Speaker
Ana Conceição
(Instrumentation Center, Physics Department, University of Coimbra)
Description
The Microstrip Gas Chamber was the first micropattern detector used for thermal neutron detection. Several studies regarding the Gas Electron Multiplier (GEM) to neutron detection have been performed. High charge gains could not be achieved at high pressures, but the readout of the scintillation produced in the electron avalanches, using a CCD camera, allowed the development of GEM-based neutron detectors.
This work investigates the viability of a hybrid structure based on Gas Electron Multiplier with a Micromegas Gap Amplifying Structure, GEM-MIGAS, as a neutron gaseous detector. The GEM-MIGAS was recently introduced by J. A. Mir as a GEM coupled to a Micromegas induction gap, i. e. a GEM having a short induction gap of a few tens of microns, typically 50 µm.The operation principle is based on the electron multiplication within both the GEM holes and in induction gap, combining the amplification properties of the GEM and Micromegas electron multipliers in a single device. This results in a more efficient extraction of the charge from the GEM and improved gas gain uniformity over large areas, as in Micromegas. In addition, lower operational voltages in the GEM can be set, with minimization of sparks and prolonged life of the GEM.
The experimental work is divided into three phases starting with the optimization of the GEM-MIGAS gaps in the 20-250 µm range whilst operating in pure CF4. Having established the best induction gap, the second phase examined the influence of the GEM hole diameter whilst keeping the induction gap fixed at 50 µm (in pure CF4). Finally, the optimum GEM-MIGAS configuration (50 µm induction gap and 30 µm GEM hole diameter) determine gains in He (2.6 bar: 1 bar) and CF4 /He (2.6 bar: 2 bar). For a 1 mm thermal neutron position resolution, a pressure of about 2.6 bar is required for CF4.
The charge gain results obtained for the different gaps operating in 2.6 bar of CF4, show that shallow gaps do not perform better at high pressures and the highest gain values were obtained using the highest gaps of 250 and 150 µm. However, some electrical instabilities were observed for these gaps and for that reason, we concluded that the 50 µm gap was the optimum for elevated pressures. For the GEM-MIGAS using a standard GEM, with 50 µm hole diameter, the gain decreased rapidly with pressure, from 1.5 x10^4 to 400 at 1 and 2.6 bar CF4, respectively. In contrast, the gain drop with pressure is less steep when using a GEM-MIGAS with GEM hole diameter of 30µm, decreasing from 3.0x10^4 to 6.0x10^3 for 1 and 2.6 bar CF4, respectively. The optimum GEM-MIGAS configuration was found to be 50 µm induction gap and a GEM with 30 µm hole diameter.
The highest gains with the optimum GEM-MIGAS using CF4 /He (2.6 bar: 1 bar) and CF4 /He (2.6 bar: 2 bar) were 3x10^3 and 2x10^3, respectively, almost two orders of magnitude above that required for thermal neutron detection. This implies that it may be possible to raise CF4 pressure above 2.6 bars, for sub-mm position resolution, until gain drops to few tens. However, this aspect remains to be determined experimentally.
It was successfully demonstrated that GEM-MIGAS, with 50 µm induction gap and the 30 µm hole GEM, is a viable choice to be used in neutron gaseous detectors based in He/CF4 mixtures. The gain achieved using 2.6 bar CF4/ 2bar He was above 10^3 and sufficient to achieve 1 mm thermal neutron position resolution. Higher CF4/He filling pressures can be used before the reduction of the gain down few tens.
Author
Ana Conceição
(Instrumentation Center, Physics Department, University of Coimbra)
Co-authors
Dr
Jamil A. Mir
(Science and Technology Facilities Council, Rutherford Appleton Laboratory)
Prof.
Joaquim Santos
(Instrumentation Center, Physics Department, University of Coimbra)
