Investigation of the possibility of gamma-ray diagnostic imaging of target compression at NIF

Abstract

The National Ignition Facility at Lawrence Livermore National Laboratory is the world’s leading facility to study the physics of igniting plasmas. Plasmas of hot deuterium and tritium, undergo d(t,n)α reactions that produce a 14.1 MeV neutron and 3.5 MeV a particle, in the center of mass. As these neutrons pass through the materials surrounding the hot core, they may undergo subsequent (n,x) reactions. For example, 12C(n,n’γ)12C reactions occur in remnant debris from the polymer ablator resulting in a significant fluence of 4.44 MeV gamma-rays. Imaging of these gammas will enable the determination of the volumetric size and symmetry of the ablation; large size and high asymmetry is expected to correlate with poor compression and lower fusion yield. Results from a gamma-ray imaging system are expected to be complimentary to a neutron imaging diagnostic system already in place at the NIF. This paper describes initial efforts to design a gamma-ray imaging system for the NIF using the existing neutron imaging system as a baseline for study. Due to the cross-section and expected range of ablator areal densities, the gamma flux should be approximately 10−3 of the neutron flux. For this reason, care must be taken to maximize the efficiency of the gamma-ray imaging system because it will be gamma starved. As with the neutron imager, use of pinholes and/or coded apertures are anticipated. Along with aperture and detector design, the selection of an appropriate scintillator is discussed. The volume of energy deposition of the interacting 4.44 MeV gamma-rays is a critical parameter limiting the imaging system spatial resolution. The volume of energy deposition is simulated with GEANT4, and plans to measure the volume of energy deposition experimentally are described. Results of tests on a pixellated LYSO scintillator are also presented.

Testing of a gamma ray imaging system at the High Intensity Gamma Source

“…This is well below our 2 mm requirement. [4] LYSO has a fast response time of 41 ns and beats our time-of-flight requirement by an order of magnitude. The LYSO is very bright with 29,200 photons/MeV produced at a maximum wavelength of 420 nm.…”

“…An analysis of the characteristics of many common scintillators results in a short list that includes LSYO, BGO, and YSO. [4] Samples of these three materials were obtained for testing of their spatial resolution, as this is the most important limitation for adequately measuring the 100-200 micron source with enough fidelity that the symmetry can be determined to within 10%. Other requirements are of issue as well.…”

“…[2] [3] As neutron yields surpass ~5x10 15 it becomes practical to image these gamma rays. [4] The images of the gamma rays released from the 12 C(n,n'γ) 12 C in the plastic ablator shell contain important information. Gammas that are produced in the ablated material can help determine the symmetry of the n ablation process.…”

Preliminary investigation fo the non-linear response of image intensifiers used for gamma-ray imaging

“…Another scintillator imaging task that might be vulnerable to gain saturation effects is the use of pixellated scintillators to image gamma rays from inertial confinement fusion targets in order to diagnose compression uniformity. Work on imaging systems of this type is just beginning, but this approach is described in a paper by Lemieux, et al [7]; such gamma-ray imaging systems use pixellated scintillator arrays read out by an image intensifier and a CCD camera. Tests of a prototype system are described in a report in this conference [8].…”

“…More recently, image intensifiers have been adapted for use in x-ray and gamma-ray imaging systems for biomedical applications [1] and other uses [2]. Figure 1 shows an example of such a device, a BazookaSPECT camera developed at the Center for Gamma-Ray Imaging at the University of Arizona [3].…”