John D. Valentine

Benchmarking the Compton coincidence technique for measuring electron response non-proportionality in inorganic scintillators

To study the light yield non-proportionality of inorganic scintillation materials, a Compton coincidence experiment has been designed and implemented. The coincidence technique is used to measure the nearly mono-energetic electron response by recording events only when energetic electrons are produced by gamma rays that are Compton scattered through a specific angle. This technique provides the ability to accurately determine the light yield non-proportionality of scintillation materials as a function electron energy while minimizing the potential effects of surface interaction and X-ray escape. To benchmark the Compton coincidence technique (CCT), the electron response for NaI(Tl) has been measured for electron energies from 2 keV to 450 keV and compared to previously-published analytical and measured light yield response data for electrons. The CCT data represent the most accurate electron response measurement to date on NaI(Tl) for energies below 20 keV. With the CCT benchmarked, the electron response non-proportionality of other scintillators can be analyzed.

Design of a Facility for Measuring Scintillator Non-Proportionality

While the original Compton coincidence technique provided accurate measurements of electron response in scintillators, the data rate was low and measurements took weeks. We present the conceptual design for a high throughput version that is predicted to collect data at 65 cps, reducing measurement times from weeks to hours. In this design, a collimated 1 mCi 137Cs source will illuminate the scintillator sample from a distance of 18 cm and 5 high-purity germanium (HPGe) detectors placed 10 cm from the scintillator will measure the energy of the scattered gamma ray. The source can be placed in either of two positions spaced 15deg apart, allowing relatively uniform scattering angle coverage from 0deg to 146deg, corresponding to electron energies in the scintillator from 0 to 466 keV. The scintillator will be coupled to a hybrid photodetector (HPD), which has extremely linear response, and the HPDs ability to resolve single photoelectrons provides a built-in calibration mechanism. The output of each HPGe detector and the HPD will be digitized with a free-running 12-bit, 200 MHz ADC, providing accurate measurement of the signal amplitudes and the ability to measure the electron response for different temporal components of the scintillator signals. The facility will be located at Lawrence Livermore National Laboratory (LLNL) and is intended to be made available to the community at large. The goals are to facilitate scintillator development and to understand the nature of the light-yield non-proportionality and its effect on the energy resolution.

Performance of a Facility for Measuring Scintillator Non-Proportionality

We have constructed a second-generation Compton coincidence instrument, known as the Scintillator Light Yield Non-proportionality Characterization Instrument (SLYNCI), to characterize the electron response of scintillating materials. While the SLYNCI design includes more and higher efficiency HPGe detectors than the original apparatus (five 25%-30% detectors versus one 10% detector), the most novel feature is that no collimator is placed in front of the HPGe detectors. Because of these improvements, the SLYNCI data collection rate is over 30 times higher than the original instrument. In this paper, we present a validation study of this instrument, reporting on the hardware implementation, calibration, and performance. We discuss the analysis method and present measurements of the electron response of two different NaI:Tl samples. We also discuss the systematic errors of the measurement, especially those that are unique to SLYNCI. We find that the apparatus is very stable, but that careful attention must be paid to the energy calibration of the HPGe detectors.

Progress in Studying Scintillator Proportionality: Phenomenological Model

We present a model to describe the origin of non-proportional dependence of scintillator light yield on the energy of an ionizing particle. The non-proportionality is discussed in terms of energy relaxation channels and their linear and non-linear dependences on the deposited energy. In this approach, the scintillation response is described as a function of the deposited energy deposition and the kinetic rates of each relaxation channel. This mathematical framework allows both a qualitative interpretation and a quantitative fitting representation of scintillation non-proportionality response as function of kinetic rates. This method was successfully applied to thallium doped sodium iodide measured with SLYNCI, a new facility using the Compton coincidence technique. Finally, attention is given to the physical meaning of the dominant relaxation channels, and to the potential causes responsible for the scintillation non-proportionality. We find that thallium doped sodium iodide behaves as if non-proportionality is due to competition between radiative recombinations and non-radiative Auger processes.

Measurements of NaI(Tl) Electron Response: Comparison of Different Samples

This paper measures the sample to sample variation in the light yield proportionality of NaI(Tl), and so explores whether this is an invariant characteristic of the material or whether it depends on the chemical and physical properties of the tested samples. We report on the electron response of nine crystals of NaI(Tl), differing in shape, volume, age, manufacturer and quality. The proportionality has been measured at the SLYNCI facility in the energy range between 3.5 to 460 keV. We observe that while samples produced by the same manufacturer at approximately the same time have virtually identical electron response curves, there are significant sample to sample variations among crystals produced by different manufacturers or at different times. In an effort to correlate changes in the electron response with details of the scintillation mechanism, we characterized other scintillation properties, including the gamma response and the x-ray excited emission spectra and decay times, for the nine crystals. While sample to sample differences in these crystals were observed, we have been unable to identify the underlying fundamental mechanisms that are responsible for these differences.

Efficiency calculation and coincidence summing correction for germanium detectors by Monte Carlo simulation

A method is presented for efficiency calculation and coincidence-summing correction of high-purity germanium (HPGe) detector spectra by using Monte Carlo N-particle transport (MCNP) code. This technique will be used in the efficiency calibration of HPGe detectors to reduce the number of standard sources to be prepared. Modeling of the detector geometry is described in detail, and differences between the simulated and measured spectra are discussed. Standard point sources traceable to the National Institute for Science and Technology were used to measure the full-energy peak and total efficiencies. The simulated full-energy peak efficiency for noncoincidence /sup 137/Cs gamma rays agreed with the measured value to within 2%, but the simulated total efficiency is about 8% lower than the measured value for 662 keV. A /sup 60/Co point source was placed in five positions above along the center line of the detector from 0.6 to 14.2 cm. For the 1173- and 1332-keV gamma rays from /sup 60/Co, their spectra were simulated using MCNP separately. Subsequently, these spectra were combined according to their coincidence relationship to form the simulated /sup 60/Co spectrum. The calculated coincidence summing factors for 1173 and 1332 keV are about 3% lower than the measured values at the closest geometry for a point source due to the underestimation of the total efficiency.

First-generation hybrid compact Compton imager

At Lawrence Livermore National Laboratory, we are pursuing the development of a gamma-ray imaging system using the Compton effect. We have built our first generation hybrid Compton imaging system, and we have conducted initial calibration and image measurements using this system. In this paper, we present the details of the hybrid Compton imaging system and initial calibration and image measurements

Imaging performance of the Si/Ge hybrid Compton imager

The point spread function (PSF) of a fully-instrumented silicon/germanium Compton telescope has been measured as a function of energy and angle. Overall, the resolution was 3deg to 4deg FWHM over most of the energy range and field of view. The various contributions to the resolution have been quantified. These contributions include the energy and position uncertainty of the detector; source energy; Doppler broadening; and the 1/r broadening characteristic of Compton back-projection. Furthermore, a distortion of the PSF is observed for sources imaged off-axis from the detector. These contributions are discussed and compared to theory and simulations

A nonlinear regression technique for the separation of kinetic compartments in dynamic SPECT imaging

The accuracy of SPECT images are compromised when the tracer distribution changes during image acquisition. Image analysis can be further complicated if there is an overlap between the kinetic compartments. For example, Tc-99m Teboroxime is an imaging agent with rapid washout from the myocardium and high liver uptake. The reconstructed myocardium in Tc-99m Teboroxime SPECT may be compromised with artifacts due to changing activity and contaminated by reconstruction artifacts due to the liver. This study proposes a nonlinear regression technique to separate the kinetic compartments, based on the knowledge of time-activity curves (TAC) coupled with dynamic SPECT scans. Dynamic scans (acquired with a rapid fanning acquisition) are reconstructed independently to generate dynamic short-axis images (representing the tracer distribution at different times), which are then transformed to allow each pixel to be evaluated over time independently. The objective function to be minimized is the deviation of the separated compartmental pixels scaled by the TAC plus a distance penalty. This minimization problem is solved subject to reference and nonnegative constraints. The reference constraint is produced from the reconstruction of the summed projections from the dynamic scans (with minimal artifacts due to changing activity). Patient studies of Tc-99m Teboroxime SPECT were used to evaluate this technique. Qualitatively, the kinetic compartments (the liver and heart) were separated producing images with reduced liver contamination in the myocardium. The quantitative accuracy of the separation depends on the accuracy of the tracer kinetics and can also be degraded by physical factors such as photon attenuation and Compton scatter.

Discriminator amplitude walk correction in gamma-ray coincidence experiments using list-mode time-stamping data acquisition

Abstract When pulse amplitude and time stamp are recorded in list-mode time-stamping data acquisition, it is possible to correct for the system amplitude walk, typically observed as the time pickoff dependence on pulse amplitude. In this study, a method of correcting for amplitude walk during post-acquisition analysis of such list mode data is developed and demonstrated. The method is demonstrated using a simple two-channel system and a photon source capable of producing coincidence events (22Na). Two leading-edge discriminators, vulnerable to the amplitude walk, were used to produce the time pickoffs. The resulting corrected data show an amplitude walk less than the detector timing resolution. The method developed can be used in list-mode data acquisition systems such as medical imaging scanners or Compton scatter cameras.