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Uses of radioactive ion beams in astrophysical research
318
Nuclear Instruments and Methods in Physics Research B10/11(1985) 378-381 North-Holland, Amsterdam
USES OF RADIOACIIVE
ION BEAMS IN ASTROPHYSICAL
RESEARCH
R.N. BOYD Department
of Physics and Department
of Astronomy
L. RYBARCYK, H.J. HAUSMAN, W. KIM and P. SCHMALBROCK Department
of Physics, Ohio State University, Columbw,
OH 43210, USA
The radioactive ion beam facility presently being constructed at The Ohio State University is described. Experiments with it will utiliz.e double scattering, the first of which produces the radioactive ion beam of interest, and the second of which produces the reaction of interest. The system is described using the ‘50(a, y)“Ne reaction as an example reaction study. A situation in nuclear burning which involves nuclear isomeric states, that involving the states of 34C1, is described, and the considerations required for experimental study of the relevant reactions are given.
1. Introduction
The uses and methods of production of beams of radioactive nuclei have received an increasing amount of attention in recent years [1,2]. One facility [3] which will be used to study nuclear reactions on radioactive nuclei, that are being built at The Ohio State University, is nearing completion. Since its basic design features are important to consideration of experiments which can be performed using it and similar facilities [4], such features will be discussed and results of recent tests on one crucial component, the time-of-flight (TOF) system, will be given. A number of situations in astrophysics [1,2] have been described which require knowledge of nuclear cross sections involving radioactive nuclei. One which has not been emphasized, however, is that in which nuclear isomers contribute. One such case, that of the participation of 26A1 in stellar burning, has been discussed in detail [5]. In the present work we will describe the reactions involving “Cl and its isomeric excited state 34c1m which might be involved in different nucleosynthesis schemes. Then the techniques for producing the radioactive beams necessary to perform the experiments to measure the relevant reaction cross sections involving “Cl and %Clm will be discussed.
2. RISOAR, the OSU radioactive ion beam facility The OSU radioactive ion beam facility, BISOAB, operates in two stages. A nuclear reaction at the fist of two gas cells, Target I, produces ions of the radioactive nucleus of interest. The radioactive nuclei are then 0168-583X/85/$03.30 0 Blsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
magnetically separated from the other nuclear species, directed through a tagging and TOF system, then focused onto a second gas cell, Target II. A detector located there observes the products of the reaction of interest. The specific components are indicated in fig. 1, using the reaction study ‘*O(a, y)19Ne as an illustrative
IGO
BEAN
VAN
DE
FROM GRAAFF
ACCELERATOR
60°
DIPOLE
MAGNET
4HE GANRA-RAY
TARGET
AND DETECTOR
Fig. 1. The basic components of RISOAR, the two-stage facility for study of nuclear reactions on short-lived radioactive nuclei.
RN. m
-3.0
Boyd et al. / Radioactive
DETECTOR
-QkV-A
\
’ 1’
\
\
\
-2.2kV+
1
Fig. 2. Schematic diagram of the TOF detection systems. Typical values of the voltages on the grids are as indicated. example. For study of that specific reaction, a beam of
I60 ions from the OSU Van de Graaff accelerator is first directed onto a 3He cell (Target I) where some of the incident nuclei are converted to I50 via the ‘He(160, 150)4He reaction. A large fraction of the “0 ions emerging from Target I can be collected and focused by the dipole magnet onto Target II which, in this example, is a 4He target. Just prior to Target II each ion will pass through a pair of thin foils. Electrons knocked out of the foils by the passing “0 ions will be detected, thus allowing for a TOF, hence energy, determination of each ion. A second time interval, that between the second TOF detector and the detection of the capture gamma ray from Target II, is also measured. It, together with dE/dx information (the ions will lose energy as they pass through the gas in Target II) determines the location in Target II at which the capture event occurred, hence of the energy at which the reaction occurred. Thus large portions of excitation functions can be measured at a single accelerator setting. Most of the components of the system are essentially as described in ref. [3]. The TOF detector system described therein, however, has been replaced with one having a different design, shown in fig. 2. Electrons knocked out of the foil, at - 3.0 kV, are accelerated to the flat grid at - 2.2 kV, then reflected and focused by a curved electrostatic mirror onto a channel plate detector. The system has been found to give a timing peak of about 400 ps fwhm for 5 MeV alpha particles. The detector efficiency, about 50%, is consistent with other systems of this type [6]; it is expected to be considerably higher for the heavier ions to be used in our radioactive ion beam experiments. 3. Nuclear astrophysics with radioactive nuclei There are a variety of stellar burning situations in which nucleosynthesis occurs at rates sufficient to in-
ion beams in astrophysical
research
379
volve short lived radioactive nuclei. That involving the rapid proton burning process, or rp process, has been discussed in detail [7], although so few of the thermonuclear reaction rates required for that study are known that most were assumed to be averaged values. Thus the actual rates could vary considerably from the estimated values, and the results of the network calculations could differ appreciably from those which actually occur. Other nuclear reaction processes could also occur which would involve rapid proton burning, e.g., where matter is being accreted onto the surface of a neutron star [8]. One reaction of particular interest in that context is that used to describe RISOAR, the “O(a, y)“Ne reaction. Another interesting case could occur in the acceleration region of stellar jets [9]. Such situations could produce effective temperatures which are much higher than those characterizing the relative motion within each of the jet constituents, i.e., the energy distributions of the constituents could differ greatly from a Boltzmann distribution. Any of these situations could produce rapid proton or alpha-particle burning. The nuclear reactions pertinent thereto can sometimes lead to curious situations in which reactions through nuclear isomeric states must be considered for a full picture of the reaction processes involved. A case in point is 34C1,which can be produced by the 33S(p y)34C1 reaction. The “Cl (ground state) has a half lif; of only 1.52 s so will usually decay before subsequent reactions can occur on it. However the 34Clm (0.146 MeV state) has a half life of 32.2 min, thus making it a possible bridge to mass 35 amu nuclei at some temperatures, and an important possibility for understanding high temperature (or high effective temperature) situations. Possible appropriate reactions are characterized in fig. 3a. Shown in fig. 3b is the subset of these nuclear reactions which dominates in each temperature region. The divisions between temperature regions were obtained by assuming the decay half life rl,2 was equal to the time between successive reactions, determined from the equation 0.693/r,,,
= n,(uu),
where np is the density of protons in the environment being studied. The thermonuclear reaction rates (au) were estimated using the statistical model prescription of Woosley et al. [lo], and np was assumed to be 100 g/crd. As can be seen from fig. 3b, for the parameters assumed, %I” becomes an important direct bridge to the Ar isotopes at a temperature considerably below that at which the ground state becomes important. As has been noted [9], nondoltzmann distributions could also cause an appreciable enhancement in the thermonuclear reaction rates; this would lower the effective temperature at which each nuclear process becomes important. One aspect not indicated in fig. 3a should be noted: some environments could have a hot dense phoIII. NUCLEAR PHYSICS/ASTROPHYSICS
RN. Boyd et al. / Raakctive
2
ion beams in astrophysical research
that beams predominantly in one or the other state can be obtained. For example, the “Cl( ‘He, u)~C~ reaction [ll] populates the 3 + isomeric state much more strongly than it does the O+ ground state. Conversely the 32S(3He, P)~C~ reaction [12] favors the O+ ground state over the isomeric state by a similar factor. Thus beams of “Cl onto a ‘He target would be expected to produce beams predominantly of 34Clm, while beams of 32S onto ‘He would give beams predominantly of the “Cl (ground state). In such considerations attention must also be given to population of higher lying excited states and to their gamma-ray branches, since, as the nuclear decay times of the excited states are usually much shorter than the transit time through any radioactive beam system (about 200 ns for RISOAR), all such states will also contribute to the ground state or isomerit state beams.
0
4. Conclusions
T z V
a-2
LEA B
-4
-6
Fig. 3. (a) Possible H burning reactions around the mass 34 amu nuclei. Proton radiative captures are indicated as solid lines, while beta decays are shown as dashed lines. 3(b) Thermonuclear reaction rates as a function of temperature. The subset of reactions for mass 34 amu nuclei germane to each region is as indicated. The value of np was assumed to be 100 g/cm’.
ton bath in which photon induced transitions between the MCI (ground state) and the “Clm(0.146 MeV state) could occur. Then thermal equihbrium between them and the other excited states of “CI would be established, as was discussed for the 26AI case [5]. The experiments required to determine explicitly the thermonuclear reaction rates involve bombardment of a hydrogen, target (Target II in the RISOAR configuration) with a %Cl beam, first in the ground state and then in the isomeric state. While it would be difficuh to prepare a beam having only one of these components, the systematics of low energy nuclear reactions suggest
We expect that RISOAR will soon be able to generate the “0 beams necessary to study the “O(a, y)“Ne reaction. In the initial configuration it will not be able to generate the uCl and wClm beams necessary to perform the other set of measurements discussed above. The present OSU Van de Graaff accelerator is not capable of generating the heavy ion beams sufficient in energy to overcome the coulomb barrier of the radioactive nucleus producing reactions. Furthermore RISOAR was designed to have a radioactive ion scattering angle at Target I which was matched to lighter ion reactions, e.g., 3He(160, 150)4He instead of 3He(35Cl, s%l)4He. Nonetheless the basic ideas presented with regard to production of beams of isomeric nuclei, and to study of reactions on them, do apply. Indeed long range plans for RISOAR include the capability for study of nuclear reactions on isomeric nuclei.
References [l] Prcc. Workshop on Radioactive Ion Beams and Small Cross section Measurements, ed., R.N. Boyd (1981). [2] Prcc. Workshop on Prospects for Research with Radioactive Beams from Heavy Ion Accelerators, ed., J.M. Nitschke, LBL Report No. LBL 18187 (1984). ]3] R.N. Boyd, L. Rybarcyk, M. Wicscher and H.J. Hausmaa, IEEE Trans. Nucl. sci. NS30 (1983) 1387. [4] R.C. Haight, G.J. Mathews, R.M. White, L.A. Aviles and S.E. Wocdard, IEEE Trans. Nucl. sci. NS30 (1983) 1160. IS] R.A. Ward and WA. Fowler, Astrophys. J. 238 (1980) 266. [6] T. Gdcnweller, H. NOB,R. Sapotta, R.E. Renfordt and R. Bass, Nucl. In&r. and Meth. 198 (1982) 263. [7] RK. Wallace and S.E. Woo&y, Astrophys. J. Suppl. 45 (1981) 389.
R.N. Boyd et al. / Radioactive [8] See, e.g., R.E. Tamm and R.E. Picldum, Astrophys. J. 224
(1978) 210. [9] R.N. Boyd, G.W. Collins II, G.H. Newsom and M. Wiescher, Science 225 (1984) 508. [lo] S.E. Woo&y, W.A. Fowler, Holmes and B.A. Zimmerman, At. Data Nucl. Data Tables 22 (1978) 371.
ion beam
in astrophysical research
381
[ll] J.J.M. van Gasteren, B. Sikora and A. van der Steld, Nucl. Phys. A231 (1974) 411. [12] H. Nann, L. Armbruster and B.H. WiIdenthaI, Nucl. Phys. Al98 (1972) 11.
III. NUCLEAR PHYSICS/ASTROPHYSICS
Nuclear Instruments and Methods in Physics Research B10/11(1985) 378-381 North-Holland, Amsterdam
USES OF RADIOACIIVE
ION BEAMS IN ASTROPHYSICAL
RESEARCH
R.N. BOYD Department
of Physics and Department
of Astronomy
L. RYBARCYK, H.J. HAUSMAN, W. KIM and P. SCHMALBROCK Department
of Physics, Ohio State University, Columbw,
OH 43210, USA
The radioactive ion beam facility presently being constructed at The Ohio State University is described. Experiments with it will utiliz.e double scattering, the first of which produces the radioactive ion beam of interest, and the second of which produces the reaction of interest. The system is described using the ‘50(a, y)“Ne reaction as an example reaction study. A situation in nuclear burning which involves nuclear isomeric states, that involving the states of 34C1, is described, and the considerations required for experimental study of the relevant reactions are given.
1. Introduction
The uses and methods of production of beams of radioactive nuclei have received an increasing amount of attention in recent years [1,2]. One facility [3] which will be used to study nuclear reactions on radioactive nuclei, that are being built at The Ohio State University, is nearing completion. Since its basic design features are important to consideration of experiments which can be performed using it and similar facilities [4], such features will be discussed and results of recent tests on one crucial component, the time-of-flight (TOF) system, will be given. A number of situations in astrophysics [1,2] have been described which require knowledge of nuclear cross sections involving radioactive nuclei. One which has not been emphasized, however, is that in which nuclear isomers contribute. One such case, that of the participation of 26A1 in stellar burning, has been discussed in detail [5]. In the present work we will describe the reactions involving “Cl and its isomeric excited state 34c1m which might be involved in different nucleosynthesis schemes. Then the techniques for producing the radioactive beams necessary to perform the experiments to measure the relevant reaction cross sections involving “Cl and %Clm will be discussed.
2. RISOAR, the OSU radioactive ion beam facility The OSU radioactive ion beam facility, BISOAB, operates in two stages. A nuclear reaction at the fist of two gas cells, Target I, produces ions of the radioactive nucleus of interest. The radioactive nuclei are then 0168-583X/85/$03.30 0 Blsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
magnetically separated from the other nuclear species, directed through a tagging and TOF system, then focused onto a second gas cell, Target II. A detector located there observes the products of the reaction of interest. The specific components are indicated in fig. 1, using the reaction study ‘*O(a, y)19Ne as an illustrative
IGO
BEAN
VAN
DE
FROM GRAAFF
ACCELERATOR
60°
DIPOLE
MAGNET
4HE GANRA-RAY
TARGET
AND DETECTOR
Fig. 1. The basic components of RISOAR, the two-stage facility for study of nuclear reactions on short-lived radioactive nuclei.
RN. m
-3.0
Boyd et al. / Radioactive
DETECTOR
-QkV-A
\
’ 1’
\
\
\
-2.2kV+
1
Fig. 2. Schematic diagram of the TOF detection systems. Typical values of the voltages on the grids are as indicated. example. For study of that specific reaction, a beam of
I60 ions from the OSU Van de Graaff accelerator is first directed onto a 3He cell (Target I) where some of the incident nuclei are converted to I50 via the ‘He(160, 150)4He reaction. A large fraction of the “0 ions emerging from Target I can be collected and focused by the dipole magnet onto Target II which, in this example, is a 4He target. Just prior to Target II each ion will pass through a pair of thin foils. Electrons knocked out of the foils by the passing “0 ions will be detected, thus allowing for a TOF, hence energy, determination of each ion. A second time interval, that between the second TOF detector and the detection of the capture gamma ray from Target II, is also measured. It, together with dE/dx information (the ions will lose energy as they pass through the gas in Target II) determines the location in Target II at which the capture event occurred, hence of the energy at which the reaction occurred. Thus large portions of excitation functions can be measured at a single accelerator setting. Most of the components of the system are essentially as described in ref. [3]. The TOF detector system described therein, however, has been replaced with one having a different design, shown in fig. 2. Electrons knocked out of the foil, at - 3.0 kV, are accelerated to the flat grid at - 2.2 kV, then reflected and focused by a curved electrostatic mirror onto a channel plate detector. The system has been found to give a timing peak of about 400 ps fwhm for 5 MeV alpha particles. The detector efficiency, about 50%, is consistent with other systems of this type [6]; it is expected to be considerably higher for the heavier ions to be used in our radioactive ion beam experiments. 3. Nuclear astrophysics with radioactive nuclei There are a variety of stellar burning situations in which nucleosynthesis occurs at rates sufficient to in-
ion beams in astrophysical
research
379
volve short lived radioactive nuclei. That involving the rapid proton burning process, or rp process, has been discussed in detail [7], although so few of the thermonuclear reaction rates required for that study are known that most were assumed to be averaged values. Thus the actual rates could vary considerably from the estimated values, and the results of the network calculations could differ appreciably from those which actually occur. Other nuclear reaction processes could also occur which would involve rapid proton burning, e.g., where matter is being accreted onto the surface of a neutron star [8]. One reaction of particular interest in that context is that used to describe RISOAR, the “O(a, y)“Ne reaction. Another interesting case could occur in the acceleration region of stellar jets [9]. Such situations could produce effective temperatures which are much higher than those characterizing the relative motion within each of the jet constituents, i.e., the energy distributions of the constituents could differ greatly from a Boltzmann distribution. Any of these situations could produce rapid proton or alpha-particle burning. The nuclear reactions pertinent thereto can sometimes lead to curious situations in which reactions through nuclear isomeric states must be considered for a full picture of the reaction processes involved. A case in point is 34C1,which can be produced by the 33S(p y)34C1 reaction. The “Cl (ground state) has a half lif; of only 1.52 s so will usually decay before subsequent reactions can occur on it. However the 34Clm (0.146 MeV state) has a half life of 32.2 min, thus making it a possible bridge to mass 35 amu nuclei at some temperatures, and an important possibility for understanding high temperature (or high effective temperature) situations. Possible appropriate reactions are characterized in fig. 3a. Shown in fig. 3b is the subset of these nuclear reactions which dominates in each temperature region. The divisions between temperature regions were obtained by assuming the decay half life rl,2 was equal to the time between successive reactions, determined from the equation 0.693/r,,,
= n,(uu),
where np is the density of protons in the environment being studied. The thermonuclear reaction rates (au) were estimated using the statistical model prescription of Woosley et al. [lo], and np was assumed to be 100 g/crd. As can be seen from fig. 3b, for the parameters assumed, %I” becomes an important direct bridge to the Ar isotopes at a temperature considerably below that at which the ground state becomes important. As has been noted [9], nondoltzmann distributions could also cause an appreciable enhancement in the thermonuclear reaction rates; this would lower the effective temperature at which each nuclear process becomes important. One aspect not indicated in fig. 3a should be noted: some environments could have a hot dense phoIII. NUCLEAR PHYSICS/ASTROPHYSICS
RN. Boyd et al. / Raakctive
2
ion beams in astrophysical research
that beams predominantly in one or the other state can be obtained. For example, the “Cl( ‘He, u)~C~ reaction [ll] populates the 3 + isomeric state much more strongly than it does the O+ ground state. Conversely the 32S(3He, P)~C~ reaction [12] favors the O+ ground state over the isomeric state by a similar factor. Thus beams of “Cl onto a ‘He target would be expected to produce beams predominantly of 34Clm, while beams of 32S onto ‘He would give beams predominantly of the “Cl (ground state). In such considerations attention must also be given to population of higher lying excited states and to their gamma-ray branches, since, as the nuclear decay times of the excited states are usually much shorter than the transit time through any radioactive beam system (about 200 ns for RISOAR), all such states will also contribute to the ground state or isomerit state beams.
0
4. Conclusions
T z V
a-2
LEA B
-4
-6
Fig. 3. (a) Possible H burning reactions around the mass 34 amu nuclei. Proton radiative captures are indicated as solid lines, while beta decays are shown as dashed lines. 3(b) Thermonuclear reaction rates as a function of temperature. The subset of reactions for mass 34 amu nuclei germane to each region is as indicated. The value of np was assumed to be 100 g/cm’.
ton bath in which photon induced transitions between the MCI (ground state) and the “Clm(0.146 MeV state) could occur. Then thermal equihbrium between them and the other excited states of “CI would be established, as was discussed for the 26AI case [5]. The experiments required to determine explicitly the thermonuclear reaction rates involve bombardment of a hydrogen, target (Target II in the RISOAR configuration) with a %Cl beam, first in the ground state and then in the isomeric state. While it would be difficuh to prepare a beam having only one of these components, the systematics of low energy nuclear reactions suggest
We expect that RISOAR will soon be able to generate the “0 beams necessary to study the “O(a, y)“Ne reaction. In the initial configuration it will not be able to generate the uCl and wClm beams necessary to perform the other set of measurements discussed above. The present OSU Van de Graaff accelerator is not capable of generating the heavy ion beams sufficient in energy to overcome the coulomb barrier of the radioactive nucleus producing reactions. Furthermore RISOAR was designed to have a radioactive ion scattering angle at Target I which was matched to lighter ion reactions, e.g., 3He(160, 150)4He instead of 3He(35Cl, s%l)4He. Nonetheless the basic ideas presented with regard to production of beams of isomeric nuclei, and to study of reactions on them, do apply. Indeed long range plans for RISOAR include the capability for study of nuclear reactions on isomeric nuclei.
References [l] Prcc. Workshop on Radioactive Ion Beams and Small Cross section Measurements, ed., R.N. Boyd (1981). [2] Prcc. Workshop on Prospects for Research with Radioactive Beams from Heavy Ion Accelerators, ed., J.M. Nitschke, LBL Report No. LBL 18187 (1984). ]3] R.N. Boyd, L. Rybarcyk, M. Wicscher and H.J. Hausmaa, IEEE Trans. Nucl. sci. NS30 (1983) 1387. [4] R.C. Haight, G.J. Mathews, R.M. White, L.A. Aviles and S.E. Wocdard, IEEE Trans. Nucl. sci. NS30 (1983) 1160. IS] R.A. Ward and WA. Fowler, Astrophys. J. 238 (1980) 266. [6] T. Gdcnweller, H. NOB,R. Sapotta, R.E. Renfordt and R. Bass, Nucl. In&r. and Meth. 198 (1982) 263. [7] RK. Wallace and S.E. Woo&y, Astrophys. J. Suppl. 45 (1981) 389.
R.N. Boyd et al. / Radioactive [8] See, e.g., R.E. Tamm and R.E. Picldum, Astrophys. J. 224
(1978) 210. [9] R.N. Boyd, G.W. Collins II, G.H. Newsom and M. Wiescher, Science 225 (1984) 508. [lo] S.E. Woo&y, W.A. Fowler, Holmes and B.A. Zimmerman, At. Data Nucl. Data Tables 22 (1978) 371.
ion beam
in astrophysical research
381
[ll] J.J.M. van Gasteren, B. Sikora and A. van der Steld, Nucl. Phys. A231 (1974) 411. [12] H. Nann, L. Armbruster and B.H. WiIdenthaI, Nucl. Phys. Al98 (1972) 11.
III. NUCLEAR PHYSICS/ASTROPHYSICS