Pulse height defect in an ionization chamber investigated by cold fission measurements

doi:10.1016/0168-9002(90)90224-T

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 286, Issues 1–2, 1 January 1990, Pages 220-229

https://doi.org/10.1016/0168-9002(90)90224-T Get rights and content

Abstract

The cold fragmentation of ²³⁵U thermal neutron fission has been studied in a twin ionization chamber working with pure methane gas. Due to the very high fragment mass resolution, a careful investigation of the pulse height defect in this detector has been achieved. No dependence of the pulse height defect on the fragment mass has been observed. Therefore, it is easy to calibrate the energy response of such a detector. An accurate calibration method is proposed.

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The FFIS spectrometer for determination of fission fragment mass distribution with the energy–velocity method
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Citation Excerpt :
Axial grid ionization chambers [17] with the electric field parallel to the trajectory of incident particles are widely used for fission fragment measurements [18,19]. On the one hand, gas detectors are immune to radiation damage and suffer minor pulse-height defects compared with solid-state surface barrier detectors [20–23], which enables grid ionization chambers with outstanding energy resolution. On the other hand, the shape of the anode signal of the axial grid ionization chamber mirrors the ionizing stopping power curve, which contains the charge information of heavy ions [5,24].
The FFIS spectrometer (Fission Fragment Identification Spectrometer) for determining independent fission yields has been developed based on the E–v method. By directly measuring the time-of-flight (TOF) and kinetic energy of fission fragments on their flight path, the post-neutron-emission mass yield distributions can be obtained. With the aim of getting the highest possible mass resolution, an axial grid ionization chamber and a TOF measurement system based on microchannel plate (MCP) detectors were designed. The ionization chamber with a silicon nitride entrance window of 100 nm thickness was tested and calibrated with a ²⁵²Cf spontaneous fission source. The intrinsic energy resolution for 1.27 MeV/u ⁶³Cu particles is measured to be 450 keV (FWHM). The time resolution was determined with two MCP timing detectors to be 157 ps (FWHM) for ²⁴¹Am radiation source, implying that the intrinsic time resolution is about 110 ps (FWHM) for a single timing detector. Thermal-neutron-induced fission of ²³⁵U was studied at the In-hospital Neutron Irradiator (IHNI). An event-by-event energy loss addback technique was developed based on Geant4 calculation to minimize the correction error. The first results of mass yield distributions from ²³⁵U(n $_{t h}$ ,f) are reported.
Theory of nuclear fission
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Atomic nuclei are quantum many-body systems of protons and neutrons held together by strong nuclear forces. Under the proper conditions, nuclei can break into two (sometimes three) fragments which will subsequently decay by emitting particles. This phenomenon is called nuclear fission. Since different fission events may produce different fragmentations, the end-products of all fissions that occurred in a small chemical sample of matter comprise hundreds of different isotopes, including $α$ particles, together with a large number of emitted neutrons, photons, electrons and antineutrinos. The extraordinary complexity of this process, which happens at length scales of the order of a femtometer, mostly takes less than a femtosecond but is not entirely over until all the lingering $β$ decays have completed – which can take years – is a fascinating window into the physics of atomic nuclei. While fission may be more naturally known in the context of its technological applications, it also plays a crucial role in the synthesis of heavy elements in astrophysical environments. In both cases, simulations are needed for the many systems or energies inaccessible to experiments in the laboratory. In this context, the level of accuracy and precision required poses formidable challenges to nuclear theory. The goal of this article is to provide a comprehensive overview of the theoretical methods employed in the description of nuclear fission.
A twin Frisch-grid ionization chamber as a selective detector for the delayed gamma-spectroscopy of fission fragments
2017, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
We present a twin Frisch-grid ionization chamber. The detector is meant to provide high selective power for the study of delayed gamma-ray spectroscopy of fission fragments produced via ²⁵²Cf spontaneous fission. A mean energy resolution on the kinetic energy of fission fragments of 675 keV (FWHM) is achieved and allows us to resolve masses of fragments for fission events where neutron emission is not energetically possible. The mean mass resolution measured for these particular events amounts to 0.54 mass units (FWHM). For fission events with neutron emission a resolution of 4 mass units (FWHM) is reported. Information on fragment emission angle is measured with a resolution of 0.1 on the difference of the cosines determined for both halves of the detector. A charge resolution of 4.5 charge units (FWHM) is also demonstrated.
Ambiguities in the grid-inefficiency correction for Frisch-Grid Ionization Chambers
2012, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Ionization chambers with Frisch grids have been very successfully applied to neutron-induced fission-fragment studies during the past 20 years. They are radiation resistant and can be easily adapted to the experimental conditions. The use of Frisch grids has the advantage to remove the angular dependency from the charge induced on the anode plate. However, due to the Grid Inefficiency (GI) in shielding the charges, the anode signal remains slightly angular dependent. The correction for the GI is, however, essential to determine the correct energy of the ionizing particles. GI corrections can amount to a few percent of the anode signal. Presently, two contradicting correction methods are considered in literature. The first method adding the angular-dependent part of the signal to the signal pulse height; the second method subtracting the former from the latter. Both additive and subtractive approaches were investigated in an experiment where a Twin Frisch-Grid Ionization Chamber (TFGIC) was employed to detect the spontaneous fission fragments (FF) emitted by a ²⁵²Cf source. Two parallel-wire grids with different wire spacing (1 and 2 mm, respectively), were used individually, in the same chamber side. All the other experimental conditions were unchanged. The 2 mm grid featured more than double the GI of the 1 mm grid. The induced charge on the anode in both measurements was compared, before and after GI correction. Before GI correction, the 2 mm grid resulted in a lower pulse-height distribution than the 1 mm grid. After applying both GI corrections to both measurements only the additive approach led to consistent grid independent pulse-height distributions. The application of the subtractive correction on the contrary led to inconsistent, grid-dependent results. It is also shown that the impact of either of the correction methods is small on the FF mass distributions of ²³⁵U(n_th, f).
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The response of silicon–silicon–CsI(Tl) and silicon–CsI(Tl) telescopes to fragments produced in nuclear interactions has been studied. The telescopes were developed within the FAZIA collaboration. The capabilities of two methods are compared: (a) the standard $Δ E - E$ technique and (b) the digital Pulse Shape Analysis technique (for identification of nuclear fragments stopped in a single Si-layer). In a test setup, nuclear fragments covering a large range in nuclear charge, mass and energy were detected. They were produced in nuclear reactions induced by a 35A MeV beam of ¹²⁹Xe impinging on various targets. It was found that the $Δ E - E$ correlations allow the identification of all isotopes up to $Z \sim 25$ . With the digital Pulse Shape Analysis it is possible to fully distinguish the charge of stopped nuclei up to the maximum available Z (slightly over that of the beam, Z=54).

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Pulse height defect in an ionization chamber investigated by cold fission measurements

Abstract

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Phys. Rev.

Z. Phys.

J. Phys. Lett.

IXe Journées d'étude sur la fission nucléaire

Rapport CENBG 8722

J. Phys. Lett.

IX^e Journées d'étude sur la fission nucléaire