RA3. Experimental Equipment -
A New Experimental Setup Optimized for (d,p-gamma) Reactions
(M. Johnson, Spokesperson)
A new setup for proton-gamma coincidence measurements in (d,p) reactions has been constructed and tested at HRIBF. One motivation for proton-gamma measurements is to extract neutron capture information for astrophysically important nuclei such as neutron-rich species which may lie on the so-called r-process path. The reactions are performed at the HRIBF by accelerating a nuclear species of interest and impinging the beam onto a deuterated polyethylene target (CD2). The energy loss of these heavy ions in the target and the unfavorable kinematics results in a practical limit to the achievable energy resolution by detection of charged particles alone. By adding high-resolution gamma-ray detectors, the energy resolution can be improved by almost an order of magnitude.
The (d,p-gamma) setup is pictured in Fig. RA3-1, mounted to the CARDS
array. The overall length of the (d,p-gamma) setup is about 3 feet.
Additional space is needed on both ends to access the Si detectors
within the chamber (discussed below). In all, 5 feet is required for
the (d,p-gamma) assembly and accessibilty. Once the Si detectors have
been assembled, the access space can be filled using KF-40 pipe for the
upstream section and 4" pipe on the downstream section.
Figure RA3-1: Photograph of the (d,p-gamma) setup looking upstream. Seen here is the CARDS array, four Ge clovers, and the vacuum chamber (center of clovers), which house the Si detectors and targets. Preamp modules for the Si telescopes described in the text are mounted (not shown here) on the aft section of the chamber (nearest the photographer). See text for more details.
The (d,p-gamma) setup was designed to use four Ge clovers. In the geometry shown in Fig. RA3-1, the absolute gamma-ray efficiency was measured to be 6.9% at 1.33 MeV. The fronts face of the clovers are approximately 6 cm from the target. The central axis of the clover is perpendicular to the beam axis and is in-line with the target plane. With the use of high-resolution Ge detectors and the close geometry, the largest contribution to the energy resolution comes from Doppler broadening. The Doppler broadening was measured to be 32 keV (FWHM) in each crystal of the clover for recoils of 4-MeV/u A~90 beams onto a CD2 target.
Protons are detected in a number of Si detectors, mounted within the
vacuum chamber. Surrounding the target location is a set of four Si
telescopes, configured in a box-like geometry as shown in Fig. RA3-2. The
thin transmission detectors closest to the target are 5x5 cm
position-sensitive strip detectors. Thick Si 5x5 cm pad detectors back
up the thin detectors. The telescope array covers angles from 90
degrees to 135 degrees. The angular resolution is about 2 degrees for
this geometry. The preamps are mounted on an additional chamber piece
and are attached downstream of the central part of the chamber (visible
in Fig. RA3-1).
Figure RA3-2: Photograph of the target location looking downstream. Seen here is a CD2 target extended into the beam axis. Three other target manipulators are retracted. The bottom manipulator holds a phosphor for beam tuning. Surrounding the target are four Si telescopes. The Ge clovers are on the outside of the chamber walls adjacent to the Si telescopes. Short cables (seen) connect the Si telescopes to the feedthrus to the preamp modules downstream.
For more backward angles, SIDAR is used (Fig.RA3-3a), covering the angular range covered by SIDAR is from 142 degrees to 165 degrees. It is mounted on a flange and slid into the vacuum chamber on two precision slide-rails (Fig. RA3-3b). The preamps for SIDAR are mounted on the flange on which it sits. The solid angle coverage of the entire Si array is estimated at 28% of 4-pi. Also, the SIDAR flange has a viewport which is in-line with the target location, and can be used to tune the beam using a phosphor (see Fig. RA3-2).
Figure RA3-3a: Photograph of SIDAR mounted on the flange. The cables connect the detector wedges to feedthrus to preamp modules which are attached (not shown) on the other side of the flange. In this photograph, the upstream direction is into the plane of the page.
Figure RA3-3b: Photograph of SIDAR being inserted into the vacuum chamber. The feedthrus for the preamp modules can be seen on the left side (exterior) of the flange. Also seen is the viewport (on top of the beamline) for beam tuning. Atop the vacuum chamber is a gate valve which can be connected to a cryo-pump. (No cryo-pump was attached here for the feasibility test discussed in the text.)
Recently, a test run (RIB-131) was performed in which different
stable beams were accelerated by the tandem accelerator to test the
performance of the (d,p-gamma) setup described above. Fig RA3-4 shows a
preliminary plot of excitation energy (derived from proton
measurements) versus gamma-ray energy for a run with a 90Zr beam onto a 1000 microgram/cm2 CD2
target. One of the benefits of proton-gamma measurements is
demonstrated by this plot: for each gamma-ray transition it can be seen
at which excitation energy the nucleus was formed. Also, a number of
directly populated states are visible, clearly separated in gamma-ray
energy, which are not resolved in proton energy alone.
Figure RA3-4: Preliminary results taken from RIB-131. The x-axis is gamma-ray energy (spectrum dispersion is 8 keV/channel), the y-axis is excitation energy extracted from proton energy deposited in the Si array and lab angle (spectrum dispersion is 20 keV/channel). The groupings at x~150, 255, and 325 are proton-gamma coincidences for populating the 1.21-, 2.04- and 2.56-MeV levels. The elongation along the y-axis is due to the comparatively poor resolution of the protons.
Currently, the test runs are being analyzed to determine the full capabilities of the (d,p-gamma) setup. Plans are being made to make new proposals for (d,p-gamma) measurements.