D. H. Lo

Response of X-UV photodiodes to 1. 5--17. 5 keV x rays and MeV alpha particles

The absolute x‐ray response of three X‐UV photodiodes was measured over an energy range of 1.5–17.5 keV so that they could be used to calibrate x‐ray imaging systems for the ASCA satellite mission. An intense electron‐beam x‐ray generator was used to test both the dc and ac x‐ray response at 1.5, 4.5, 8.0, and 17.5 keV, and an 55Fe source was used to examine one of the photodiodes at 5.9 keV. The x‐ray response was determined by comparing the X‐UV diode signal to that of a previously calibrated silicon surface barrier diode (SBD). The X‐UV detector response was similar to the SBD response at low energies (1.5 and 4.5 keV). At 8 keV, the X‐UV detectors exhibited about 70% of the SBD response, and at 17.5 keV, about 50%. This result is surprising, because the X‐UV diodes actually have a greater silicon thickness than the SBD. In contrast to our findings for SBDs in the past, this implies that not the entire physical volume of these detectors comprises the active volume. The X‐UV detector x‐ray response was ...

Charged fusion product diagnostics in Alcator C‐Mod

With a plasma current of up to 3 MA, toroidal field of up to 9 T, the confinement of the charged fusion products (CFPs) in Alcator C‐Mod is expected to be excellent. For example, at maximum current and field, classical losses of the 0.8 MeV 3He, 1 MeV triton, 3 MeV proton, 3.5 and 3.7 MeV alphas are expected to be less than 5%. For the study of the global confinement of CFPs, we plan to measure the burnup of the 1 MeV triton (from the D–D reaction) using a proton recoil detector (NE‐213) for the detection of the 14 MeV neutron resulting from the secondary fusion reaction (D–T). On the other hand, loss measurements of CFPs will be made inside the first wall with two detectors (one at the bottom, one at the midplane) using silicon based detectors. The midplane detector will be used to diagnose D–3He plasmas by looking at the unconfined 14.7 MeV proton. In addition to the derivation of fusion yield, energy distribution of the escaping protons will provide information about the ion velocity distribution of th...

Response of SBDs to MeV protons, tritons, and alphas: Evidence that the charged‐particle sensitive depth is not generally the depletion depth

As part of an on‐going effort to develop diagnostics for energetic charged particles from laboratory and space experiments, we examined the possibility that particle identification could be expedited by varying the applied bias voltage on silicon surface barrier detectors (SBDs). Using MeV protons, tritons, and alphas, we performed spectroscopy experiments whereby we observed changes of the energy spectrum as a function of the bias voltage. These particles were either generated via a Cockcroft–Walton linac as fusion products, or emitted from radioisotopes. The results indicate that, contrary to commonly held belief, the detector sensitive depth is not generally the depletion depth. Indeed for partially depleted SBDs the performance is not greatly degraded even for zero bias.

A fusion‐product source

A Texas Nuclear Cockcroft–Walton neutron generator was refurbished for use as a general fusion‐product source. This well‐calibrated source is now used routinely for characterizing energetic charged‐particle detectors, for the development of nuclear fusion diagnostics, for studying radiation damage, and for calibrating x‐ray detectors for laboratory and space plasmas. This paper is an overview of the facility. We describe the main accelerator operating systems, the primary fusion reactions studied, and several diagnostics used to characterize the fusion‐product source.

14 MeV neutron yields from D-T operation of the MIT Cockcroft-Walton accelerator

As part of our fusion-product diagnostic development program, we have begun a series of experiments with 14 MeV neutrons generated in a Cockcroft-Walton accelerator. Two different detectors have been used to measure the neutron yield: a silicon SBD and a Cu foil. The energy of the emitted neutrons has been determined by using two spectrometers: the SBD and a3He proportional counter. The reaction rate is monitored, with about ±5% accuracy, by detecting the α particles from D + T →n +α. The neutron yields obtained from the Si detector and the Cu activation had associated uncertainties of about ±15% and agreed well with the predicted values from α measurements.

Escaping charged fusion product spectrometer on Alcator C-Mod

A spectrometer for studying escaping charged fusion products has been designed, built, and installed on the Alcator C‐Mod tokamak. The ultimate goal of deploying this diagnostic is to evaluate the effectiveness of 3He minority heating through the broadened spectrum of unconfined 14.7 MeV protons from D‐3He reactions. Other physics issues that will be addressed with this diagnostic include: first‐orbit losses, central ion temperature, 3He burnup, etc. The spectrometer is located 25.8 cm below the midplane and at least 5 cm behind the shadow of the rf limiter. The diagnostic is equipped with a 300 μm thick ion‐implanted‐silicon detector that is bakeable up to 200 °C. Eighteen apertures of various dimensions and viewing angles, with aluminum foils of various thicknesses, allow for a choice of different charged fusion products and a control of the signal level. In‐vessel calibration is provided by two weak radioactive sources of α particles.

PIXE x rays: From Z=4 to Z=92

A high‐intensity, charged‐particle‐induced x‐ray (PIXE) source has been developed for the purpose of characterizing x‐ray detectors and optics, and measuring filter transmissions. With energetic proton beams up to 165 keV, intense line x radiations (0.5 A≤λ≤111 A) have been generated from the K, L, M, and N shells of elements 4≤Z≤92. The PIXE spectrum has orders‐of‐magnitude lower background continuum than a conventional electron beam or radioactive α‐fluorescence source [C. K. Li, R. D. Petrasso, K. W. Wenzel et al. (to be published)].

A proton activation diagnostic to measure D–3He reaction yields

We are developing activation diagnostics for monitoring energetic charged-particle fluxes in space and laboratory plasmas. More immediately, we plan to use activation to measure the time-integrated proton flux from D--{sup 3}He fusion reactions in Alcator C-MOD, providing a measure of the time-averaged D--{sup 3}He fusion rate. We demonstrated the techniques feasibility by inducing significant gamma activity in a titanium sample exposed to D--{sup 3}He protons created in our Cockcroft--Walton generator. The titanium target received a fluence of 5.5{times}10{sup 9} protons at 14.7 MeV (of order what a 3-cm{sup 2} target should receive from one shot in Alcator C-MOD) and became activated by the{sup 48}Ti({ital p},{ital n}){sup 48}V reaction. The activitys spectrum from a high-purity germanium (HPGe) detector showed the characteristic 0.984- and 1.312-MeV lines of {sup 48}V. The measured activity agreed reasonably well with theory. An absence of activity at those energies before D--{sup 3}He activation eliminated background or D--D product-induced activity as the gamma source. We intend to repeat the experiment with a chromium target to evaluate that materials diagnostic potential.

MIT modular x‐ray source systems for the study of plasma diagnostics

Two new x‐ray source systems are now on line at our facility. Each provides an e‐beam to 25 kV. Targets are interchangeable between machines, and four x‐ray detectors may be used simultaneously with a target. The gridded e‐gun of the RACEHORSE system gives a 0.5–1.0‐cm pulsable spot on target. The nongridded e‐gun of the SCORPION system provides a 0.3‐mm or smaller dc microspot on target. RACEHORSE is being used to study and characterize type‐II diamond photoconductors for use in diagnosing plasmas, while SCORPION is being used to develop a slitless spectrograph using photographic film. Source design details and some RACEHORSE results are presented.

γ‐ray and neutron calibration facility (abstract)

High‐temperature inertial and magnetically confined plasmas are copious emitters of γ rays and neutrons. Some γ‐ray and neutron reactions of interest are D+He3→Li5+γ (16.7), D+P→He3+γ (5.5 MeV), D+T→He5+γ (16.6 MeV), D+D→He4+γ (23 MeV), and D+D→He3+n (2.45 MeV). In order to test and absolutely calibrate γ‐ray and neutron diagnostics for such reactions, we are investigating the feasibility of renovating a Cockcroft–Walton accelerator which resides in our laboratory. We will report on the status of this work. This work was supported by U. S. Department of Energy contract No. DE‐AC02‐78ET51013.