Elsevier

Physics Letters A

Volume 255, Issues 4–6, 17 May 1999, Pages 221-229
Physics Letters A

The deuteron binding energy and the neutron mass

https://doi.org/10.1016/S0375-9601(99)00078-XGet rights and content

Abstract

A new value for the deuteron binding energy of S(d)=2.38817007(42)×10−3 u is reported based on an absolute wavelength determination of the 2.2 MeV n–p capture gamma-ray using a crystal diffraction spectrometer. A new more precise value for the neutron mass mn=1.00866491637(82) u is obtained by summing this binding energy and the 2H−1H mass difference.

Introduction

In 1986 a new value for the deuteron binding energy S(d) measured with a flat crystal spectrometer and calibrated crystals was published [1]. The measured energy had a relative standard uncertainty (relative estimated standard deviation) ur of about 1×10−6 and was approximately 8×10−6 S(d) larger than previous determinations. This measurement was one of the first results obtained at the GAMS4 crystal diffraction facility which was being developed at the high flux reactor of the Institut Max von Laue-Paul Langevin (ILL) 2, 3. When this measurement was combined with accurate atomic mass measurements, an improved value for the neutron mass was obtained with an uncertainty which was dominated by the then available mass measurements [4].

Since that time, two significant developments related to the deuteron binding energy have occurred. First, new mass spectroscopy techniques have been developed which are more than an order of magnitude more accurate than previous atomic mass determinations 5, 6. When these new mass measurements are combined with the 1986 deuteron binding energy measurement to obtain a value for the neutron mass, the uncertainty is completely dominated by the gamma-ray measurements. Second, the GAMS4 crystal diffraction facility has been continually improved during the last ten years toward the goal of providing gamma-ray wavelength measurements for 0.1 MeV<E≤6 MeV with ur≈1×10−7. There have been a number of improvements which, we believe, reduce or eliminate some of the errors which were present in the earlier measurement. Both of these developments justify the remeasurement of the deuteron binding energy.

In two experimental campaigns, separated by about 3 years (February 1995 and March 1998), we have remeasured the 2.2 MeV gamma-ray emitted in the reaction n+p→d+γ, which after correction for recoil, is the deuteron binding energy. The uncertainty has been reduced by a factor of five, which leads to a reduction in the uncertainty of the neutron mass by 2.5.

In addition to its contribution to the neutron mass, this gamma-ray is an essential input in the determination of the values of high energy gamma-ray wavelengths from the atomic mass scale. In a typical neutron capture reaction, n+AXA+1X+γ's, the binding energy of the captured neutron can be determined in two ways: (1) by measuring and summing, after correction for recoil, the appropriate gamma-rays; and (2) by measuring the relative atomic mass difference Ar(n)−[Ar(A+1X)−Ar(AX)]. For reactions where the mass difference is measured with greater accuracy than the gamma-rays, the mass difference route is employed to set the gamma-ray scale. However, the mass difference route requires knowledge of a value for the neutron mass which, in turn, depends on the deuteron binding energy.

Section snippets

The experiment

The GAMS4 crystal diffraction facility is a precision gamma-ray metrology laboratory coupled to a reactor port which is specially equipped to transport and hold sources in a position tangential to the reactor core. It is an ideal facility for studying prompt gamma-rays. The gamma-rays are diffracted by a two axis flat crystal spectrometer used in transmission. The precision wavelength measuring capability of GAMS4 results from the incorporation of two unique features. First, the crystals are

The results

In each configuration, a group of 4 profiles, recorded in the sequence +,−,−,+ (where + denotes positive and − denotes negative n) were used to determine θBragg at 22.5°C from the optical angle interferometer fringe numbers and the value of the calibration constant at that temperature and time. Table 2 gives the date, configuration, number of Bragg angle measurements, and mean first order Bragg angle and standard deviation of the mean for each date and configuration. In order to obtain a final

Discussion

In order to compare the value for S(d) obtained in this work with the value for S(d) published in 1986, it is necessary to apply two corrections to the 1986 value. First, in the appendix to Ref. [1], we noted that there was an apparent fractional discrepancy in the then available absolute lattice spacing measurements of about 1.8×10−6. This discrepancy is now understood and the lattice spacing used in the 1986 publication needs to be reduced by 1.73×10−6

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