Detector development I.

For the fission-fragment angular distribution measurements, a multi-target detector array will be developed, consisting of position sensitive gas detector modules based on the state-of-the-art THGEM technology. THGEM is a robust, simple to manufacture, high-gain, gaseous electron multiplier. Its operation is based on gas multiplication within small, sub-millimeter to millimeter diameter holes, in standard double-face Cu-clad printed circuit boards (PCB). The hole structure of the THGEM together with a segmented readout electrode provides a true pixelated radiation localization. The electron multiplication of the THGEM is based on the large potential difference between the two sides of the board resulting in a strong dipole field within the holes. Electrons, deposited by ionizing radiation in a conversion region above the THGEM, are focused into the holes by the dispersed electric field. Then, the electrons are multiplied within the holes under the high electric field (25–50 kV/cm). A small fraction of the resulting avalanche electrons are collected on a bottom electrode, while the significant part is transferred to a collecting anode or to a second multiplier element. Each hole acts as an independent multiplier.

At the low-pressure operation mode the signals are very fast, having a rise time of t ~ 3-4 ns. With the standard design (1 mm holes pitch), a position resolution of 2 mm can be achieved by using segmented anode planes with 1-2 mm wide anode pads. In our design the detector covers almost a full solid angle and has an angular resolution of about 5 degrees. The background sensitivity and the radiation damage is negligible, however, the extremely low counting rates in photofission experiments at deep sub-barrier energies require sufficient α particle discrimination. This goal can be achieved by the coincident detection of both fission fragments.

It has to be emphasized, that the foreseen unprecedented sub-millimetre γ beam-spot size allows to develop considerably more compact photofission detectors than those of before when only bremsstrahlung γ sources were available with a beam spot diameter of ~4-5 cm. Furthermore, due to the small diameter of the targets, we can even use highly-radioactive target materials (e.g. ²³⁹Pu) without encountering radiation safety problems, which was not possible before. The well-focused γ beam also defines a distinct fission position, so a remarkably improved angular resolution can be achieved compared to previous bremsstrahlung photofission experiments.

The photo of such a TH-GEM board (manufactured at CERN) is presented at the bottom of the page, as well as the results of the first tests by using a 252Cf fission source.

TH-GEM board manufactured at CERN

2D position distribution of the fission fragment from a 252Cf fission source