Abstract
The development of a switch circuit to gate charge sensitive preamplifiers for use at pulsed radiation sources will be presented. This development was used for the 16O(n,α)13C reaction measurement with a Double Frisch Grid Ionization Chamber (DFGIC) at the neutron time-of-flight facility CERN n_TOF in Geneva, Switzerland. Intense instantaneous radiation which is produced in the spallation target of the n_TOF facility (γ-flash) can saturate charge sensitive preamplifiers and prevent signals from being registered in the detection system. The switch circuit made it possible for the first time to perform a measurement with the DFGIC with γ-flash gated off at n_TOF. Nano-second gating of charge sensitive preamplifiers has a wide range of applicability at pulsed radiation sources, where short bursts of radiation must be gated off to avoid saturation, e.g. with HPGe detectors for γ-ray detection. Nano-second gating requires the stray-capacitance of the wideband reflective switch to be compensated to avoid a strong signal during the switch operation. Spectral analysis of the switch circuit shows that additional noise is insignificant.
Highlights
The n_TOF facility at CERN [1] uses the CERN PS proton beam (7 ⋅ 1012 protons per pulse with an energy of 20 GeV) to produce an intense burst of neutrons by spallation in a large lead target
In EAR2 this prompt flash is not visible because the decay of pions is forward focused due to their relativistic velocity, but a delayed and wider flash is present made of the very intense flux of fast neutrons which produce a piled up signal when they scatter in the sample and detectors
The central component is the wide band reflective switch ADG902 from Analog Device [8] built in complementary metal oxide semiconductors (CMOS) technology
Summary
The n_TOF facility at CERN [1] uses the CERN PS proton beam (7 ⋅ 1012 protons per pulse with an energy of 20 GeV) to produce an intense burst of neutrons by spallation in a large lead target. While charged particles are deflected out of the neutron beam using a magnet, γ-rays coming mainly from the in-flight decay of neutral pions, reaching energies up to GeV, can fly up to EAR1 which is at an angle of 10 degrees relative to the proton beam This generates a prompt flash made of the scattered photons and of the created charged leptons on the window materials of the neutron beam pipe, the so-called γ-flash. In EAR2 this prompt flash is not visible because the decay of pions is forward focused due to their relativistic velocity, but a delayed and wider flash is present made of the very intense flux of fast neutrons which produce a piled up signal when they scatter in the sample and detectors This neutron-flash is harmful when studying reactions above 1 MeV, but it can be mitigated with the switch electronics, to a lesser extent due its wider time spread. In this sense the switch circuit solves the problem where transistor reset preamplifier [4,5,6] and optical feedback techniques [5] used e.g. for high resolution gamma-ray spectroscopy fail to do it [7]
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