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.
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| TH-GEM board manufactured at CERN |
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| 2D position distribution of the fission fragment from a 252Cf fission source |
