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

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Summary

Introduction

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]

Development of the switch circuit
The core chip ADG902
Compensation circuit
Reduction of the gate voltage
Charge compensation with a second switch
Coupling of the switch circuit
Limitations of the switch circuit
Full scheme of the switch circuit
Noise figure
Applications of the switch circuit at pulsed beam sources
Direct applications of the switch circuit
Other possible applications of the switch circuit
Findings
Summary

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