A fast neutron spectrometer for D-D fusion neutron measurements at the Alcator C tokamak

Abstract

A neutron spectrometer using a high pressure 3 He ionization chamber has been designed and used to measure the neutron spectrum from an ohmically heated deuterium plasma. The resolution of the spectrometer at 2.45 MeV is determine to be 46 keV full width at the half maximum (FWHM). Particular attention has been paid to optimizing the detector shielding and collimation to reject thermal and epithermal neutrons scattered from the tokamak structure. As a result, measurements indicate that the ratio of the number of counts in the 2.45 MeV peak to the total number of detected neutron events is 1/67.For the 8 Psec amplifier time constant used, a count rate as high as 44 counts per second has been achieved in the thermonuclear peak.The observed spectra have been compared with calculated spectra using the MCNP Monte Carlo Neutral Particle Transport code and they show good agreement. There is little evidence of neutrons produced from photoneutron reactions or electrodisintegration. It has been possible to confirm that the shape of the thermonuclear peak is consistent with the Gaussian shape predicted and that the ion temperature as determined from the line width is consistent with other Alcator C ion temperature diagnostics, and follows the trends predicted by the theory of Doppler line broadening.

Fast-neutron spectrometry using a 3He ionization chamber and digital pulse shape analysis

“…The algorithm 4 developed for this project demonstrated a greater level of discrimination then shown in prior work, especially in its 5 ability to eliminate lower-energy recoil events, but this also served to reduce the full energy peak acceptance 6 rate. [19,20,31] The absolute efficiency for C-S type spectrometers, without pulse shape discrimination, has been 7 reported to be (1.7 ± 0.5) u 10 -4 at 2.45 MeV. [19] With the implementation of pulse shape discrimination prior work 8 with analog pulse analysis circuitry has been shown capable of achieving up to 60% acceptance for 6.3-MeV 9 neutrons and up to 80% acceptance for 8.1-MeV neutrons.…”

“…In the realm of nuclear physics C-S detectors have been used to measure delayed 14 neutron spectra from fission products, to evaluate nuclear energy levels following charged particle nuclear reactions, 15 and to study neutron spectra from fusion. [14][15][16][17][18][19][20] They have also been used to quantify the neutron spectra from 16 radioisotope neutron sources and to evaluate neutron radiation fields for health physics applications. [7,[21][22][23] The 17 best method to evaluate the performance of a neutron spectrometer is to use a charged-particle accelerator to 18 produce calibrated monoenergetic neutron fields of known energy and known intensity across a wide range of 19 energies; an alternate approach is to use the 252 Cf transmission technique to make relative analyses of the 20 performance of a C-S neutron spectrometer.…”

“…Strong reliance was also placed on 10 prior reports by Sailor and Prussin, Evans and Brandenberger, and Fisher et al, which provide an excellent overview 11 of the layout and operating principles of the FNS-100 detector. [4,10,19] An Ortec Model 456 power supply was 12 used to bias the detector. 13 14…”

“…Post processing was carried out on a computer using algorithms 18 developed using National Instrument's LabView programming language. In essence, the algorithm used for this 19 work was similar to the discrimination techniques implemented previously using analog components; it worked by 20 analyzing the rise time of the waveforms from the 3 He ionization chamber and separating longer rise-time events 21 from shorter rise-time events. However, taking advantage of the full capabilities for complete waveform analysis 22 using dPSA, improvements to this simple approach were developed which included the application of additional 23 filters including methods to remove noise and to discriminate pile-up events in the waveform data.…”

“…This information was then used to generate a simulated response function for 18 the spectrometer. 19 …”