A Secondary Electron Detector for PRISMA
A UK collaboration, consisting of the NPG together with the Universities of Manchester and the West of Scotland, is building a large area Secondary Electron Detector (Se-D) for the focal plane detector system of PRISMA at LNL.
Incoming ions pass through a thin emissive foil (0.6µm thick Mylar foil corresponding to about 80µg/cm2) producing secondary electrons which are accelerated under an electric potential, transported by a magnetic field and detected in a MWPC located out-of-plane with respect to the path of the incoming ions. A MWPC is chosen rather than the usual Micro-Channel Plates (MCP) because they are more costly and difficult to implement over such a large focal plane (1m wide). The position is generated by measuring the centroid of the charge deposition across the wires on each plane. This requires individual wire-by-wire readout rather than the usual delay-line readout of the MWPC. Development work involved attempts to minimize electron straggling before the active volume of the out-of-beam MWPC using magnetic field electron transport and accelerating potentials in order to obtain position and time resolutions of the order of 1-2mm and 200-300ps, respectively.
A small area (7x7cm2 active area) prototype of Se-D was designed and constructed. A schematic view of the detector is shown in Fig. 1. It consists of an in-beam emissive foil with an out-of-beam MWPC (Fig. 2) to detect the secondary emission, followed by an in-beam MWPC to evaluate efficiency and time resolution of the out-of-beam device. Typical values for the accelerating potential and the magnetic field are of the order of -20 kV and 100 Gauss, respectively. Guard rings are use to preserve the uniformity of the electric field in between the emissive foil and the detector. The MWPC has a three electrode structure with the central cathode composed of a 1.5 µm thick mylar foil and anodic planes (X and Y position) made of gold-plated tungsten wires (20 µm diameter) with a spacing of 1 mm.
|
|
|
|
||
Figure 1: Schematic view of the set-up used for bench and in-beam tests. |
Figure 2: The Se-D prototype (left) with delay line readout (top-right) and the Gassiplex ASIC cards (bottom-right). |
|
Bench tests
Preliminary bench-tests have been performed by using fission fragments from a 252Cf source. Isobutane (C4H10) was used as filling gas of the Se-D at a working pressure of 5 hPa. In the first tests, a delay line readout was used to obtain the position information from the anodic planes of the MWPC. The prototype was tested successfully tested with two GAS32 ASIC cards (see Fig. 2) daisy chained together and coupled to a custom designed V4FADC board (fast analogic-to-digital converters). Typical position resolutions of the order of 1 mm in both X and Y directions have been measured.
For the large area (1 m wide) Se-D design, a smaller footprint ASIC board (named GAS16) has been designed. Each GAS16 (Fig. 3) includes one 16-channel Gassiplex ASIC embedding a preamplifier, a shaping time amplifier and a Track & Hold per channel. With a 5 MHz readout clock, the V4FADC module (shown in Fig. 5) collects and converts data from the GAS16 boards and sends it via the 1Gbit ethernet to a controlling VME processor under the control of a MIDAS GUI. The event rate using a pulse generator is above 1,000 / sec. The GAS16 card has been tested using a PPAC prototype (Fig. 4) in order to check its performances. Six of those GAS16 board have now been built and are currently being tested. The 500 wires in the X plan and 90 in the Y plan will require a total of 38 GAS16 boards.
 |
|
|
Figure 3: One GAS16 card to read 16 PPAC wires. |
Figure 4: Two GAS16 boards mounted on a PPAC prototype. |
Figure 5: V4FADC module. |
The 16 channels in a GAS16 board are time multiplexed onto one analogue output which is then converted by the flash ADC in the V4FADC module. This V4FADC has been designed to suit a wide range of ASICs electronics. It includes a Xilinx FPGA operating system which allows to operate the ASICs control and readout, the FADC conversion and the data storage. Each V4FADC module can steer up to eight GAS16 boards so five of them are required for the final design of the large area Se-D. Ten have already been built in Daresbury. All of them are first being tested individually before to be synchronised altogether via a metronome for an ultimate test.
In-beam tests
In-beam tests of the small area Se-D prototype were performed in October 2007 at LNL by using 250 MeV 80Se ions elastically scattered from a 100 mg/cm2 thick 12C target. The Se-D was placed at an angle of 52° with respect to the beam direction (Fig. 6) in order to check the performance of the detector with heavy ions at energies around 0.5A MeV. Isobutane (C4H10) was used as filling gas at a working pressure of 5 hPa. A constant gas flow was used to avoid the gas contamination during the tests. X and Y position resolutions (Fig. 7) of the order of 1 mm have been measured with slits in front of emissive foil. The overall time resolution, measured using the in-beam MWPC as Start detector, is of the order of 1 ns including the energy straggling in the C target, the kinematical broadening and the time resolution contribution of the in-beam MWPC.
|
|
Figure 6: Experimental set-up installed at LNL for the in-beam commissioning. |
Figure 7: X and Y distributions measured with 80Se ions elastically scattered onto a 12C target. |
Then, in-beam tests confirmed that the overall behaviour of the detector was essentially identical to that observed with fission fragments from the 252Cf source. There were no unforeseen beam-related issues.
The prototype detector is currently being scaled up to provide a 1 m wide device for use at the PRISMA focal plane. A working detector design has been developed, and effort is now concentrated on the provision of the magnetic field for electron transport. A large electromagnet is being developed
to provide around 100 Gauss across the full extent of the PRISMA focal plane.
Contact