DEVELOPMENT OF AN ION TIME-OF-FLIGHT SPECTROMETER FOR NEUTRON DEPTH PROFILING

Open Access
Author:
Cetiner, Mustafa Sacit
Graduate Program:
Nuclear Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 03, 2008
Committee Members:
  • Kenan Unlu, Committee Chair
  • Jack Brenizer Jr., Committee Member
  • Arthur Thompson Motta, Committee Member
  • Vijaykrishnan Narayanan, Committee Member
  • R Gregory Downing, Committee Member
  • Tim Z Hossain, Committee Member
  • Michael S Gordon, Committee Member
Keywords:
  • time-of-flight
  • ion time-of-flight spectrometry
  • neutron depth profiling
  • coincidence spectrometry
Abstract:
Ion time-of-flight spectrometry techniques are investigated for applicability to neutron depth profiling. Time-of-flight techniques are used extensively in a wide range of scientific and technological applications including energy and mass spectroscopy. Neutron depth profiling is a near-surface analysis technique that gives concentration distribution versus depth for certain technologically important light elements. The technique uses thermal or sub-thermal neutrons to initiate (n,p) or (n,alpha) reactions. Concentration versus depth distribution is obtained by the transformation of the energy spectrum into depth distribution by using stopping force tables of the projectiles in the substrate, and by converting the number of counts into concentration using a standard sample of known dose value. Conventionally, neutron depth profiling measurements are based on charged particle spectrometry, which employs semiconductor detectors such as a surface barrier detector (SBD) and the associated electronics. Measurements with semiconductor detectors are affected by a number of broadening mechanisms, which result from the interactions between the projectile ion and the detector material as well as fluctuations in the signal generation process. These are inherent features of the detection mechanism that involve the semiconductor detectors and cannot be avoided. Ion time-of-flight spectrometry offers highly precise measurement capabilities, particularly for slow particles. For high-energy low-mass particles, measurement resolution tends to degrade with all other parameters fixed. The threshold for more precise ion energy measurements with respect to conventional techniques, such as direct energy measurement by a surface barrier detector, is directly related to the design and operating parameters of the device. Time-of-flight spectrometry involves correlated detection of two signals by a coincidence unit. In ion time-of-flight spectroscopy, the ion generates the primary input signal. Without loss of generality, the secondary signal is obtained by the passage of the ion through a thin carbon foil, which produces ion-induced secondary electron emission (IISEE). The time-of-flight spectrometer physically acts as an ion/electron separator. The electrons that enter the active volume of the spectrometer are transported onto the microchannel plate detector to generate the secondary signal. The electron optics can be designed in variety of ways depending on the nature of the measurement and physical requirements. Two ion time-of-flight spectrometer designs are introduced: the parallel electric and magnetic (PEM) field spectrometer and the cross electric and magnetic (CEM) field spectrometer. The CEM field spectrometers have been extensively used in a wide range of applications where precise mass differentiation is required. The PEM field spectrometers have lately found interest in mass spectroscopy applications. The application of the PEM field spectrometer for energy measurements is a novel approach. The PEM field spectrometer used in the measurements employs axial electric and magnetic fields along the nominal direction of the incident ion. The secondary electrons are created by a thin carbon foil on the entrance disk and transported on the microchannel plate that faces the carbon foil. The initial angular distribution of the secondary electrons has virtually no effect on the transport time of the secondary electrons from the surface of the carbon foil to the electron microchannel plate detector. Therefore, the PEM field spectrometer can offer high-resolution energy measurement for relatively lower electric fields. The measurements with the PEM field spectrometer were made with with the Tandem linear particle accelerator at the IBM T. J. Watson Research Center at Yorktown Heights, NY. The CEM field spectrometer developed for the thesis employs axial electric field along the nominal direction of the ion, and has perpendicular magnetic field. As the electric field accelerates and then decelerates the emitted secondary electron beam, the magnetic field steers the beam away from the source and focuses it onto the electron microchannel plate detector. The initial momentum distribution of the electron beam is observed to have profound effect on the electron transport time. Hence, the CEM field spectrometer measurements suffer more from spectral broadening at similar operating parameters. The CEM field spectrometer measurements were obtained with a Po-210 alpha source at the Penn State Radiation Science and Engineering Center, University Park, PA. Although the PEM field spectrometer suffers less from electron transport time dispersion, the CEM field spectrometer is more suited for application to neutron depth profiling. The multiple small-diameter apertures used in the PEM field configuration considerably reduces the geometric efficiency of the spectrometer. Most of the neutron depth profiling measurements, where isotropic emission of charged particles is observed, have relatively low count rates; hence, high detection efficiency is essential.