Start Detector Research Articles

A mass spectrometer for fission fragments based on time-of-flight and energy measurements

With the aim of determining masses of fission fragments by measuring velocities and kinetic energies, specially adapted time-of-flight detectors and ionization chambers were developed. The start and stop detectors for the time-of-flight sense the electrons emitted from a thin foil upon passage of fragments. The time resolution achieved is ⋍ 100 ps . The ionization chambers have the electric field arranged parallel to the particle trajectory, and with isobutane as the counting gas, the intrinsic energy resolution for fragments with m ⋍ 100 amu is typically 400 keV. The combination of both the time-of-flight and ionization chamber, to produce an energy-time-of-flight spectrometer, was tested with fragments from the 235U(n, f) reaction on an external thermal neutron beam at the ILL/Grenoble. In the light group of fission fragments, all masses are resolved individually with a mass resolving power m/ δm = 170 for m = 95.

Time-of-flight system for profiling recoiled light elements

The Elastic Recoil Detection method developed at University of Montreal to profile light elements in solids has been improved significantly. In this method, the light elements are ejected out of the sample by an incident 30 MeV 35Cl beam. In the original technique, the concentration profiles are extracted from the energy spectra of the recoiling particles. Some ambiguity occurs when two or more recoil particles have the same energy. This problem can be solved using a time-of-flight technique to measure the masses of the recoil particles. We use a thin carbon foil and a micro-channel plate as a start detector and a Si detector as a stop detector. This system gives a time resolution of 250 ps fwhm for 5.48 MeV alpha particles. It has been used successfully to separate the profiles of H, B, C, N and O. This method should prove useful to complement the profiles of heavy elements obtained by RBS.

Fission of polonium, osmium, and erbium composite systems

Fission cross section excitation functions were measured from near threshold to \ensuremath{\sim}10 MeV/nucleon using $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16,18}\mathrm{O}$, $^{24,26}\mathrm{Mg}$, $^{32}\mathrm{S}$, and $^{64}\mathrm{Ni}$ beams. The systems studied included $^{210}\mathrm{Po}$ formed in $^{12}\mathrm{C}$ and $^{18}\mathrm{O}$ induced reactions; $^{186}\mathrm{Os}$ formed in $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{26}\mathrm{Mg}$ reactions; and $^{158}\mathrm{Er}$ formed in $^{16}\mathrm{O}$, $^{24}\mathrm{Mg}$, $^{32}\mathrm{S}$, and $^{64}\mathrm{Ni}$ reactions. In addition, the composite systems $^{204,206,208}\mathrm{Po}$ formed with $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$ projectiles were studied. In the experiment the velocities and emission angles of two coincident fission fragments were measured using position-sensitive multiwire proportional counter stop detectors and a thin (\ensuremath{\sim}200 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) gas start detector. The measured fission excitation functions along with previous data from $^{4}\mathrm{He}$ and $^{11}\mathrm{B}$ bombardments for the $^{186}\mathrm{Os}$ and $^{210}\mathrm{Po}$ systems and recent data on the $^{200}\mathrm{Pb}$ system are compared to statistical model calculations using recent angular momentum dependent fission barriers calculated by Sierk. Comparisons of calculated and measured fission excitation functions show good overall agreement between data and theoretical predictions. These comparisons give good agreement for both a wide range of mass of the composite system and for a wide range of mass asymmetry in the entrance channel. It is concluded that the fission barriers of Sierk give a good description of both the mass and angular momentum dependence of fission barriers in this region.

Precise angular distribution measurements for high energetic ions

The reliability of a position sensitive parallel plate avalanche counter as a start detector at the entrance of a magnetic spectrometer is demonstrated. Implications for experiments with high energetic heavy ions are drawn from the results.

Detection efficiency of a heavy ion time-of-flight spectrometer with thin carbon foils in the start detector

Detection efficiency of a heavy ion time-of-flight spectrometer with thin carbon foils in the start detector

A time-of-flight method for stopping power measurements of bunched ion beams

Capacitive probes are used as start and stop detectors for a non-destructive time-of-flight spectrometer. The method is demonstrated for measuring the energy losses in thin solid targets. The stopping power results of 3.6, 4.7, 5.9, 8.2, and 10.0 MeV/u Kr-ions in Al, Ti, Ni, Zr, Ag, and Au are presented and compared to available experimental and tabulated results.

Time-of-flight telescope for mass identification of heavy ions

A time-of-flight telescope, employing microchannel plate electron multipliers as start detector and a silicon surface barrier as stop detector, is presented. The measured intrinsic time resolution is 160 ps FWHM for262Cf α-particles and 235 ps FWHM for252Cf fission fragments selected in the energy rangeE = (79÷80) MeV. The contribution of each fragment mass is estimated to be 120 ps. The measured detection efficiencies are ≿95%, depending on ion masses.

Particle identification for heavy ions in a time-of-flight spectrometer

A time-of-flight (TOF) spectrometer has been constructed at the JAERI 20 MV tandem accelerator facility. A position-sensitive start detector, which consists of a thin carbon foil, microchannel plates and a resistive plate, was developed for the TOF measurements through the spectrometer. The time and position resolutions obtained were 120 ps and 0.3 mm for α particles from 241Am, respectively. A two-dimensional position-sensitive detector was also developed to measure the solid angle of the spectrometer and the maximum solid angle obtained was 9.5 msr. As a particle detector a Bragg curve ionization chamber was developed. From the Bragg curves of heavy ions in the detector, energies, ranges and Bragg curve peaks were measured and used for particle identification. The resolving power Z/ ΔZ of the atomic number was about 50.

Detection efficiency of a heavy ion time-of-flight spectrometer with thin carbon foils in the start detector

We report measurements of the effect of heavy-ion scattering due to the passage through carbon foils with thicknesses of 4.5, 6.7, and 16.9 μg/cm2 in a wide energy range. For comparison of the results with theoretical predictions the actual foil thicknesses were determined by scaling energy loss measurements with tabulated stopping powers. Using these thicknesses the measurements in general agree well with theory. However, particularly for the 4.5 μg/cm2 foil, significant deviations from the specified thickness have been found very likely due to additional cover layers. The results are used to determine the loss in detection efficiency for low-energy heavy ions in instruments based on the time-of-flight method with thin carbon foils acting as converters for secondary electrons.

An all electrostatic mass spectrometer

An all electrostatic mass spectrometer capable of very high mass resolution, broad band mass analysis and high sensitivity is proposed. The system achieves its high mass resolution by a novel time of flight detector in which ion flight times are essentially isochronous with respect to small variations in ion energy, incident angle and initial displacement at the start detector. This “isochronator” would be used in conjunction with a small tandem accelerator as a molecular disintegrator, electrostatic analyzer and an electrostatic ion injector. The capability of this system and some of its applications are discussed.

Magnetic quadrupole spectrometer for time-of-flight measurements

A time-of-flight spectrometer has been constructed at the JAERI 20 MV tandem accelerator facility. A position-sensitive start detector (PSSD), which consists of a thin carbon foil, microchannel plates and a resistive plate, was developed for the time-of-flight measurements through the spectrometer. The time and position resolutions were found to be 120 ps and 0.3 mm (fwhm) respectively for α particles from 241 Am. The position sensitivity of the PSSD is useful for correcting not only the flight-path difference, but also the kinematic effect of the reaction products. A two-dimensional position-sensitive detector was also developed to measure the solid angle of the spectrometer and the maximum solid angle was obtained to be 9.5 msr.

Large area position-sensitive channel-plate detector system and ΔE-E ionization chamber for a time-of-flight spectrometer

For the magnetically focussed time-of-flight spectrometer installed at GSI, Darmstadt, a detector system consisting of two channel-plate time-pick up units and an ionization chamber for ΔE-E measurement has been developed. The design of the time-pick up system utilizes secondary electrons, which are produced by heavy ions passing through a thin carbon foil, and are deflected magnetically by 180° onto a channel-plate with position readout, providing a position resolution of 0.6 and 0.4 mm fwhm for the start and stop detector respectively. The intrinsic angular resolution of the spectrometer, determined by the positions at the two detectors, is Δθ < 0.02°. Due to target effects and beam conditions this deteriorates to typically 0.1°. The overall time-resolution is 200 ps fwhm at an efficiency of 95%. The energy- and Z-resolution of the ionization chamber for 64Ni-ions of 5–7.9 MeV/amu are ΔE/E = 0.5% and ΔZ/Z = 1.3%, respectively. The general properties, the construction and the time- and position-resolutions achieved in a recent experiment are described.

A position-sensitive transmission time detector

A position-sensitive channel plate time detector for heavy ions was developed. Secondary electrons from a thin carbon foil (15 × 50 mm 2) are accelerated in an electric field and then deflected by 90° onto a channel plate with a position-sensitive anode using an electrostatic mirror. This arrangement allows the foil to be perpendicular to the particle trajectories. The time resolution was found to be 150 ps, the position resolution 1.3 mm (fwhm). The device serves as start detector for the time-of-flight measurement through a magnetic spectrometer. From the position information the angle of incidence into the spectrometer is derived.

A low-mass, fast ionization chamber for detection of fission fragments

A low-mass, fast ionization chamber for use as start detector in time-of-flight measurements of the 252Cf spontaneous fission spectrum has been developed. For this chamber (mass approximately 0.9 g) a time resolution of 0.5 ns (fwhm) and a total efficiency of >99% for the fission fragments at a bias level well above the α-pulse heights could be achieved at a source strength of 15 × 10 3 fissions/s. It is estimated that these propertie can be maintained up to about 5 × 10 5 fissions/s. Due to the very high efficiency the chamber can be expected to have only a very small dependence of its start efficiency on neutron energy and direction. This important feature was also directly confirmed by comparison of neutron spectrum measurements at different directions relative to the chamber axis. Finally the paper gives detailed calculations of the scattering corrections due to the chamber materials.

Performance of a heavy-ion time-of-flight spectrometer

A heavy-ion time-of-flight spectrometer has been brought into operation at the Brookhaven National Laboratory (BNL) MP Tandem-Van-de-Graaff facility. The start detector consists of a very thin plastic scintillation detector and the stop detector is a commercial surface-barrier detector. The design and operation of the spectrometry are discussed. An overall time resolution of ∼0.3 ns is found to be routinely attainable under realistic experimental conditions. Plans to include a magnetic quadropole lens in the flight path as well as the ultimate application of the spectrometer are discussed.