Blister resistant targets for nuclear reaction experiments with alpha-particle beams.
Solid targets for nuclear reaction measurements that use alpha-particle beams commonly experience a form of degradation known as blistering. The effect can prevent the use of solid targets for high intensity alpha-particle experiments, often necessitating complex gas target systems. To combat this problem, we designed three different blister resistant target backings for use in direct reaction measurements with high intensity alpha-particle beams. The blister resistant target designs utilize gas diffusive properties of fused silica, sintered metal, and porous evaporated metal. Each target was implanted with 22Ne ions and bombarded with alpha-particle beam to test blister resistance. Targets were characterized and monitored at the Triangle Universities Nuclear Laboratory using the 22Ne(p,gamma)23Na nuclear reaction to determine the degradation of implanted material, and compare them to typical implanted noble gas targets. We found that all targets studied exhibit resistance to blistering, with the porous evaporated metal targets displaying the least amount of target material degradation.
The figure shows Atomic Force Microscopy (AFM) images of our backings: (a) Blank titanium backing; (b) Evaporated titanium layer; note the interlocking crystalline structure, indicating a porous material; (c) Evaporated titanium layer after implantation with 22Ne ions; the surface layer has been sputtered away; (d) Evaporated layer that has been implanted with 22Ne and then exposed to a 30 microamp alpha-particle beam of 900 keV energy for a total accumulated charge of 0.5 C. For more information, see Hunt et al., NIM A 921, 1 (2019). Sean Hunt is a former UNC graduate student.
LENA: the highest proton beam intensity low-energy accelerator in the world.
LENA is noted internationally for its precision nuclear reaction rate measurements of rare astrophysical processes, which are critical to stellar nucleosynthesis. Creating stellar conditions at LENA requires very intense beams and highly selective detection techniques. The acceleration system at LENA has already produced proton currents on target up to 2.5 mA, the most intense beams worldwide in any laboratory where such studies are underway. We are now in the process of commissioning an improved acceleration column capable of accelerating 30 mA of protons to energies up to 240 keV. The new column is depicted on the left between the yellow solenoidal magnet around the ion source and the blue solenoidal magnetic lens used to focus the emerging proton beam toward the target. Both the ion source and new colu
mn were designed and constructed locally, using precision parts fabricated in our departmental instrument shop at UNC-CH. Three axial insulating rods are used to support the column and compress O-ring vacuum seals between its electrodes and ceramic insulators. Internal transverse magnetic fields along the entire column length suppress backstreaming electrons which had previously produced dangerous levels of bremsstrahlung X-rays. Two channels of chilled flowing deionized water along the column now provide simultaneously both a high resistance path for column current to establish the uniform acceleration field gradient, and active cooling for the intervening column electrodes. More information can be found in Cesaratto et al., NIM A623, 888 (2010) and Cooper et al., Rev. Sci. Instr. 89, 083301 (2018). Both first authors are former UNC graduate students.
Characterization of a 10B-doped liquid scintillator as a capture-gated neutron spectrometer.
We used a 250 MHz digitizer to characterize the pulse shape discrimination (PSD) of a BC-523A 10B-doped liquid scintillator with capture-gating capabilities. Our results are compared to recent work claiming pulse shape discrimination between fast and thermal neutron signals. The capture event is identified, and we explain the origin of signals that are often misinterpreted. We use the time-of-flight method to measure the detector energy resolution for fast incident monoenergetic neutrons and the intrinsic neutron detection efficiency. Monte Carlo simulations are performed and we find agreement between measured and simulated results. These steps are important for understanding 10B-doped capture-gated spectroscopy in mixed radiation environments, as efficiencies using capture-gating are rarely reported in the literature.
The figure above shows PSD histograms of the tail integrated charge over total integrated charge, obtained using an AmBe neutron source. (a) For all pulses triggering the digitizer; regions occupied by neutrons and gamma-rays are labeled. The third branch has been falsely identified as the capture peak in the previous literature. (b) For events obtained when the moderation pulse triggered the digitizer, but with PSD only applied to the part of the signal arriving 150 ns after the triggering pulse. The oval shaped region is occupied by capture pulses with no energy deposition of the 478-keV photon produced in the 10B(n,α)7Li reaction. The branch off the oval shaped region is caused by capture pulses with partial energy deposition by the 478-keV photon. For more information, see Hunt, Iliadis, and Longland, Nucl. Instr. Meth. A 811, 108 (2016). Sean Hunt is a former UNC graduate student.
The LENA state-of-the-art gamma-ray spectrometer.
The gamma-ray coincidence spectrometer, shown on the left, is the main detection system used at LENA. It consists of a 140% HPGe detector (yellow), placed in close geometry to the target. Both the HPGe detector and the target are surrounded by a NaI(Tl) scintillator annulus, which is made up of 16 optically isolated segments. The idea behind this device is that in most fusion reactions of astrophysical interest, the nucleus is created in an excited state at an energy of several MeV or more. 
In the majority of cases, the nucleus de-excites via emission of more than one photon (i.e., giving rise to a gamma-ray cascade). Detecting the photons belonging to a given cascade in time-coincidence, and imposing, in addition, energy requirements (for example, by setting coincidence gates in a 2-dimensional histogram of NaI(Tl) energy versus HPGe energy) greatly reduces the environmental background and improves the detection sensitivity significantly. This improvement in sensitivity is crucial for measuring weak cross sections or yields of astrophysically important reactions. This technique was first described in Rowland et al., NIM A480, 610 (2002) and the spectrometer is characterized in Longland et al., NIM A566, 452 (2006) and Howard et al., NIM A729, 254 (2013). The first authors are former UNC graduate students or postdocs.