Alpha-induced reactions for the astrophysical p-process: the case of Eu
The cross sections of Eu(Tb and Eu(,n)Tb reactions have been measured with the activation method. Some aspects of the measurement are presented here to illustrate the requirements of experimental techniques needed to obtain nuclear data for the astrophysical p-process nucleosynthesis. Preliminary cross section results are also presented and compared with the predictions of statistical model calculations.
Institute of Nuclear Research (ATOMKI), H-4001 Debrecen, POB.51., Hungary
Kocaeli University, Department of Physics, TR-41380 Umuttepe, Kocaeli, Turkey
University of Basel, Department of Physics, CH-4056 Basel, Switzerland
The chemical elements with atomic number higher than about 30 are very rare in nature. In the Solar System these trans-iron elements represent only roughly 1 atom in every 10 million. The synthesis of about 99 % of the nuclei in this mass region can be explained by neutron capture reactions in the astrophysical s- and r-processes under various astrophysical conditions [bus99, arn07]. There are, however, more than 30 isotopes on the proton rich side of the nuclear chart which can be synthesized by none of these two processes. There are still many open questions regarding the synthesis of these so called p-nuclei. Their production mechanism is the astrophysical p-process which may involve several different sub-processes [arn03]. The most important mechanism is thought to be the -process which proceeds by -induced reactions starting from pre-existing s- or r-process seed nuclei. Consecutive (,n) reactions drive the material towards the neutron-deficient region, though charged-particle emitting () and (,p) reactions also contribute to the process. Figure LABEL:fig:pprocflow schematically shows the path of the -process in the case of the Tin isotopic chain.
Table 1 lists those isotopes with mass number higher than 70 for which experimental proton or alpha capture reactions cross sections are available. It is apparent that in the case of () reactions the available experimental database is very limited especially in the heavier mass region where the p-process models show the highest sensitivity to the () reaction rates. Therefore it is highly needed to extend the experimental database of () cross sections to the heavier mass region.
2 Experimental methods for charged particle capture cross section measurements
The conventional method for capture cross section measurements is the in-beam -spectroscopy. In the mass and energy range relevant for the -process, however, severe problems are encountered in the application of this technique. As it is usual in nuclear astrophysics, low cross sections from millibarn down to the nanobarn range must be measured and the beam-induced background produced on low Z target impurities may hinder the cross section measurement. Moreover, the compound nucleus created in a capture process is typically excited to an energy range where the nuclear energy level density is very high. This results in a complicated decay scheme involving many transitions. All these transitions must be measured with their complete angular distributions which requires huge experimental effort (see e.g. [gal03, has09b]). With the application of a 4 summing crystal some of these problems can be avoided [spy07], but none of the in-beam -spectroscopy methods succeeded so far to reach the heavier (A 110) mass region of the p-nuclei.
The activation method has been proved to be a very fruitful technique for the determination of cross sections relevant for the -process. Most of the isotopes listed in table 1 have been studied with activation. This method is applicable only if the reaction product of the capture leads to a radioactive isotope with convenient half-life and strong enough decay signature (like a high intensity -radiation). Most of the problems encountered in in-beam -spectroscopy can be avoided in an activation experiment. Since the off-line measurement of the induced activity leads to the determination of the total number of capture reactions, there is no need to consider individual -transitions, angular distributions, etc. The activation technique is also less sensitive to target impurities and in certain cases more isotopes of a given element can be studied simultaneously.
In the present work the cross section of the Eu(Tb and Eu(,n)Tb reactions have been measured with the activation method. (The study of (,n) reactions – although not directly relevant to the p-process – provides additional information to the investigation of different optical potentials, see below.) In the next section some details of the measurement are given emphasizing the special requirements of an activation experiment.
3 Cross section measurement of Eu(Tb and Eu(,n)Tb reactions
As can be seen in table 1, the only isotope for which -capture cross section measurement has been carried out near the A = 150 mass region is Sm [som98]. For this isotope it was found, that the statistical model calculations using different -nucleus optical potentials give extremely different cross section results. One can choose the best optical model parametrization for a given reaction only if experimental data are available for comparison. Therefore, it is important to collect more experimental data in this mass region to have a larger experimental database to check the performance of different global optical potentials. Our next step towards this goal is the study of Eu.
Alpha capture on Eu (Q = -1.98 MeV) leads to Tb which decays to Gd by electron capture with a half-life of 5.32 d. The decay is followed by a few relatively strong -radiations which can be used for the cross section measurement. Moreover, the (,n) reaction on Eu (Q = -10.14 MeV) also produces a radioactive residual nucleus, Tb, so the Eu(,n) cross section can also be measured by activation. The decay scheme is, however, more complicated. Besides its ground state decaying to Gd with 21.5 h half-life, it has two long-lived isomeric states with half-lives of 9.4 h and 22.7 h decaying by , electron capture and/or internal transition. Fortunately all these different decays are followed by many strong -radiations, a few of which can be attributed exclusively to the decay of a given state. Thus, partial cross sections to the ground and the two isomeric states can be determined separately and their sum gives the total (,n) cross section. Figure LABEL:fig:reaction shows the relevant part of the chart of nuclides with the studied reactions and the decay of the reaction product.
It should be noted that the precision of the decay parameters (half-life, -intensity) directly influences the precision of the half-life determination. In the case of the m1 isomer in Tb, the half-life has a relatively large uncertainty (9.4 0.4 h) and ambiguous values can be found in the literature. Therefore, we have performed a high precision experiment to determine this half-life value. We have obtained T = 9.994 0.039 h [gyu09]. With this measurement we reduced the uncertainty of this half-life by one order of magnitude and avoided one possible source of systematic uncertainty in the Eu(,n)Tb cross section determination.