Reviews the neutron measurement techniques that are applied to the study of tokamak plasmas. The range of neutron energies of primary interest is limited to narrow bands around 2.5 and 14 MeV, and the variety of measurements that can be made for plasma diagnostic purposes is also restricted. To characterize the plasma as a neutron source, it is necessary only to measure the total neutron emission, the relative neutron emissivity as a function of position throughout the plasma, and the energy spectra of the emitted neutrons. In principle, such measurements might be expected to be relatively easy. That this is not the case is, in part, attributable to practical problems of accessibility to a harsh environment but is mostly a consequence of the time scale on which the measurements have to be made and of the wide range of neutron emission intensities that have to be covered; for tokamak studies, the time-scale is of the order of 1 to 100 ms and the neutron intensity ranges from 1012 to 1019 s-1.

https://iopscience.iop.org/article/10.1088/0741-3335/36/2/002/pdf

Neutron measurement techniques for tokamak plasmas

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

Neutron detection techniques from μeV to GeV

Triton production from one branch of the deuteron-deuteron fusion reaction is routinely measured at 6 ms time intervals in JET plasma discharges by recording the 2.5 MeV neutrons produced in the other branch using a set of calibrated fission chambers. The burnup of the tritons is measured by detecting the 14 MeV t-d neutrons with a 0.2 cm3 Si(Li) diode. The 2.5 MeV neutron flux can be used in a simple time dependent calculation based on classical slowing-down theory to predict the 14 MeV neutron flux. The measured flux and the triton slowing-down time are systematically lower than the values estimated from the key plasma parameters but the differences are within the experimental errors.

https://iopscience.iop.org/article/10.1088/0029-5515/28/12/001/pdf

Time resolved measurements of triton burnup in JET plasmas

Determination of the neutron spectra around an 18 MV medical LINAC with a passive Bonner sphere spectrometer based on gold foils and TLD pairs

The 1.01 MeV triton burnup in FT deuterium discharges has been investigated by the measurement of 14.1 MeV and 2.45 MeV neutron emission. At large values of qL, the burnup is found to increase with plasma current as expected; around qL = 3, a reduction is observed, while for lower qL values the burnup seems to increase again, but is more erratic.

https://iopscience.iop.org/article/10.1088/0029-5515/27/6/017/pdf

Measurements of triton burnup in low q discharges in the FT tokamak

The design of a six‐channel one‐dimensional neutron/x‐ray camera (multicollimator) for the Frascati tokamak upgrade (FTU) machine is described only with regard to the aspects concerning the neutron measurements. The multicollimator, viewing a poloidal cross section of FTU, provides the radial profile of neutron emission from D–D plasmas. The ion temperature profile can be derived in ohmic plasmas if independent data on density are available, while energetic ion tails can be located in the presence of additional rf heating. Information on the position and shape of FTU circular plasmas can also be obtained. The constraints imposed on the collimation and shielding by the compactness of FTU and the expected neutron fluxes are analyzed in detail. The detection system and the performance of the multicollimator are also discussed.

Design of the neutron multicollimator for Frascati tokamak upgrade

Neutron transport calculations are performed using the Monte Carlo particle trajectory sampling method (the MCNP code) to determine the neutron fluences and energy distributions at different locations in the Joint European Torus (JET) tokamak. The primary purpose of these calculations is to calibrate the activation measurements used to determine the absolute fusion neutron yield from the JET plasma. In this paper the calculated neutron activation response coefficients are presented for three different materials used in neutron yield determination for plasmas producing 2.5- and 14-MeV neutrons. The accuracy of the calibration derived from the neutron transport calculations is tested by comparing the present results for the total 2.5-MeV neutron yield with those obtained with calibrated fission chamber: overall agreement at the +-15% level is demonstrated.

Calibration of Neutron Yield Activation Measurements at Joint European Torus

This paper describes the in situ calibration of the BF3 detector system used for measuring the total neutron yield from the FTU tokamak. Two different neutron sources, 252Cf (7.23×107 n/s) and AmBe (2.18×106 n/s), were used. The calibration was performed by placing the two sources at different radial and toroidal positions inside the vacuum vessel. Data on the response of the different detectors as a function of the source position are presented. The effect of the different spectra of the neutron sources is pointed out. Finally, the procedure for the computation of the relationship between the local flux at the detectors and the total neutron yield from an extended plasma is presented.

Calibration of the neutron yield measurement system on FTU tokamak

A benchmark experiment has been performed for the absolute calibration of the FTU tokamak neutron activation system with the purpose of comparing experimental and numerical absolute reaction rates. Indium samples have been activated by a 252Cf neutron source located on the torus axis at three different toroidal angles with respect to the irradiation ends. The same experimental arrangements have been simulated with the Monte Carlo neutron and photon transport code MCNP, in order to reproduce numerically the experimental results. The comparison was intended to assess the accuracy of the numerical simulation and of the modeling of the FTU device. The activation technique will be routinely used as a complementary method to the use of neutron counters for the determination of the absolute neutron yield. An agreement within 22% between experiment and calculation has been found, showing that the model used in MCNP provides an acceptable representation for FTU.

Experimental and numerical calibration of the neutron activation system on the FTU tokamak

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