- Email: [email protected]
Determination of (γ,n) reaction rates for the astrophysical γ process
NuclearPhysicsA718 (2003)575c-577~
ELSEVIER
www.elsevier.com/locate/npe
Determination of (r,n) reaction rates for the astrophysical y process K. Vogt” *, P. Mohr”, T. Rauscherb, K. Sonnabenda, A. Zilges” a Institut fiir Kernphysik, Technische Universitlt D-64289 Darmstadt, Germany b Institut fiir Physik, Universitlt Switzerland
Darmstadt, Schlossgartenstrasse 9,
Basel, Klingelbergstrasse 82, CH-4056 Basel,
(r,n) cross sections and reaction rates in the astrophysical y-process have been measured for the nucleus 204Pb using the method of photoactivation. Some peculiarities of the (r,n) cross section of this nucleus are discussed. 1. Introduction
Most of the heavy nuclei are produced in the astrophysical s- and r-process, except for some 30 neutron-deficient nuclei that are produced in the so called y-process, probably during supernova explosions. This process consists of a series of (r,n), (y,p), and (~,a) reactions that starts on heavy stable seed nuclei stemming from the s- and r-process [l--3]. 2. Experimental
method
The samples are irradiated at the injector of the superconducting electron accelerator S-DALINAC [4-61. Th e monoenergetic electrons hit a rotating copper wheel and are completely stopped within this wheel. The resulting bremsstrahlung photons then cause (r,n) reactions in the target. For the calibration of the absolute intensity of the photon flux, resonant photon scattering measurements using a boron target are performed simultaneously with the activation. Fig. 1 shows a typical spectrum measured during an activation run. The well known lines stemming from ‘lB(y,y’) can be observed clearly, as well as some lines stemming from ‘08Pb(y,y’). Although no lines from the low abundant zo4Pb can be observed in this spectrum, the lines from the decay of the unstable nuclei produced by the ‘04Pb(y ,n)203Pb reaction can be analyzed in the corresponding activation spectrum [7]. For the determination of the shape of the bremsstrahlung spectra, we performed Monte Carlo simulations using the computer code GEANT. Comparison with the results of the ‘lB(y,y’) measurements showed the necessity of some corrections in the high energy region. Details of the experimental method and these corrections can be found in Refs. [&lo]. *email: [email protected] 0375-9474/03/$- seefront matter0 2003 ElsevierScienceB.V All rightsreserved doi:lO.l016/S0375-9474(03)00857-l
576~
K. lbgt et al. /Nuclear
Physics A 718 (2003) 575-577~
3. Data analysis It has been discussed elsewhere in this volume [ll], that one way of calculating the astrophysical reaction rate from our experimental yield is to make an assumption about the threshold behaviour of the (r,n) cross section. We parametrized this cross section as P
a(E)
= o. x
(E-S> 2
sn
Usually, we assume a value of p = 0.5, which is justified as long as the compound nucleus decays by s-wave neutron emission. If the parametrization (1) describes the threshold behaviour correctly, the parameter 00 should be constant for measurements at different endpoint energies of the bremsstrahlung spectrum. It can be seen in Fig. 2 that there are large fluctuations in the data. Especially the point corresponding to the lowest endpoint energy is significantly too low. A possible explanation for this behaviour is that the ground state of ‘03Pb can not be reached by s-wave emission from l- intermediate state because of conservation of spin and parity. The first excited state that can be populated by s-wave decay is at E = 125.6 keV. In order to take this into account, we used an effective neutron separation energy of SE* = S&+125.6 keV in our calculations. This will be discussed in more detail elsewhere [7]. In Fig. 3, the parameter aa is plotted versus all measured endpoint energies using the effective neutron separation energy Sgff = S,+125.6 keV. It can be clearly seen that the values in Fig. 3 show less fluctuations than the values in Fig. 2. The ground state reaction rate at Ts = 2.5 calculated using the parametrization (1) with S$ = S,+125.6 keV is X = (1.56 f 0.25)/s. Acknowledgements We thank the S-DALINAC group around H.-D. Grlf for the reliable beam during the photoactivation and A. Richter and H. Utsunomiya for encouraging discussions. This
I I “B
4m
6ooO
E, (kW
8iXiI
10000
Figure 1. Typical (7,~‘) spectrum measured for photon flux calibration during the activation of a combined lead, gold, and boron target.
K. Vogt et al. /Nuclear
Physics A718 (2003) 57.S577~
600
3w
200
400 22 E b”
s E b” 100
o-
8500
i
200
9ocm
9500
loco0
Energy (keV)
Figure 2. Comparison of the values of the parameter o. measured at different endpoint energies. The dotted line is the weighted average of the different values: 00 = 143 & 31 mb.
0 8500
!moo
9500 Energy (ire’/)
Figure 3. Comparison of the values of the parameter a0 measured at different endpoint energies using a modified reaction threshold of SEff= S,+125.6 keV. The dotted line is the weighted average of the different values: as = 250 f 40 mb.
work was supported by Deutsche Forschungsgemeinschaft (Zi 510/2-l and FOR 272/2-l) and Swiss NSF (grants 2124-055832.98, 2000-061822.00, 2024-067428.01). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
S. E. Woosley and W. M. Howard, Astrophys. J. Suppl. 36, 285 (1978). M. Rayet, N. Prantzos, and M. Arnould, Astron. Astrophys. 298, 517 (1995). D. L. Lambert, Astron. Astrophys. Rev. 3 (1992) 201. A. Richter, Proc. 5th European Particle Accelerator Conference, Barcelona 1996, ed. S. Myers et al., IOP Publishing, Bristol, 1996, p. 110. P. Mohr, J. Enders, T. Hartmann, H. Kaiser, D. Schiesser, S. Schmitt, S. Volz, F. Wissel, and A. Zilges, Nucl. Inst. Meth. Phys. Res. A 423, 480 (1999). A. Zilges and P. Mohr, Prog. Part. Nucl. Phys. 44, 39 (2000). K. Vogt et al., to be published. P. Mohr et al., Phys. Lett. B 488, 127 (2000). K. Vogt et al., Phys. Rev. C 63, 055802 (2001). K. Vogt et al., Nucl. Phys. A707, 241 (2002). P. Mohr, T. Rauscher, K. Sonnabend, K. Vogt, and A. Zilges, this volume.
ELSEVIER
www.elsevier.com/locate/npe
Determination of (r,n) reaction rates for the astrophysical y process K. Vogt” *, P. Mohr”, T. Rauscherb, K. Sonnabenda, A. Zilges” a Institut fiir Kernphysik, Technische Universitlt D-64289 Darmstadt, Germany b Institut fiir Physik, Universitlt Switzerland
Darmstadt, Schlossgartenstrasse 9,
Basel, Klingelbergstrasse 82, CH-4056 Basel,
(r,n) cross sections and reaction rates in the astrophysical y-process have been measured for the nucleus 204Pb using the method of photoactivation. Some peculiarities of the (r,n) cross section of this nucleus are discussed. 1. Introduction
Most of the heavy nuclei are produced in the astrophysical s- and r-process, except for some 30 neutron-deficient nuclei that are produced in the so called y-process, probably during supernova explosions. This process consists of a series of (r,n), (y,p), and (~,a) reactions that starts on heavy stable seed nuclei stemming from the s- and r-process [l--3]. 2. Experimental
method
The samples are irradiated at the injector of the superconducting electron accelerator S-DALINAC [4-61. Th e monoenergetic electrons hit a rotating copper wheel and are completely stopped within this wheel. The resulting bremsstrahlung photons then cause (r,n) reactions in the target. For the calibration of the absolute intensity of the photon flux, resonant photon scattering measurements using a boron target are performed simultaneously with the activation. Fig. 1 shows a typical spectrum measured during an activation run. The well known lines stemming from ‘lB(y,y’) can be observed clearly, as well as some lines stemming from ‘08Pb(y,y’). Although no lines from the low abundant zo4Pb can be observed in this spectrum, the lines from the decay of the unstable nuclei produced by the ‘04Pb(y ,n)203Pb reaction can be analyzed in the corresponding activation spectrum [7]. For the determination of the shape of the bremsstrahlung spectra, we performed Monte Carlo simulations using the computer code GEANT. Comparison with the results of the ‘lB(y,y’) measurements showed the necessity of some corrections in the high energy region. Details of the experimental method and these corrections can be found in Refs. [&lo]. *email: [email protected] 0375-9474/03/$- seefront matter0 2003 ElsevierScienceB.V All rightsreserved doi:lO.l016/S0375-9474(03)00857-l
576~
K. lbgt et al. /Nuclear
Physics A 718 (2003) 575-577~
3. Data analysis It has been discussed elsewhere in this volume [ll], that one way of calculating the astrophysical reaction rate from our experimental yield is to make an assumption about the threshold behaviour of the (r,n) cross section. We parametrized this cross section as P
a(E)
= o. x
(E-S> 2
sn
Usually, we assume a value of p = 0.5, which is justified as long as the compound nucleus decays by s-wave neutron emission. If the parametrization (1) describes the threshold behaviour correctly, the parameter 00 should be constant for measurements at different endpoint energies of the bremsstrahlung spectrum. It can be seen in Fig. 2 that there are large fluctuations in the data. Especially the point corresponding to the lowest endpoint energy is significantly too low. A possible explanation for this behaviour is that the ground state of ‘03Pb can not be reached by s-wave emission from l- intermediate state because of conservation of spin and parity. The first excited state that can be populated by s-wave decay is at E = 125.6 keV. In order to take this into account, we used an effective neutron separation energy of SE* = S&+125.6 keV in our calculations. This will be discussed in more detail elsewhere [7]. In Fig. 3, the parameter aa is plotted versus all measured endpoint energies using the effective neutron separation energy Sgff = S,+125.6 keV. It can be clearly seen that the values in Fig. 3 show less fluctuations than the values in Fig. 2. The ground state reaction rate at Ts = 2.5 calculated using the parametrization (1) with S$ = S,+125.6 keV is X = (1.56 f 0.25)/s. Acknowledgements We thank the S-DALINAC group around H.-D. Grlf for the reliable beam during the photoactivation and A. Richter and H. Utsunomiya for encouraging discussions. This
I I “B
4m
6ooO
E, (kW
8iXiI
10000
Figure 1. Typical (7,~‘) spectrum measured for photon flux calibration during the activation of a combined lead, gold, and boron target.
K. Vogt et al. /Nuclear
Physics A718 (2003) 57.S577~
600
3w
200
400 22 E b”
s E b” 100
o-
8500
i
200
9ocm
9500
loco0
Energy (keV)
Figure 2. Comparison of the values of the parameter o. measured at different endpoint energies. The dotted line is the weighted average of the different values: 00 = 143 & 31 mb.
0 8500
!moo
9500 Energy (ire’/)
Figure 3. Comparison of the values of the parameter a0 measured at different endpoint energies using a modified reaction threshold of SEff= S,+125.6 keV. The dotted line is the weighted average of the different values: as = 250 f 40 mb.
work was supported by Deutsche Forschungsgemeinschaft (Zi 510/2-l and FOR 272/2-l) and Swiss NSF (grants 2124-055832.98, 2000-061822.00, 2024-067428.01). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
S. E. Woosley and W. M. Howard, Astrophys. J. Suppl. 36, 285 (1978). M. Rayet, N. Prantzos, and M. Arnould, Astron. Astrophys. 298, 517 (1995). D. L. Lambert, Astron. Astrophys. Rev. 3 (1992) 201. A. Richter, Proc. 5th European Particle Accelerator Conference, Barcelona 1996, ed. S. Myers et al., IOP Publishing, Bristol, 1996, p. 110. P. Mohr, J. Enders, T. Hartmann, H. Kaiser, D. Schiesser, S. Schmitt, S. Volz, F. Wissel, and A. Zilges, Nucl. Inst. Meth. Phys. Res. A 423, 480 (1999). A. Zilges and P. Mohr, Prog. Part. Nucl. Phys. 44, 39 (2000). K. Vogt et al., to be published. P. Mohr et al., Phys. Lett. B 488, 127 (2000). K. Vogt et al., Phys. Rev. C 63, 055802 (2001). K. Vogt et al., Nucl. Phys. A707, 241 (2002). P. Mohr, T. Rauscher, K. Sonnabend, K. Vogt, and A. Zilges, this volume.