Determination of the gamma emission probabilities of 239Np

239 Np is an important nuclide as the decay daughter of 239 U and it decays to 239 Pu by emitting beta particles and gamma rays with a half life of 2.356 days. The data of the emission probabilities of its gamma-rays in the open references are consistent except for the main gamma-ray of 106.1 keV, the emission probability of which varies from 25.9% to 27.2%. To verify the emission probability of 106.1 keV gamma-ray of 239 Np, a N-type coaxial HPGe detector was calibrated using 241 Am, 133 Ba, 60 Co, 152 Eu and 155 Eu reference gamma sources to get the accurate efficiency of the 106.1 keV gamma-ray. 239 Np was purified from solution containing 243 Am, where 239 Np is the alpha decay daughter of 243 Am. The specific activity of 239Np solution was determined by a 4πβ (PC)-γ coincidence counting device. There were 6 gamma sources prepared to measure with the HPGe detector, and the activity of 239 Np in each gamma source was calculated with the weights of the solution contained in it. The emission probability of 106.1 keV of 239 Np is measured to be (25.4 ± 0.3)%, which is consistent with 25.34%, the value evaluated in 2014.


Introduction
The neptunium isotope 239 Np is a short-lived γ and β particle emitting radionuclide, with a half-life about 56.5 hours. It decay towards 239 Pu with β and γ particle emission. It can be produced by disintegration of 243 Am. 239 Np is of great importance as a chemical yield tracer for the radiochemical determination of 237 Np in samples [1]. The absence of standardized solutions and the poor quality of the associated decay scheme data inhibited the use of 239 Np. For this purpose, gamma emission probabilities of 239 Np were determined in this work. Table 1 shows the half-life and γ -ray emission probabilities of 239 Np given by several different nuclear databases [2][3][4][5]. It can be seen that there is obvious differences between these results. In this paper, we focus on the γ -ray emission probability determination.

Sample chemical separation
The original 243 Am solution contains both 243 Am and 239 Np. In order to get pure 239 Np solution, chemical separation should be done by the following steps: After getting a solution of 239 Np, six sources were prepared to determined the specific activity of 239 Np solution with the coincidence method, and three sources were measured by a HPGe detector whose full-energy peak efficiency curve had already been obtained.

Measurement of 239 Np activity
Measurement of specific activity of 239 Np concentration was achieved by 4πβ(PC)-γ coincidence counting system which is a standard activity counting system. The system consists of a beta counting channel constructed from a proportional counter(PC) with P10 gas used as the counting medium and a gamma counting channel from a NaI detector. Corrections for background, dead time and accidental coincidence were carried out using classical formulae. A linear extrapolation is usually needed in this method to estimate source activity when the counting efficiency of beta channel approaches 100%. The description of linear extrapolation is as follow [6] Where N β , N γ and N c are the counting rates of the beta channel, gamma channel and coincidence channel. Six sources with masses between 68.5 mg and 92.7 mg were prepared onto metalized VYNS foils. Each source was measured with linear extrapolation by varying the threshold of beta channel to change the counting efficiency.  Table 2 shows the activity of each source measured by the 4πβ(PC)-γ coincidence system. Table 3 shows uncertainty components in the standardization of 239 Np solution. The specific activity of 239 Np solution at the reference time was determined to be (3.12 ± 0.05) × 10 4 Bq/g. The uncertainties represent two standard deviations, coverage factor k = 2.

Gamma ray emission probabilities
Gamma ray emission rate of 239 Np was determined by a N-type coaxial HPGe detector. The detector was calibrated by 241 Am, 133 Ba, 60 Co, 152 Eu and 155 Eu. Activity of all these sources had already determined by standardized measurement system with the relative standard uncertainties less than 1%. The decay data of radioactive nuclides was selected from gamma ray decay data standards for detector calibration recommended by the IAEA in 2007. The full-energy efficiency function of the HPGe is shown in Fig. 1. The full-energy peak efficiencies of gamma-ray from decay of 239 Np was obtained by intercepting the absolute full-energy peak efficiency curve also with the relative standard uncertainties less than 1% [7].  The gamma ray emission rate of 239 Np was calculated from the gamma-ray spectrum, and then the gamma ray emission probabilities of 239 Np can be determined by Eq. (3) [8].
Where N r is the is the full-energy peak areas of each gamma-ray, ε r is the full-energy efficiency of each gammaray, A is the activity of 239 Np at time zero, λ is the decay constant of 239 Np, T R is the real time of measurement, T L is the live time of measurement, T is the time from time zero to the beginning of measurement. Table 4 shows emission probabilities and uncertain of main γ -ray emitted by 239 Np. The uncertainties represent one standard deviation, coverage factor k = 1. Three sources were measured by HPGe to determine the γ -ray emission probabilities and each source contains about 200 mg 239 Np solution.

Conclusions
In this work, the principal γ emission of six energies in 239 Np decay was experimentally determined as shown in Table 4. The use of absolute counting techniques to determine the activity and calibrated HPGe γ detector has been of great importance in this work. There are still some differences between the values proposed in this work and the currently evaluated data. The emission probability of 106.1 keV of 239 Np is measured to be (25.4 ± 0.3)%, which is consistent with 25.34%, the value evaluated in 2014.