Design of the spectrometer
In the background of the picture, five of 12 polyethylene spheres (CH2) are shown. The support which is suited to the individual
spheres ensures that the position of the central thermal detector is in every case at the same level, independently of the diameter of the sphere.
Three spherical proportional counters (3.2 cm in diameter) filled with 3He gas are shown in the foreground, partly hidden by special PE inserts.
The forground of the picture shows parts of the modified spheres. Both the polyethylene and the lead (left) or copper (right) shells are fabricated as half-shells. So only small masses have to be displaced (and lifted), and the combination of spherical shells of different thickness allows different sensitivities to be achieved with a small number of single components.
Response of the spectrometer
The two diagrams show the repsonse of the spectrometer Rd as a function of the neutron energy En. The index "d" stands for the different diameters of the spheres. The curves are computed values which completely determine the shape of the curve. The absolute amount of the response was subsequently checked in several calibration measurements. All experiments furnished a uniform calibration factor.
The upper diagram shows the response functions for the pure polyethylene spheres with diameters of 7.62 cm (3") to 45.72 cm (18") in the energy interval of 1 meV to 20 MeV. The plotted response functions of the "bare" counting tube as well as of the cadmium-shielded counter have been multiplied by the factor 0.5.
The lower diagram shows the response functions for the spheres modified with lead and copper (magenta, red, cyanic and green) for the energy range up to 10 GeV. For comparison, some response functions from the upper diagram have also been plotted (in blue).
The response functions computed for the specific parameters of the spheres such as the exact diameter and the polyethylene density, can be matched to other system using a suitable interpolation procedure. After appropriate calibration of the detector system, the ample calculations carried out at the PTB can thus be directly used for other multisphere spectrometers.
Contact
Dr. Burkhard Wiegel
Phone: +49 (0) 531-592-6531
e-mail: Burkhard.Wiegel@ptb.de
