Underground nuclear astrophysics studies with CASPAR

The drive of low-energy nuclear astrophysics laboratories is to study the reactions of importance to stellar burning processes and elemental production through stellar nucleosynthesis, over the energy range of astrophysical interest. As laboratory measurements approach the stellar burning window, the rapid drop off of cross-sections is a significant barrier and drives the need to lower background interference. The natural background suppression of underground accelerator facilities enables the extension of current experimental data to lower energies. An example of such reactions of interest are those thought to be sources of neutrons for the s-process, the major production mechanism for elements above the iron peak. The reactions C(α,n)O and Ne(α,n)Mg are the proposed initial focus of the new nuclear astrophysics accelerator laboratory (CASPAR) currently under construction at the Sanford Underground Research Facility, Lead, South Dakota


Introduction
A new underground accelerator laboratory for studying low energy nuclear reaction cross sections of relevance for nuclear astrophysics, is currently under construction.The Compact Accelerator System for Performing Astrophysical Research (CASPAR), is at present in the installation phase at the Sanford Underground Research Facility (SURF), in Lead, South Dakota.The main focus of this new system, is on the measurements of the stellar neutron sources that dictate the production of heavy elements through the weak and the main s-process in stars.This is a long-standing, potentially transformational question of relevance for the understanding of the chemical evolution of our Universe in early stars and also later star generations [1].It is also crucial for the definition and interpretation of the solar r-process abundance curve [2] that is defined as the key signature and guiding principle for r-process measurements at present and future radioactive beam facilities [3].

Extended gas target
The inclusion of an extended, recirculating gas target system enables the use of isotopically enriched material at higher purity levels than solid target materials, and avoids the problem of beam heating in solid samples, which can lead to instability in deposited layers of target material.The target designed and developed at CSM allows for full re-circulation of enriched target gas, with a cleaning stage and re-compression.The system is designed to insure a maintained vacuum level of better than 1 x 10 -6 Torr at the target entrance, while creating an extended target in the Torr range of selected target gases.

Detection
The neutrons produced in the different (α,n) reactions in stars have an energy of less than 1 MeV, less than 2.5 MeV for 13 C(α,n).The reaction neutrons will be measured with a (nearly) 4π detector consisting of twenty 3 He counters embedded in two circular rings in a polyethylene moderator matrix around the target [7].These detectors have a high counting efficiency of around 50% for low energy neutrons created at the centre of the detector.Detectors of this kind are the NERO detector at MSU [8] and the detector used at Stuttgart for previous 22 Ne(α,n) reaction measurements [9].Extensive tests of the newly developed system at different underground locations demonstrated a negligible internal neutron background from radium impurities in the aluminium 3 He cylinder walls.Figure 2 shows a Monte Carlo simulation for a neutron thermalization and detection process.Neutron background measurements at several locations and differing environments were undertaken to study the overall contribution of surrounding rock formations.The internal background in the 3 He detection device was negligible and indistinguishable from the noise background level in the detector as demonstrated by measurements in the pure NaCl salt environment at WIPP (Waste Isolation Pilot Plant).

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Co r r e sp o n d in g a u t h o r : d r o b e r t 4 @n d .ed u DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 201

Figure 2 .
Figure 2. Mo n t e Ca r lo sim u la t io n fo r a n e u t r o n t h e r m a liza t io n a n d d e t e ct io n p r o ce ss in t h e p r o p o se d CASPAR n e u t r o n d e t e ct o r .

Figure 3 .
Figure 3. Iso m e t r ic la y -o u t o f SURF a t t h e 4 8 5 0 ft le v e l o f t h e Ho m est a k e Min e in So u t h Da k o t a .Im a ge p r o vid e d b y SURF.