Y. Yamaguchi

Fine-pitch and thick-foil gas electron multipliers for cosmic x-ray polarimeters

We have produced various gas electron multiplier foils (GEMs) by using laser etching technique for cosmic X-ray polarimeters. The finest structure GEM we have fabricated has 30 μm-diameter holes on a 50 μm-pitch. The effective gain of the GEM reaches around 5000 at the voltage of 570 V between electrodes. The gain is slightly higher than that of the CERN standard GEM with 70 μm-diameter holes on a 140 μm-pitch. We have fabricated GEMs with thickness of 100 μm which has two times thicker than the standard GEM. The effective gain of the thick-foil GEM is 104 at the applied voltage of 350 V per 50 μm of thickness. The gain is about two orders higher than that of the standard GEM. The remarkable characteristic of the thick-foil GEM is that the effective gain at the beginning of micro-discharge is quite improved. For fabricating the thick-foil GEMs, we have employed new material, liquid crystal polymer (LCP) which has little moisture absorption rate, as an insulator layer instead of polyimide. One of the thick-foil GEM we have fabricated has 8 μm copper layer in the middle of the 100 μm-thick insulator layer. The metal layer in the middle of the foil works as a field-shaper in the multiplication channels, though it slightly decreases the effective gain.

Measurement of basic features of Thick-GEM and Resistive-GEM

Recently, Thick-GEM (TGEM) whose geometry is at the millimeter-scale has been developed to overcome the disadvantages of micro-scale Gas Electron Multiplier (GEM) such as fragility against a discharge, difficulty in obtaining enough gain at low pressure and so on. In addition, TGEM with resistive electrodes (RETGEM) also has been developed to make TGEM more tolerant with a discharge. Active research is carried out worldwide for applications such as liquid Argon detectors, Cerenkov light detectors, sampling elements in calorimeter and low-pressure time projection chamber. TGEM and RETGEM look very promising, but their basic properties have not yet been studied thoroughly. TGEM and RETGEM with different geometries were made and the basic properties such as the voltage dependence of gain, the energy resolution and the gain variation as a function of time at different gain levels were measured. A gain of about 104 was obtained with Ar(90%) + CH4 (10%) gas mixture for both TGEM and RETGEM. The gain variation as a function of time of TGEM at the gain ~ 2000 was within 6% after the correction by pressure and temperature. On the other hand, the gain variation as a function of time of TGEM was large at higher gain and can not be explained by only the correlation between the gain and the value of pressure over temperature. The gain variation as a function of time of RETGEM was within 4% after the correction by pressure and temperature. The achieved energy resolution was ~ 13% (TGEM) and ~ 10% (RETGEM). The energy resolution of RETGEM is comparable to that of micro-scale GEM.

Gain properties of gas electron multipliers (GEMs) for space applications

Fine-pitch and thick-foil GEMs have been produced using a laser etching technique for photoelectric X-ray polarimeters onboard future missions. The finest hole pitch of the thick-foil GEM is 80 μm with a hole diameter of 40 μm, and a thickness of the insulator is 100 µm. The maximum effective gain in a 70%-30% mixture of argon and carbon dioxide reaches 3×104 at voltage of 750 V between GEM electrodes. No significant gain increase or decrease was observed during 24 hours test in which applied high voltage was ramped up and down frequently. The measured gain stability was less than 4%. An accelerated test of the high voltage ramp up and down for two years LEO operations were carried out. During the 6500 times voltage ramp up and down, the GEM kept its gain within 4% variation and no unexpected behavior was observed.

Measurement of One- and Two-Neutron Transfers in Reaction of 6He+9Be at 25 MeV/u

The cross sections of one- and two-neutron transfers induced by 6He at 25 MeV/u on a 9Be target were measured in RIKEN. Clear identification of the recoiled Be isotopes was achieved. In total five 11Be and 371 10Be events, the corresponding two- and one-neutron transfers were obtained and analysed for transfer reaction cross sections. The results are useful to determine the spectroscopic factors of the internal halo structure of the 6He nucleus.

Measurement of the reaction cross section of (18)C and observations of fragments from (17)C and (18)C at 80A MeV

The one- and two-neutron removal reactions from (17)C and (18)C as well as the reaction cross section of (18)C have been studied using a carbon target at 80A MeV. The longitudinal momentum distributions of (15,16)C fragments from (17)C and (16,17)C fragments from (18)C were measured by a direct time-of-flight method. The width of (15)C fragments from (17)C is fairly smaller than that from other C isotopes. The experimental data are discussed within the framework of the Glauber model.

Development of a pipelined ADC chip for the gas electron multiplier readout

A 10-bit 10 Msps A/D converter chip with low power consumption is designed for gas electron multiplier (GEM) readout based on the n-well 0.35 /spl mu/m CMOS process. A pipelined ADC architecture consisting of an array of 1.5-bit unit ADCs is used. The opamp sharing between two stages is adopted in order to reduce the power consumption. The expected power consumption is about 12 mW. In this paper, the progress of the design development is reported.

Measurements of soft and intermediate pt photons from hot and dense matter at RHIC-PHENIX

Abstract The measurements of direct photons in 1.0 p T 5.0 GeV/ c for p+p and Au+Au collisions have been made through the virtual photon method at PHENIX. The fraction of the direct γ ∗ component to the inclusive e + e − yield is determined by a shape analysis using the e + e − mass spectra in m e e 300 MeV/ c 2 for 1.0 p T 5.0 GeV/ c . The real direct photon spectra in p+p and Au+Au collisions are obtained from the virtual direct photon fractions, and a significant excess over a binary-scaled p+p result is seen in Au+Au collisions. Hydrodynamical models which reproduce the Au+Au result indicate the initial temperature of the matter is higher than the critical temperature of QGP. The d+Au data taken in 2008 are promising to evaluate the contribution of the nuclear effects due to its large statistics.

Development and performance evaluation of Thick-GEM

A Gas Electron Multiplier with a thick insulator such as a 100 mum or 150 mum thick insulator (Thick-GEM) has been developed by dry etching successfully in Japan. The electric field inside a hole of the Thick-GEM was calculated and the basic properties of the Thick-GEM were measured. A much stronger electric field can be realized inside the hole of the Thick-GEM than that of a GEM with a 50 mum thick insulator (Standard-GEM). The Thick-GEM can attain much higher gain than the Standard-GEM and has a good gain stability within 1.0% for 9 hours. In this paper, the characteristics of the Thick-GEM are described compared with the Standard-GEM.

Single-Proton Pickup Reaction of the Halo Nucleus 6He on a 9Be Target at 25 MeV/nucleon

The inclusive differential cross sections of the 7Li nucleus in a reaction induced by 6He on a 9Be target are measured at an incident energy of 25 MeV/nucleon. Finite-range distorted-wave Born approximation calculations suggest that these 7Li particles are formed in a direct single-proton pickup reaction 9Be(6He,7Li)8Li. The experimental data can be well reproduced by taking into account of the contributions of both the ground states and the first excited states of 7Li and 8Li.

Measurement of the Breakup Cross Sections in 6He+9Be Reaction at 25 MeV/Nucleon

The breakup reaction cross sections were measured for the reaction of 6He at 25 MeV/nucleon from 9Be target with intensity of 105 pps. By fitting the energy spectra of breakup α particles with Gaussian functions, the angular distribution of differential cross sections in the laboratory system has been extracted and compared with the Serber model calculations. The good agreement between the calculation and the experimental data favours a dominant configuration of the 4He core plus valence neutrons for the structure of 6He.