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Development of an Ionization Chamber for the SPIDER Fission Fragment Detector
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Nuclear Data Sheets 119 (2014) 389–391 www.elsevier.com/locate/nds
Development of an Ionization Chamber for the SPIDER Fission Fragment Detector K. Meierbachtol,1, ∗ F. Tovesson,1 C.W. Arnold,2 A.B. Laptev,1 T.A. Bredeweg,2 M. Jandel,2 R.O. Nelson,1 and M.C. White3 1 LANSCE-NS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3 Materials and Physical Data, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2
The ionization chamber component of the SPIDER detector has been designed to measure energy loss and kinetic energy of fragments produced through neutron-induced fission with energy resolutions of <1% and a time-dependent signal collection. Important design elements implemented are an axial configuration of the electrodes for improved energy loss and measurement and a thin silicon nitride entrance window to minimize both energy loss and energy straggling of the incoming fragments. High energy resolution and improved charge resolution from the ionization chamber are combined with the high precision of the upstream time-of-flight component of SPIDER to achieve resolutions in mass and nuclear charge of 1 AMU and Z=1. A discussion of the present resolution capabilities of the ionization chamber will be presented.
I.
INTRODUCTION
The SPectrometer for Ion DEtermination in fission Research (SPIDER) at Los Alamos neutron science center (LANSCE) at Los Alamos National Laboratory is a new detector that will measure neutron-induced fission fragment yields. The incident neutron energy range of 0.01 eV to 20 MeV that will be studied significantly expands on current fission yield measurements. The accuracy of fission fragment yield measurements and corresponding correlated data are important for nuclear energy and global security applications as well as the basic understanding of the fission process. The SPIDER detector has the goal of measuring 97% of fission produced fragments with 2-5% accuracy. Many challenges exist to measuring fission fragment yields with high mass resolution, high accuracy, and high efficiency. The two velocity, two energy, or ‘2V-2E’ method, has been shown to achieve high mass resolution and high accuracy with moderate efficiency. More details can be found in C.W. Arnold et al. proceedings from this conference. The SPIDER detector will use the 2V2E method to study fission fragment yields with the goal of achieving 1 atomic mass unit mass resolution and 1 charge unit charge resolution. The SPIDER detector will consist of multiple detector components that will measure the velocity, total energy, and energy loss of fission fragments emitted during binary fission. Detector pairs will be 180 degrees from
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.108 0090-3752/© 2014 Elsevier Inc. All rights reserved.
each other in order to simultaneously measure both binary fragments. II.
IONIZATION CHAMBER
A gas-filled axial ionization chamber will serve as the detector for measuring both the total kinetic energy and the energy loss as a function of position (dE/dx). After passing through a thin entrance window the fission fragment enters the gas volume and the energy deposited by the fragment in the detector gas is measured. A.
Measuring Energy and Charge
The time-dependent signal collected by the anode is a reflection of the characteristic specific ionization distribution or Bragg curve for each unique fragment. The total area under the curve is the total energy of the fragment. Similar axial ionization chambers [1] have measured the energy of light fission fragments to < 0.5% in energy resolution and to better than 1.0% energy resolution for the heavy fission fragment region. The charge of a fragment can be determined in two ways; both the peak of the Bragg curve and the particle track length are dependent on the Z of the fragment. High energy resolution combined with a time-dependent signal can evaluate either dependence to resolve individual charges. Optimization of operating conditions of the ionization chamber are crucial to achieving high energy and charge resolution.
Development of an Ionization . . .
NUCLEAR DATA SHEETS
K. Meierbachtol et al.
FIG. 2. Prototype silicon nitride membrane entrance window. Blue support structure is a thin silicon wafer.
FIG. 1. Schematic cutaway view of the prototype axial ionization chamber for the SPIDER detector.
III.
B.
The entrance window of the ionization chamber creates an important barrier between a high vacuum region necessary for the timing detectors of SPIDER and the gas-filled ionization chamber. Traditionally these windows have been a source of large energy loss and, more importantly, energy straggling of the incoming ions both of which can reduce the potential for high energy resolution. Silicon nitride membranes fifty to a few hundred nanometers thick have been shown to have significantly less energy loss for heavy ions than traditional mylar windows while still providing the necessary strength and pressure differential over the desired area of the entrance window [2]. Silicon nitride membranes 100 and 200 nm thick mounted onto a silicon wafer as a support structure (shown in Fig. 2) will be tested for pressure differential capabilities, particle energy loss and straggling, and particle transmission.
PROTOTYPE AXIAL IONIZATION CHAMBER
A prototype ionization chamber has been built to test detector design options, optimize operating conditions, and establish baseline energy and charge resolution capabilities. Insight into chamber design characteristics such as chamber dimensions and entrance window material, size, and thickness as well as operating parameters such as fill-gas type, gas pressure, gas flow rate, and applied electric field strengths will influence the construction of the full scale SPIDER detector’s ionization detectors. Figure 1 is a schematic of the prototype axial ionization chamber. The two-chambered design was built to have a gas-filled ionization side and a vacuum side to test both the gas-filled ionization chamber as well as entrance window options.
C. A.
Entrance Window
Gas System
Internal Structure
The internal structure of the ionization chamber contains a cathode and an anode separated by guard rings and a Frisch grid to create a uniform electric field to direct the drift of ionization electrons toward the anode for time-dependent readout (shown schematically in Fig. 3). Different Frisch grid - anode separation distances and electric field strengths are known parameters that effect electron drift and collection times which in turn directly effect the energy resolution. The time-dependent readout of the anode and the subsequent data acquisition will also be investigated. Bit resolution of the digitizer as well as pulse processing components are important parameters that will be evaluated to obtain high resolution of the total energy signal and the energy loss signal.
The gas for an ionization chamber is a vital component to obtaining high resolution measurements. A gas system has been built to provide a continuous exchange of gas, either P10 mixture (90% argon, 10% methane) or pure isobutane, in the chamber by actively monitoring the pressure inside the chamber as well as gas flow rates into and out of the chamber. The ability to continuously fine tune the gas flow rate was built into the system. A constant exchange of fill gas is known to minimize recombination and radiation damage, both of which can decrease signal amplitude and achievable energy resolution. 390
Development of an Ionization . . .
NUCLEAR DATA SHEETS
K. Meierbachtol et al.
differences of 1 AMU and charge differences of ΔZ=1. Future work with regards to the axial ionization chamber will include optimization work on the detector design, baseline energy and charge resolution capabilities, and data acquisition and analysis. Specifically, a prototype detector has been built to investigate entrance window options for the ionization chamber as well as operating parameters of the detector, both of which are known to play a role in achieving high energy resolutions. Knowledge gained by this work will also aid in the design and scale up to the final SPIDER detector for measuring fission fragment yields with one atomic mass unit mass resolution. Acknowledgements: This work was performed under the auspices of the US Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396 and grants for LDRD Project 20110037DR. LA-UR-13-21662.
FIG. 3. Internal structure of the ionization chamber.
IV.
FUTURE WORK
Work continues on developing an axial ionization detector with energy resolution capabilities to resolve mass
[1] A. Oed et al., Nucl. Instrum. Methods 205, 455 (1983).
[2] C. Kottler et al., Nucl. Instrum. Methods B248, 155 (2006).
391
Nuclear Data Sheets 119 (2014) 389–391 www.elsevier.com/locate/nds
Development of an Ionization Chamber for the SPIDER Fission Fragment Detector K. Meierbachtol,1, ∗ F. Tovesson,1 C.W. Arnold,2 A.B. Laptev,1 T.A. Bredeweg,2 M. Jandel,2 R.O. Nelson,1 and M.C. White3 1 LANSCE-NS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3 Materials and Physical Data, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2
The ionization chamber component of the SPIDER detector has been designed to measure energy loss and kinetic energy of fragments produced through neutron-induced fission with energy resolutions of <1% and a time-dependent signal collection. Important design elements implemented are an axial configuration of the electrodes for improved energy loss and measurement and a thin silicon nitride entrance window to minimize both energy loss and energy straggling of the incoming fragments. High energy resolution and improved charge resolution from the ionization chamber are combined with the high precision of the upstream time-of-flight component of SPIDER to achieve resolutions in mass and nuclear charge of 1 AMU and Z=1. A discussion of the present resolution capabilities of the ionization chamber will be presented.
I.
INTRODUCTION
The SPectrometer for Ion DEtermination in fission Research (SPIDER) at Los Alamos neutron science center (LANSCE) at Los Alamos National Laboratory is a new detector that will measure neutron-induced fission fragment yields. The incident neutron energy range of 0.01 eV to 20 MeV that will be studied significantly expands on current fission yield measurements. The accuracy of fission fragment yield measurements and corresponding correlated data are important for nuclear energy and global security applications as well as the basic understanding of the fission process. The SPIDER detector has the goal of measuring 97% of fission produced fragments with 2-5% accuracy. Many challenges exist to measuring fission fragment yields with high mass resolution, high accuracy, and high efficiency. The two velocity, two energy, or ‘2V-2E’ method, has been shown to achieve high mass resolution and high accuracy with moderate efficiency. More details can be found in C.W. Arnold et al. proceedings from this conference. The SPIDER detector will use the 2V2E method to study fission fragment yields with the goal of achieving 1 atomic mass unit mass resolution and 1 charge unit charge resolution. The SPIDER detector will consist of multiple detector components that will measure the velocity, total energy, and energy loss of fission fragments emitted during binary fission. Detector pairs will be 180 degrees from
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.108 0090-3752/© 2014 Elsevier Inc. All rights reserved.
each other in order to simultaneously measure both binary fragments. II.
IONIZATION CHAMBER
A gas-filled axial ionization chamber will serve as the detector for measuring both the total kinetic energy and the energy loss as a function of position (dE/dx). After passing through a thin entrance window the fission fragment enters the gas volume and the energy deposited by the fragment in the detector gas is measured. A.
Measuring Energy and Charge
The time-dependent signal collected by the anode is a reflection of the characteristic specific ionization distribution or Bragg curve for each unique fragment. The total area under the curve is the total energy of the fragment. Similar axial ionization chambers [1] have measured the energy of light fission fragments to < 0.5% in energy resolution and to better than 1.0% energy resolution for the heavy fission fragment region. The charge of a fragment can be determined in two ways; both the peak of the Bragg curve and the particle track length are dependent on the Z of the fragment. High energy resolution combined with a time-dependent signal can evaluate either dependence to resolve individual charges. Optimization of operating conditions of the ionization chamber are crucial to achieving high energy and charge resolution.
Development of an Ionization . . .
NUCLEAR DATA SHEETS
K. Meierbachtol et al.
FIG. 2. Prototype silicon nitride membrane entrance window. Blue support structure is a thin silicon wafer.
FIG. 1. Schematic cutaway view of the prototype axial ionization chamber for the SPIDER detector.
III.
B.
The entrance window of the ionization chamber creates an important barrier between a high vacuum region necessary for the timing detectors of SPIDER and the gas-filled ionization chamber. Traditionally these windows have been a source of large energy loss and, more importantly, energy straggling of the incoming ions both of which can reduce the potential for high energy resolution. Silicon nitride membranes fifty to a few hundred nanometers thick have been shown to have significantly less energy loss for heavy ions than traditional mylar windows while still providing the necessary strength and pressure differential over the desired area of the entrance window [2]. Silicon nitride membranes 100 and 200 nm thick mounted onto a silicon wafer as a support structure (shown in Fig. 2) will be tested for pressure differential capabilities, particle energy loss and straggling, and particle transmission.
PROTOTYPE AXIAL IONIZATION CHAMBER
A prototype ionization chamber has been built to test detector design options, optimize operating conditions, and establish baseline energy and charge resolution capabilities. Insight into chamber design characteristics such as chamber dimensions and entrance window material, size, and thickness as well as operating parameters such as fill-gas type, gas pressure, gas flow rate, and applied electric field strengths will influence the construction of the full scale SPIDER detector’s ionization detectors. Figure 1 is a schematic of the prototype axial ionization chamber. The two-chambered design was built to have a gas-filled ionization side and a vacuum side to test both the gas-filled ionization chamber as well as entrance window options.
C. A.
Entrance Window
Gas System
Internal Structure
The internal structure of the ionization chamber contains a cathode and an anode separated by guard rings and a Frisch grid to create a uniform electric field to direct the drift of ionization electrons toward the anode for time-dependent readout (shown schematically in Fig. 3). Different Frisch grid - anode separation distances and electric field strengths are known parameters that effect electron drift and collection times which in turn directly effect the energy resolution. The time-dependent readout of the anode and the subsequent data acquisition will also be investigated. Bit resolution of the digitizer as well as pulse processing components are important parameters that will be evaluated to obtain high resolution of the total energy signal and the energy loss signal.
The gas for an ionization chamber is a vital component to obtaining high resolution measurements. A gas system has been built to provide a continuous exchange of gas, either P10 mixture (90% argon, 10% methane) or pure isobutane, in the chamber by actively monitoring the pressure inside the chamber as well as gas flow rates into and out of the chamber. The ability to continuously fine tune the gas flow rate was built into the system. A constant exchange of fill gas is known to minimize recombination and radiation damage, both of which can decrease signal amplitude and achievable energy resolution. 390
Development of an Ionization . . .
NUCLEAR DATA SHEETS
K. Meierbachtol et al.
differences of 1 AMU and charge differences of ΔZ=1. Future work with regards to the axial ionization chamber will include optimization work on the detector design, baseline energy and charge resolution capabilities, and data acquisition and analysis. Specifically, a prototype detector has been built to investigate entrance window options for the ionization chamber as well as operating parameters of the detector, both of which are known to play a role in achieving high energy resolutions. Knowledge gained by this work will also aid in the design and scale up to the final SPIDER detector for measuring fission fragment yields with one atomic mass unit mass resolution. Acknowledgements: This work was performed under the auspices of the US Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396 and grants for LDRD Project 20110037DR. LA-UR-13-21662.
FIG. 3. Internal structure of the ionization chamber.
IV.
FUTURE WORK
Work continues on developing an axial ionization detector with energy resolution capabilities to resolve mass
[1] A. Oed et al., Nucl. Instrum. Methods 205, 455 (1983).
[2] C. Kottler et al., Nucl. Instrum. Methods B248, 155 (2006).
391