A note on the interpretation of activation measurements on the Princeton large torus

Nuclear Instruments and Methods 214 (1983) 551-552 North-Holland Publishing Company

551

Letter to the Editor A NOTE ON THE INTERPRETATION OF ACTIVATION PRINCETON LARGE TORUS

MEASUREMENTS ON THE

J.D. S T R A C H A N Plasma Physics Laboratory, Princeton University, Princeton, NJ 08544, USA

G. Z A N K L Max-Planck Institut fiir Plasmaphysik, 8046 Garching bei Munchen, Germany Received 11 April 1983

Foil orientation is found to be unimportant in the interpretation of nSIn activation measurements around the PLT fusion experiment.

Indium activation foils have been used [1] on the Princeton Large Torus (PLT) to calibrate absolutely the neutron detectors. As part of that work, the poloidal symmetry of the 2.5 MeV neutron flux at the vacuum vessel was measured and the flux was about 10% larger towards the large major radius side (outside) of the plasma (fig. 8, ref. 1). The poloidal asymmetry was interpreted as a plasma outshift consistent in magnitude with the expected Shafranov shift of the plasma center. Recently, Pedretti [2] re-examined the PLT data from the point-of-view that the poloidal asymmetry could be caused by an anisotropic neutron emission. Pedretti claimed that eq. (3) of ref. 1, describing the isotropic

emission, was incorrect since it neglected the reduction in the neutron fluence to the indium disk by the factor cos "t where y is the angle between an element of the plasma and the normal to the disk face. The result of including the cos y factor is that the calculated plasma outshift is reduced to about one half of the value quoted in ref. 1. It is the purpose of this note to point out that the calculation of ref. 1 is correct and that the cos y factor should not be included in the calculation of the foil activation. Although the number of neutrons encountering a foil is reduced by cos ~, for neutrons incident from off-normal directions, the activation induced by each

In Foil

Normal ~ncident Neutrons

Path Through Foil

Path Through Foil

J Off -Normal Incident Neutrons

Fig. 1. Normal incident neutrons pass through a distance, d, of the foil while off-normal neutrons pass through the longer distance d/cos ~,. 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

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Fig. 2. Poloidal cross-section of PLT indicating the location of the four indium foils•

552

J.D. Strachan, G. Zankl / Interpretation of activation measurements

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Fig. 3. Relative activation o f t h e four indium foils as a function of the poloidal angle, "y, between the normal of the foil and the horizontal midplane of the plasma. The dashed line is the calculation o f r e f : 1 and the shaded region is the calculation of ref. 2.

neutron is increased by (cos 7 ) - 1 since these neutrons travel further through the foil (fig. 1). So long as self-absorption of neutrons by the foil is not important, the shape of the indium does not matter. In our case, we choose indium disks for ease in measuring the subsequent 336 keV gammas from the nSmln decay.

To test this idea, four indium foils were simultaneously exposed to a series of PLT plasmas heated by 1.5 M W of deuterium neutral beams. About 2 × 1013 n e u t r o n s / s were emitted from the plasma due to b e a m - t a r g e t d(d, n)3He fusion reactions [3]. The foils were all located on the horizontal midplane of PLT with the center of mass of each foil being about 5 cm from the outside edge of the PLT vacuum vessel. The poloidal angle between the face of the foil and the center of the plasma was different for each foil so that the angle, 7, of ref. 2 corresponded to 0 °, 30 °, 60 °, or 90 ° (fig. 2). The measured llSmln activation in each foil was essentially identical (fig. 3) in agreement with eq. (3) of ref. 1 but in sharp contrast to the expectation of ref. 2. The expectations of ref. 2 are shown as a shaded region since the minor radial profile of the neutron emission becomes important when 7 = 90 °. The limits of the shaded region are for an infinitesimally narrow line source and a uniform source inside one half the plasma minor radius. We conclude that the analysis of ref. 1 for isotropic neutron emission is essentially correct. The authors thank G. Estepp, J. Hosea and the PLT experimental group, and G. Schilling and the PLT neutral beam group for help in performing the experiment. This work is supported by D o E Contract ~ DE-AC0276-CHO-3073.

References

[1] G. Zankl et al., Nucl. Instr. and Meth. 185 (1981) 321. [2] E. Pedretti, Nucl. Instr. and Meth. 203 (1982) 409. [3] J.D. Strachan et al., Nucl. Fus. 21 (1981) 67.