Melvin S. Freedman

Correction for Phosphor Backscattering in Electron Scintillation Spectrometry

An iterative numerical integration procedure has been developed to correct both beta spectra and differential beta‐gamma directional correlations for the effects of finite instrument energy resolution including the effect of backscattering from a scintillation detector.The method has been tested and found to reproduce the true spectral shape in the cases of Re186, Au198, Tl204, and to allow separation of composite beta spectra as in the case of W187. Application of the method is limited to maximum beta energies exceeding 200 kev.

Response of NaI, anthracene and plastic scintillators to electrons and the problems of detecting low energy electrons with scintillation counters☆

Abstract The electron response of NaI(Tl) has been measured with magnetic-spectrometer-energy-selected electrons incident on cleaved crystal surfaces, in the energy range 5–1000 keV. The light output/keV (≡ response) shows a maximum in the neighborhood of 15–20 keV which is some 20% larger than that at 1 MeV; below 15 keV the response falls sharply reaching 80% of the peak value at ∼ 5 keV. The electron response of anthracene and a plastic scintillator (Pilot B) were measured under the same experimental conditions for comparison. Both anthracene and Pilot B show nonproportional response in the low energy region. At 500 keV the ratio of the responses NaI(Tl)/anthracene/Pilot B are 1.6/1.0/0.38; at 20 keV the ratios are 2.5/1.0/0.41. Decay curve analysis yields for some of the long lived states (excited by incident electrons) in NaI(Tl), 200–300 μsec, 7–9 msec, 40–80 msec, 4 sec, 1 min and 12 min; for anthracene 36 μsec, 120 μsec and 570 μsec. Relatively intense long lived states and more backscattering make NaI(Tl) not generally useful as a low energy electron detector. Uncovered anthracene mounted directly on a photomultiplier and the combination cooled to prevent sublimation of the phosphor in the high vacuum of the beta spectrometer provides a counter which has efficiencies measurable from the pulse height distributions down to ∼6 keV. At backgrounds of 300–500 cpm it has usable efficiencies down to 1 keV.

Internal conversion coefficients with relativistic hartree-fock atomic model: comparison with 67Zn

Abstract Improved agreement of measured internal conversion coefficients for 67Zn with new calculations based on a relativistic Hartree-Fock treatment with the inclusion of finite nuclear size effects was obtained, in comparison to the earlier internal conversion coefficient tabulation.

Large Volume Degausser with Gradient Compensation

A solution is presented to the practical problem of adapting known theoretical multicoil magnetic field generators for compensation of slightly inhomogeneous fields. Three concentric orthogonal sets of coils are used as uniform field generators. The two sets used to compensate the strong field components are each an array of four parallel, equal‐size square coils, separated by ⅓ of a side length, i.e., uniformly spaced on the surface of a cube. Each of these sets generates a magnetic field uniform to 2×10−4 within the central sphere of diameter equal to 0.2 of the side length of a coil. The outer pair of coils (22 turns each) and the inner pair (13 turns each) are series connected to a single current supply. By shunting small fractions of the current from one‐half of two of the three coil sets, and by appropriate location of two pairs of small coils near the main degausser coils (also series connected), field gradients that result in variations of the order of 2×10−3 of the uncompensated field over the ce...

Measurement of low energy neutrino absorption probability in thallium 205

A major aspect of the P-P solar neutrino flux determination using 205Tl is the very difficult problem of experimentally demonstrating the neutrino reaction cross section with about 10% accuracy. Based on an idea of P. Kienle, such measurements now become possible. Moreover, the proposed measurements of weak interaction rates are in the isotopic states of interest; hence they will be much more readily acceptable than would indirect measurements via strong or electromagnetic interactions or theoretical calculations. One will soon be able to completely strip the electrons from atomic 205Tl and to maintain the bare nucleus in this state in the heavy ion storage ring to be built at GSI Darmstadt. This nucleus can decay by emitting a beta-minus particle into the bound K-level of the daughter 82205Pb12−81+ ion as the only energetically open decay channel (plus, of course, an antineut This single channel beta decay explores the same nuclear wave functions of initial and final states as does the neutrino capture in atomic 205Tl, and thus its probability or rate is governed by the same nuclear matrix elements that affect both weak interactions: 81205Tl12+81+→ 82205Pb12−81++ν ν81205Tl12+0→ 82205Pb12−1++e− Measuring the rate of accumulation of 205Pb81+ ions in the circulating beam of 205Tl81+ ions gives directly the cross section of reaction (b). The calculations of the expected rates under realistic experimental conditions will be shown to be very favorable for the measurement, e.g., ∼104 ions per hour in a 100 mA beam. The great sensitivity of the rate to the 205Pb-205Tl mass difference will appear (12%/keV), showing the need to materially improve the uncertainty in this value, and several experiments will be proposed. A special calibration experiment to verify the method and check the theoretical calculations will be suggested. Finally the neutrino cross section calculation based on the observed rate of reaction (a) will be shown. Demonstrating bound state beta decay may be the first verification of the theory of this very important process that influences beta decay rates of several isotopes in stellar interiors, e.g., 187Re, that play important roles in geologic and cosmologic dating and nucleosynthesis.

Ionization by Nuclear Transitions

Ionization and excitation in an atom induced by nuclear processes [1,2] within that atom exemplify the so-called single step and two-step mechanisms. The former is characterized as a sudden shaking transition of the composite nucleus-atom system, analogous to shaking in photoemission. In the latter process the charged particles emitted in a nuclear decay transfer energy by a relatively slow “direct collision” final state interaction with atomic electrons. These are not truly distinct mechanisms; rather they are labels for the extremes of the time spans involved in different types of nuclear decays. In some theories they are even coherent interactions. For either the shaking or direct collision processes, we can give a phenomenological description of an ionization event that will apply to both types.

Wide Range, Continuously Variable High Current Generator Control

A circuit is described for controlling the output current of a high current (100 amp) dc generator driving a low impedance (2.5 ohm, 40 mh) load. The circuit features precise continuous control of the load current by a single dial potentiometer over the range from zero to full output, with 0.03 percent linearity of current vs potentiometer setting. A method is presented for reduction of current ripple from the generator in the low impedance load.

The Scintillation Efficiency of Anthracene for Low Energy Electrons

The efficiency of an anthracene scintillation counter used as the detector in a double magnetic lens beta‐ray spectrometer for electrons in the range 0–20 kev has been determined. Monoenergetic electrons generated by an electron gun were focused on the anthracene crystal, the beam being concurrently monitored by an electron collector and electrometer. The results are shown in Fig. 4.

K-electron ejection in β− decay: Pk or 12Pk?

Abstract It is argued that a recent claim that there is an error of a factor of two in the Law- Campbell theory of K-electron shake-off in β − decay is wrong.

Electron detection efficiency of a Nal(Tl) scintillator for electrons and for gamma rays above a few keV

Abstract A technique is developed for the analysis of photoelectron pulse amplitude distributions from gamma rays or electrons of selected energy observed with a GaP first dynode photomultiplier, taking into account the time-spread of photon emission from the Nal(Tl) scintillator. The observed amplitude distribution is decomposed into a unique sum of experimentally based, constructed amplitude distributions (with relative intensities fn), one for each number n of photoelectrons emitted in each detected event. It is experimentally verified that, for unique energy deposition in the scintillator, the pulse amplitude distribution gives rise to a number distribution fn, which is Poisson Pm(n), and also that the loss of detected events above a very low discrimination level is accurately given by the Poisson prediction for events which fail to release even one photoelectron, i.e., Pm(0). For a spectrum of energies actually deposited in the crystal due to various degrees of back-scattering of an impinging electron with given energy, the derived fn distribution below n = 10 is analyzed into a quasi continuum of weighted Poisson distributions. The efficiency loss for any given discrimination level can be obtained from this continuous function. The method is shown to work down to an intrinsic detection efficiency of 10% for very low energy gamma rays and down to at least 5 keV for electrons. Reasons for disagreement between observation and expectation in the range 2.6-5 keV for electrons are discussed.