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Details on the JULIOS detector
JULIOS stands for 'JUelicher LInear Ortsauflösender Szintillationsdetektor'. It is a linear position-sensitive scintillation detector and it operates on the basis of a modified Anger technology using a 1 mm 6Li glass scintillator and a linear row of 24 photomultipliers. The detector is position- as well as time-resolving. The detector system has been designed in collaboration between Forschungszentrum Jülich and Bonn University. Development and construction has been performed in the Zentrallabor für Elektronik (ZEL) in Jülich.
|Type||linear position-sensitive scintillation detector,|
|Outer dimensions (mm)||900 x 450 x 125 (standard unit)|
1100 x 475 x 180 (large unit)
|Scintillator||Ce activated 6Li glass|
|Size of scintillator (mm)||680 x 25 (length x height)|
940 x 75 (length x height)
|Thickness of scintillator (mm)||1.0|
l =1.0 Å
l =2.0 Å
|Number of photomultipliers||24|
|Linear spatial resolution (mm)||2.3 (standard) 3.3 (large unit)|
|Electronic time resolution||1 microsec|
|Count rate capability (KHz)||100|
|Maximum no. position channels||4096|
|Optional no. time channels||65536|
|Max counts per channel||4 Byte|
|Gamma sensitivity (E > 1 MeV)||104|
The detector has to convert the information about position and time of the arrival of the neutron in the scintillator into an electrical signal. Since neutrons are neutral and only weakly absorbed by materials it is not possible to detect them directly. The neutron is used, however, to generate charged particles by means of a nuclear reaction which, in turn, can be detected. In the JULIOS detector neutrons are converted in the 6Li glass scintillator glass via the reaction
6Li + n ---> 3H + a + 4.6 MeV
The triton (3H) and alpha (a ) particles carry most of the released energy of 4.6 MeV. Most of it is dissipated into heat, but a small part o the decay products energy is converted into light by a scintillation process. Every captured neutron produces about 2000 photons in the scintillator. The light is passed to photomultipliers where is converted into an electrical signal.
The position analysis of the absorbed neutron is based on the determination of the light distribution on 24 photomultipliers (PM’s) at the back of the scintillator. The geometrical arrangement of scintillator, air gap, light coupler, and photomultipliers (see figure) allows to confine the light from a neutron capture into a cone, which irradiates only 3 adjacent PM’s. A dedicated hardwired computer uses the digitized PM anode signals to calculate the centre of gravity of the light cone, i.e. to get the position of neutron capture. The hardware of the JULIOS detectors includes also modules for automatic gain stabilisation of the photomultipliers and for correction of positional non-linearities and inhomogenities of the scintillator material. Selection of the active position window used in a measurement and division of this window into position channels is carried out by software. The maximum electronic position resolution of the detector is 12 bit, but a division of the scintillator length of 744 mm into 256 channels (8 bit) is sufficient for most applications, i.e. one position channel has a width of 2.906 mm.
The detector is synchronised with the time structure of the pulsed neutron source: A high resolution 16 bit timing module measures the time between the external ISIS synchronisation pulse trigger, which marks the time of neutron generation in the target, and the time of neutron absorption in the scintillator. Neutrons are registered in the time frame 0-20 ms (50 Hz ISIS pulse rate). This time frame is divided into time channels by software, the minimum width of a time slice being 1 ms.
The JULIOS detectors can be operated as a stand-alone system without any mainframe computer or the data acquisition can be done using the standard ISIS data acquisition system (DAE).
As stand-alone system the JULIOS electronics is connected to a 32 bit PC with EISA-bus architecture. Position encoded and time stamped JULIOS events are transferred via a so-called BMAC (Bus master ADC Controller) PC interface card into the memory area of the PC. Memory contents in the reserved position-time matrix are incremented, much like a multi channel analyser. The detector and its PC based processing electronics and data acquisition system is operated under the quasi-multitasking PC-shell of MS-Windows. A life-display of the data in the memory is used for interactive on-line screening, whereas command files are used for automatic measurements. A PC program package is used to perform all necessary normalisations, corrections and focusing operations.
The JULIOS are currently being connected to the ISIS DAE. 12 bit position information are transferred, plus handshake signals which synchronise the data transfer with the DAE timing. A VMS ALPHA workstation serves as FEM (Front End Minicomputer) to control sample environment, data collection and perform data analysis.
W. Schäfer, E. Jansen, A. Szepesvary, R. Skowronek, G. Will, R. Reinartz, K.D. Müller
"The scintillation PSD JULIOS and its use in TOF-diffractometry at ISIS"
Proc. ICANS XII 1993, eds U. Steigenberger, T. Broome, G. Rees, A. Soper, RAL report 94-025 (1994) Vol. 1, 1-200.
W. Schäfer, E. Jansen, G. Will, A. Szepesvary, R. Reinartz, K.D. Müller
" Update on the Jülich linear and area neutron scintillation detectors"
Physica B 213&214 (1995) 972-974.
P. Convert, J.B. Forsyth (ed.)
"Position-sensitve detection of thermal neutrons"
Academic Press (1983)
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