp. Teor. Fiz. 33 (1957) 1295 [Soy. Phys. JETP 6 (1958) 995]; K. Wildermuth and Y.C. Tang, A unified theory of the nucleus (Vieweg, Braunschweig, 1977).
256
17 December 1979
[3] V.V. Balashov, AIP Conf. Proc. No. 47 (Winnipeg, Canada, 1978) p. 252; see also other contributions at this conference. [4] R.J. Ellis et al., Proc. 2nd Intern. Conf. on Meson-nuclear physics (Houston, March 1979); H.J. Ziock et al. Univ. of Virginia preprint. [5] Yu.R. Gismatulin et al., Yad. Fiz. 26 (1977) 243 [Sov. J. Nucl. Phys. 26 (1977) 126]. [6] J.P. Albanese et al., Nuct. Instrum. Methods 158 (1979) 363. [7] H.S. Plendl et al., Proc. 2nd Intern. Conf. on Nucl. reaction mechanisms (Varenna, Italy, 1979).