Baba  M.

Itoga  T.

Kamata  S.

Nakhostin  M.

Oishi  T.

Otsuki  T.

Unno  Y.

Nakhostin, M.

Baba, M.

Oishi, T.

Itoga, T.

Kamata, S.

Unno, Y.

Otsuki, T.

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Development of a Novel Fast-response Neutron Beam Monitor

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Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Development of a Novel Fast-response NeutronBeam Monitor著者 Nakhostin  M., Baba  M., Oishi  T., Itoga  T.,Kamata  S., Unno  Y., Otsuki  T.journal orpublication titleCYRIC annual reportvolume 2005page range 27-30year 2005URL http://hdl.handle.net/10097/50308CYRIC Annual Report 2005    II. 6.  Development of a Novel Fast-response Neutron Beam Monitor    Nakhostin M.1, Baba M.1, Oishi T.1, Itoga T.1, Kamata S.1, Unno Y.1, and Otsuki T.2 Cyclotron and Radioisotope Center, Tohoku University Laboratory for Nuclera Study, Tohoku Univeristy    Introduction During the last year we have been involved with the development of fast response neutrons detectors.  Such detectors are required for couple of applications. A prominent example of such applications is intense neutron beam monitoring in next generation of accelerator-based neutron sources.  Such accelerators are now under construction in several countries.  For example, in Japan, Japan proton accelerator research complex (J-PARC) is being built.  In this project very intense thermal neutron beams will be available and therefore appropriate neutron detectors are required to provide information on the neutron beam such as intensity, spatial and time distribution.  In such a harsh environment, detector radiation durability is of crucial importance as well. This report is devoted to the preliminary results of a novel fast neutron beam monitor, which is based on the parallel plate avalanche chamber (PPAC).  Choice of PPAC was made due to its fast response, radiation resistance, high-count rate and position measurement capability.  Parallel plate avalanche counters A PPAC consists simply of two thin metalized foils mounted in parallel, with a small gap of a few mm in between (Fig. 1).  PPAC is employed at a few torr of a hydrocarbon gas like isobutene, and under a strong electric field E in the gap.  Due to the high-reduced electric field, E/P, (P is gas pressure) released electrons by ionizing radiations initiate electron avalanche and form a very fast signal  (~1 ns rise time) due to the high drift velocity of electrons.  27Detector can be easily made position sensitive, if an electrode is made of a grid of parallel thin wires or parallel strips.  Then, position data can be obtained by means of conventional methods like delay line or charge division method.  The neutron-sensitive PPAC Our prototype detector consists of three parts:  The converter sheet, which produces recoil charged particles, the PPAC and detector housing. Converter; the converter is a thin layer of 6LiF evaporated on aluminum backing. In order to obtain a good pulse height spectrum converter sheet is placed a few centimeters far from PPAC.  PPAC; a schematic illustration of our PPAC has been shown in Fig. 2.  It consists of an aluminized Mylar foil electrode as anode and cathode made of a grid of gold-plated tungsten wires (30 μm) with 2 mm spacing.  Anode-cathode gap is 2 mm and position information is obtained by using delay-line technique.  Detector housing; detector assembly is mounted in the detector housing, which is made of aluminum.  The detector is operated in the gas flow mode with isobutene gas.  Test experiments results Test experiments were carried out with using an Am-Be neutron source. Fast neutrons were thermalized by employing appropriate thickness of polyethylene.  A typical detector pulse height spectrum is shown in Fig. 2.  Figure 3 shows a typical signal waveform, obtained with fast current sensitive preamplifier (VT120, Ortec). The signal rise time of ~ 4 ns and ~ 6 nsec pulse width (FWHM) is easily obtainable.  Such signals are well suited for the time dependent beam monitoring applications under high count rates. A projection of position spectrum is shown in Fig. 4.  The obtainable spatial resolution is equal with wires spacing (2 mm).  Delay line system was composed of delay chips with 3 ns delay between adjacent wires and 100 ps time resolution.  Summary and conclusion A novel neutron beam monitor with very fast response and position measurement capability has been successfully developed.  Fast signals with ~ 4 ns rise time and ~6 ns pulse  28width together with 2 mm position accuracy were easily obtained.  This detector looks very promising for beam monitoring in J-PARC project and so on.     G.N.DRCSignalCharged particlePositionH.VFast signalFigure 1.  PPAC principle of operation. Figure 2.  Schematic representation of our PPAC.    0 50 100 150 200 25-20020406080100120Pulse height (arbitrary unit)Time (ns)  PPAC  Plastic scintillator    0 1000 2000 3000 400002040 Channel number Figure 4.  Typical signal waveform, compared with scintillator.  Scintillator shows much longer decay time. Figure 3.  Typical pulse height spectrum.        290 500 1000 1500 2000 25000510152025  Count per channel Figure 5.  Results of position measurement according to delay line method.  30

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Development of a Novel Fast-response Neutron Beam Monitor

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