HIGS AS A FACILITY FOR NUCLEAR ASTROPHYSICS

H.R. Weller

Duke University and Triangle Universities Nuclear Laboratory (TUNL)

 There are at present a number of facilities which produce polarized g rays for nuclear physics studies.  All of those facilities which employ Compton backscattering techniques operate by scattering conventional laser light from electrons circulating in a storage ring. The High Intensity Gamma-ray Source (HIGS) is a joint project between TUNL and the Duke Free Electron Laser Laboratory (DFELL). This facility utilizes intra-cavity back-scattering of the UV-FEL light in order to produce a g-flux enhancement of approximately 103 over the existing sources.  The Duke storage ring was designed to operate at energies from 200 MeV to 1.1 GeV. At present, gamma-ray beams with energies ranging from 2-to-50 MeV are available with intensities of 105-107 g/s and 100% linear polarization.  An upgrade is presently underway which will allow for the production of g rays up to an energy of about 225 MeV having intensities in excess of 108 /sec/MeV. The primary component of the upgrade is a 1.2 GeV booster-injector which will permit the operation of the ring at full-flux and full energy. The current electron source for the ring is a 240-280 MeV S-band radio-frequency linear accelerator. This system is adequate to support the initial stage of operation during the construction of the 1.2 GeV booster injector. The construction of the booster will not interfere with normal operation of the HIGS facility. The booster injector utilizes standard technology and will provide for efficient injection at any chosen operating energy of the storage ring from 200 MeV to 1.2 GeV.  In addition, an upgrade of the present OK-4 FEL to a helical undulator system (OK-5) is underway.  This new system has many advantages over the present one, including making switchable linear and circularly polarized beams available, an increase in power and a decrease in mirror-damaging radiation.  The present schedule calls for having OK-5 operating by the end of 2002.  The full system, including the booster injector, is expected to be ready for use by mid-2005.
A schematic diagram of the DFELL arrangement is shown in Figure 1.

 

Figure 1.  Schematic diagram of the HIGS - the DFELL-TUNL  g-ray facility.

 There are a number of distinct advantages of this system as compared to presently available ones.  The first is the high-intensity beams which will allow us to measure nuclear processes having low cross sections with high precision in realistic times.  The second advantage is the fact that tagging is not needed. The high quality of the electron beam permits energy definition by the use of collimation alone. This means that, unlike many tagged sources, there will be NO untagged high-energy g rays in the vicinity of the target and detectors.  In addition, the energy of these beams can be continuously tuned from 2 to 225 MeV.  The energy resolution of these beams will be exceptional.  For example, a 1 mm radius collimator located 30 meters from the collision point should allow us to produce 100 MeV g rays having a FWHM energy spread of less than 400 keV.  These g-ray beams will be 100% polarized, and the beam environment will be exceptionally clean; backgrounds will be negligibly small.
 
The Research Program at HIGS

TUNL researchers, in collaboration with outside theoretical and experimental colleagues, have proposed a broad based research program in nuclear physics which is designed to exploit the unique flux, energy resolution and polarization of the HIGS beams.  Included in this program are proposals for experiments relevant to nuclear astrophysics.  Key experiments include:

1. d(g,n)p  Preliminary results have been obtained in the case of d(g,n)p in the threshold region. This is an important region , relevant to Big-Bang nucleosynthesis.  The HIGS 100% linearly polarized beam allows for a separation of the M1 and the E1 parts of the cross section.
2.  A study of 12C (a,g)16O by means of the inverse reaction 16O(g,a)12C.
3. 9Be(g,n)aa to study the inverse reaction.
4. 26Mg (g,n) 25Mg / 26Mg (g,a)22Ne as a means to study the n/a branching ratio in the 22Ne + a reaction.
5. 28Si + g  to study the process of Silicon burning.
6. 180Ta (g,g’)—Study the details of the de-excitation of the isomeric state of  180Ta.
7. Nuclear Resonance Fluorescence: the HIGS beams have the intensity and energy resolution to allow for a new level of NRF spectroscopy.  Preliminary results on 138Ba are very exciting and indicate the power of this facility.

A description of the presently available facility and the anticipated facility following the present upgrade will be given, along with a summary of that part of the research program relevant to nuclear astrophysics.