Photoactivation studies on p-process nuclei - Why and How

Motivation - Search for heavy element synthesis recipe

It always remained a fascination to understand and explore processes in the universe which govern how it works –to study the unimaginably small nucleus and interpret where the stars get their fuels from. Combining Nuclear Physics and Astrophysics is a powerful tool to study the means and modes of stellar nucleosynthesis and get an idea of how heavy nuclei are being produced.

Most of the light elements up to iron are being formed from successive rounds of thermonuclear fusion burning in the stellar interiors. The nuclei heavier than iron are being synthesized mainly by neutron-capture reactions – the astrophysical r and s processes. But the 35 neutron deficient stable isotopes between Se and Hg are shielded from the rapid neutron capture by stable isobars. The possible production site for these so-called p-nuclei are astrophysical scenarios like supernova explosions and they are thought to be made of photodisintegrations of the type (γ, n), (γ, p) or (γ, α) on heavy seed nuclei. The natural abundances of the p-nuclei are in the order of 0.01 – 1%. We are focussing on the photo-dissociation studies of various medium-mass nuclei and our aim is to get more accurate experimental cross sections which will surely be able to improve the present atrophysical network calculations. For a current and detailed review of these ‘nuclear astrophysics p-nuts’, see [1].

Bremsstrahlung facility at ELBE - Experimental setup and observables

To study the p-process nuclei relevant to nuclear astrophysics by the method of bremsstrahlung induced activation, the superconducting electron accelerator ELBE delivers beams up to 40 MeV energy with average currents up to 1mA. The whole experimental area is designed so as to minimize the production of neutrons and scattering from surrounding materials. The floorplan of the setup is as shown in the figure. The primary electron beam is focused onto a thin aluminum foil made from niobium and then the beam is separated from photons and dumped into the electron beam dump, behind which the photoactivation site is located. For in-beam studies with bremsstrahlung and for beam energy determination, a photon beam is formed by the collimator made from high-purity aluminum placed inside the concrete shielding of the accelerator hall. The photons scattered are observed with high-purity germanium detectors surrounded by escape-suppression shields made of bismuth-germanate (BGO) scintillation detectors.

The sample is activated in the high photon flux inside the electron beam dump together with an activation standard sample. At the same time another activation standard sample is irradiated at the target position inside the bremsstrahlung cave. The absolute photon flux is determined by comparing the activations of the known standard sample at both activation sites. This procedure allows for an efficient reduction of the detection threshold for nuclei with low dissociation cross sections.

Decay measurement site - The Lead Castle

After the irradiation, nuclear samples are taken to a low background installation (The Lead Castle) where the decay is measured using high-purity germanium detectors with high efficiency and energy resolution. An example of the decay spectrum of a virgin sample of NatMo(no probe) overlayed on the decay spectrum of the same sample after irradiation in the beam dump is as shown. The photoactivation yields are derived from the full-energy peak in a spectrum measured with the high-purity germanium-detector setup in the Lead Castle.

References and books of related interest

[1] M. Arnould, S. Goriely, Phys. Rep.384 (2003) 1- 84

[2] A. Wagner, R. Beyer, M. Erhard, et al., J. Phys. G 31 (2005) 1969

[3] M. Erhard, A.R. Junghans, R. Beyer et al., Eur. Phys. J. A 27 (2006) 135-140

[4] Cauldrons in the Cosmos, Nuclear Astrophysics, Claus E. Rolfs and William S.Rodney, The University of Chicago Press (1988)

[5] Gamma and X-ray Spectroscopy with Semiconductor Detectors, K. Debertin and R. G. Helmer, North Holland Publications (1988)