Frank J. Lynch

Time Dependence of Scintillations and the Effect on Pulse-Shape Discrimination

The time dependence of the light following excitation by gamma rays and neutrons was measured for stilbene and for the liquids NE-213, NE-213M, and NE-218.? In addition, the number of photoelectrons produced at the cathode of an RCA-8575 photomultiplier per keV energy loss in the scintillator has been determined for each of these scintillators. A straightforward technique for pulse-shape discrimination is described; it consists in measuring the time difference between the start and the instant at which the integrated photomultiplier pulse reaches a specified fraction of its final amplitude. From the measured scintillation decay, the probability distribution of these rise times has been calculated for different fractions and for different photon and neutron energies on the assumption of Poisson statistics. These calculated distributions are in agreement with the experimental results obtained with the above-mentioned technique. The pulse-shape discriminating system is very stable and will accept 80% of the neutrons at 350 keV and only approximately 1% of the gamma rays producing scintillations with equal integrated light.

Improved Timing with NaI(Tl)

By timing on the earliest component of the anode current pulse of the photomultiplier, the time resolution obtained with NaI(TI) at 300°K can be made somewhat superior to that reported for pure NaI at 77°K. For example, the time distribution for x rays (31. 6 keV) relative to the conversion electrons from a Cs137 source, shows a full width at half maximum of 1.6 nsec and a slope corresponding to a half life T? = 0.63 nsec. This implies that ? /R is much smaller than commonly believed, where ? is the mean life of the scintillation and R is the average number of photoelectrons per event. This small value is not due to a small ? because only the usual slow component is observed when the shape of the scintillation is measured by a probability technique. Therefore R must be larger than expected; the observed time resolution requires at least 8.7 photoelectrons/ keV. This is consistent with the value 9.3 p.e./ keV inferred from the pulse-height resolution obtained with artificial light pulses which are equivalent in amplitude to a 662-keV event in NaI(Tl). It is also consistent with the value (9. 1 p.e./keV) obtained by combining the absolute efficiency of NaI(Tl) measured by Van Sciver and Bogart with the quantum efficiency that RCA lists for the bi-alkali photocathode.

Basic Limitation of Scintillation Counters in Time Measurements

The time resolution of a scintillation detector can be calculated from the scintillator illumination function, and the response of the photomultiplier to an instantaneous pulse of light. Such calculations using the parameters of present scintillators and photomultipliers agree with measured resolutions and predict improvements which can be achieved. Parameters of commercial and some promising experimental fast scintillators are given, as well as measurements on transit time spread of some fast photomultipliers.

New Liquid Scintillators with Higher Speed and Efficiency

The variation of light intensity with time following excitation by gamma rays has been measured for toluene solutions of quaterphenyl, quinquiphenyl, and sexiphenyl compounds whose solubilities were enhanced by substitution of alkyl groups. The decay curves, measured by use of a light-intensity-sampling technique, were analyzed to yield the mean lives ?1 for energy transfer from solvent to solute, ?2 for decay of the fast component (from the singlet state), and ?3 for decay of the slower component (probably from excimers). Preliminary estimates of ?2 for quaterphenyl, quinquiphenyl, and sexiphenyl are 1.27, 1.22, and 1.02 nsec, respectively. As expected, ?1 appears to vary inversely with solute concentration, and at the highest concentration of quaterphenyl in toluene (~ 65 g/liter, y = 1.12 × 10-2) was < 0.1 nsec. At this concentration, the amount of light in the slower component was only about 1/6 of the total light. This indicates that formation of excimers is inhibited. In addition, the photoelectrons produced in an RCA-8575 photomultiplier per keV of energy lost in the scintillator was measured to be 2.1 for quaterphenyl, 2.2 for quinquiphenyl, and 1.9 for sexiphenyl solutions. The improvement in efficiency and speed is reflected in a corresponding improvement in the time resolution achievable with these scintillators. For a 300-keV energy loss in the quaterphenyl solution, the FWHM was 0.20 nsec and the effective T1 /2 was 0.035 nsec.

Ultra-Short Pulies of Heavy Ions

The bunching requirements for a heavy-ion tandemlinac accelerator are defined and a bunching system to satisfy these requirenents is outlined. This discussion introduces an experimrent on the bunching of 45 MeV 16O ions by means of a ¿/2 superconducting-helix resonator. The measured ion-bunch width is 64 psec, a value daminated by the resolution width of the ion detector. By correcting for the detector-resolution width one infers that the ion bunch itself is <40 psec wide.

New Fluorescence Photometer

An instrument for microanalytical analysis by a fluorescence method which is sensitive, linear, and stable is described. Both an unknown and a reference sample of the same constitution are excited to fluorescence by ultraviolet light emitted in pulses by an ac powered mercury arc lamp. The resulting pulsed fluorescent light from each sample is incident on a separate vacuum phototube. The ratio of the fundamental ac components of the photoelectric currents flowing in the two phototubes is measured automatically by a servo‐mechanism. In such a system, the measurement is independent of the intensity of the exciting ultraviolet light, the thermionic currents of the two phototubes, and the temperature of the fluorescent samples. A simplified optical system provides good efficiency. Either solid or liquid samples may be rapidly measured. For uranium analysis the instrument may be calibrated to read full scale for 5·10−10, 5·10−9, 5·10−8, and 5·10−7 gram of uranium. Although it is designed primarily for uranium ...

Development and Operation of a Prototype Superconducting Linac for Heavy-Ion Acceleration

A prototype superconducting-helix accelerator is described and design considerations are discussed. The results obtained during 120 hours of beam acceleration are given. These include a wealth of practical engineering experience, the demonstration of stable operation with external phase control, and measurements of various kinds of accelerator-physics data.

Direct Position and Time Digitizer for Position-Sensitive Proportional Counters

A circuit for directly digitizing the location and flight time of neutrons detected in a 2-dimensional position-sensitive proportional counter has been constructed and is now in use here in diffraction experiments with the pulsed neutron source. The position along each coordinate is digitized directly by counting pulses from a 250 MHz clock during the time interval corresponding to the difference in rise times of pulses from the opposite ends of a resistive cathode. In addition, a slower time base is included for digitizing the neutron time-of-flightlight. The circuits, on a single CAMAC board, provide 8 binary bits of x position, 8 bits of y position and 12 bits of time without adding to the system deadtime. All-digital circuitry assures linearity, stability and reliability.

NEW LIQUID SCINTILLATORS WITH FASTER RESPONSE AND HIGHER EFFICIENCY.

Abstract The variation of light intensity with the time following excitation by gamma rays has been measured for a quaterphenyl compound whose solubility was enhanced by substitution of alkyl groups. The decay curves, measured by use of a probability-sampling technique, were analyzed to yield preliminary estimates of the time constants τ1 related to energy transfer from solvent to solute, τ2 related to the rapidly decaying component from the singlet state, and τ3 related to the slower component which is probably due to excimers. A solution (∼65 g/liter, γ = 1.12 × of 11, 44-bis(2-butyloctyloxy)-p-quaterphen (QP-G12) in toluene shows τ1 ∼ 0.05 nsec, τ2 ∼ 1.4 nsec, and τ3 ∼ 11.4 nsec. The intensity of the slow component (mean life τ3) is about 20% of that of the fast component (mean life τ2). This is about a third of the slow/fast ratio measured for 2,5-diphenyloxazone (PPO) in toluene at the same mole fraction. This indicates that QP-GI2 does not form excimers as readily as PPO does. For bothsolutes, measu...

Precision Integrator for Ion Beams

The integrator consists of a circuit which operates relays to turn off data‐storage devices, timers, etc., when the charge conducted by an ion beam reaches a specified quantity. A vibrating‐reed electrometer (without feedback) senses when the potential drop across a capacitor into which the beam current flows becomes equal to the potential of several standard cells in series. Errors in sensing the potential are less than 0.001%, and the over‐all time delay of the relay operation is about 15 msec. The major sources of error are associated with soakage and changes in capacitance with temperature, less than 0.05% and about −0.01% per °C, respectively. It should be feasible to attain a considerable reduction in all the errors of integration caused by these known sources of error.