• Started on: 2002-03-11
• Field area: Detectors
• Cientific coordenator: Francisco Fraga
• funding:
• url:

O projecto em curso envolve o desenvolvimento de detectores de radiação gasosos com sistemas de leitura ópticos, neste caso fotomultiplicadores. A cintilação é emitida pelas avalanches produzidas em microestruturas e permite a localização das interacções. São considerados especialmente um detector para imagiologia com neutrões térmicos de 32x32 cm2 e resolução de 1 mm, uma câmara de traços (TPC) e um espctrómetro de neutrões rápidos.

Estes trabalhos são em parte desenvolvidos numa actividade financiada pelo 6º Quadro Comunitário de Apoio - JRA2 - MILAND: Millimetre resolution Large Area Neutron Detector NMI3 - HII3-CT-2003-505925.

During the second year of the MILAND (millimeter large area neutron detector) collaboration we worked in the simulation of the Anger type readout, assembly and calibration of the PMT systems. Two data taking experiments were carried with neutron beams the ILL reactor (July 2005) and the ISIS spallation source at Rutherford Appleton – Laboratory (October 2005). Measurements of primary scintillation of HeCF4 were carried in Coimbra, as well as of secondary light emitted in MSGCs. In October 2005 a new student started developing software for the data acquisition and analysis of the pulses collected by the PMT arrays, in order to study the time evolution of these signals, anticipating its use for an optical readout TPC, as outlined in the Gaseous Active Scintillators for Imaging activity. We also participated in discussions about future neutron detectors using gas scintillation to be considered in FP7.

The choice of the final version of the MILAND detector was made at the end of the year. Considering the status of development of the several possible candidates and the management of risks a classical MWPC chamber with individual readout was chosen, discarding the other possibilities, modular MSGC, delay line readout MSGC and scintillation Anger camera .

The Monte Carlo simulations were used to find the optimum distance between the GEM plane and the detector array. These results also have shown that the position resolution is dominated by the number of photons collected by PMTs, and therefore, to improve resolution we need to increase the number of detected photons.

For both beam tests the detector was fitted with a MSGC and operated with 2 bar 3He / 2 bar CF4, although a second detector using a GEM at 1 bar 3He / 1 bar CF4 was also tested at the ILL. The first tests were carried with a sensitive position Hamamatsu and a position resolution of 3mm was obtained. The next tests at ISIS, using a more promising readout built at LIP using 30mm diameter PMTs were less successful, as the MSGC had to be operated at limited gain at 1 bar CF4 pressure, that limited the achievable resolution to 5 mm.

Several other topics have been also been addressed, such as the measure the depth of interaction (deduced from the drift time, the time between the primary and secondary light) the drift speed vs. drift field, the electron attachment to impurities or CF4 itself and the separation between neutron and gamma events by evaluating not only pulse height but also pulse duration, which is expected to be longer due to the longer range of the electron in the gas.

Also from the data collected at the ILL in June with a single PMT it was found that the GEM at 1 bar CF4 can emit typically between one or two orders of magnitude more light than the microstrip safe operational GEM voltage, but could not be operated with reliability at higher pressures.

The measurements have also shown that the scintillation Anger camera read out system works and we could currently achieve better results than a similar device developed at Jülich using a solid scintillator that has a resolution of 6 mm. Areas for improvement were clearly identified are being considered.

Spectroscopy measurements of the primary light of the gas mixture excited with an 241Am alpha source having a linear energy deposit in the gas similar to the proton-triton track energy density were made with He-CF4 mixtures were made with a monochromator (Applied Photophysics m. 7300, with a 1200 g/mm grating blazed at 300 nm, using a photomultiplier (RCA C31034), cooled to -20ºC, operating in single photon counting mode. The effect of drift field on primary light was also measured to check for the possibility of electron recombination, that was found to be non-existent. Both the 300 and 620 nm bands were always found, and relative intensities changed with pressure, explaining different results reported in the past by authors using readouts with distinct spectral sensitivities.

Considering that the existing secondary scintillation data, taken exclusively with GEMs, should be complemented with data collected with MSGCs, several measurements of secondary light in detectors operated in pure CF4 and 3He-CF4 were also carried.

All these results were reported in detail in the JRA2 - MILAND: Millimetre resolution Large Area Neutron Detector NMI3 - HII3-CT-2003-505925 2005 Annual Report,

O projecto em curso envolve o desenvolvimento de detectores de radiação gasosos com sistemas de leitura ópticos, neste caso fotomultiplicadores. A cintilação é emitida pelas avalanches produzidas em microestruturas e permite a localização das interacções. São considerados especialmente uma câmara de traços (TPC) com GEMs e os desenvolvimentos utilizando microstrips com substracto transparente.

Estes trabalhos são em parte desenvolvidos na actividade "Optimization and readout of gaseous active scintillators" POCI/FP/81974/2007

Activity plan 2008

Tests of the optically read TPC will be carried either with a 3x3 or 2+3+2 PMT arrays (square or hexagonal mount 40 mm PMTs). This detector will be operated with a double GEM for high dE/dx particles and triple GEM for MIPs or low luminosity gas mixtures. We intend to research on signal processing of the PMTs signals, working mainly on the correction of distortions due to incomplete light collection and timing. It is a new type of readout that has never been used for 3D reconstruction, and we need new algorithms to analyze the performance of the TPC.

Following the successful tests of MSGC readouts for thermal neutron readout made at the ILL Grenoble in 2007, during 2008 tests will be carried using a new type of microstrip developed at the Tokyo University and supplied by Hiroyuki Takahashi. As the microstrip electrodes are manufactured using indium-tin-oxide (ITO) and are 90% transparent, this type of MSGCs can be optically readout from the back side and the substrate can be used as detector output window.