Experimental evidences of 95 mTc production in a nuclear reactor

https://doi.org/10.1016/j.apradiso.2018.02.001Get rights and content

Highlights

  • 95 mTc has been identified in solutions of 99 mTc obtained in a nuclear reactor.

  • Characterization was performed by gamma spectrometry and half-life determination.

  • The possible ways that lead to the 95 mTc production are discussed.

Abstract

95 mTc has been identified as by-product in some solutions of 99 mTc obtained by irradiation of molybdenum trioxide in a reactor neutron flux. The characterization was carried out using both measurements by gamma spectrometry and half-life determination. The possible ways that lead to the 95 mTc production in a nuclear reactor are discussed.

Introduction

Among the long-lived technetium radioisotopes, the most frequently used is, possibly, 95 mTc. Because of its convenient 61 d half-life (National Nuclear Data Center, 2016) and the multiple gamma transitions associated with its decay, 95 mTc is employed as tracer, in order to study the dissemination of the element in the environment and in many animal and vegetal systems.

The production of 95 mTc rests on the use of cyclotrons, by bombardment of molybdenum with protons or deuterons, or alpha particles on niobium. In connection with reactor neutrons, the only mechanism that can lead to 95 mTc is through the reactions: 96Ru(n,d + n,np + n,pn)95 mTc, which require high-energy neutrons and are seldom produced. The authors could not find in the literature any mention regarding the use of this hypothetical mode of production, nor respecting any other alternative mode in nuclear reactors. The possibility of its occurrence has been mentioned in connection with the use of 14 MeV neutrons (Nagai and Hatsukawa, 2009).

Although the considerations formulated in the precedent paragraph seem to suggest that 95 mTc could not be obtained in nuclear reactors, the authors found that, under some special conditions, this radionuclide actually appears as by-product of the 99 mTc production. The present work deals with the experimental evidences of this finding and the possible explanations of the results.

Section snippets

General considerations

99 mTc is by far the nuclide preferentially used in nuclear medicine with diagnostic purposes. Some of the reasons that explain this priority are its chemical versatility, its very convenient half-life and the easily detectable gamma transition from its isomeric transition decay, respectively 6.0067 h and 140.511 keV (National Nuclear Data Center, 2016).

The production of 99 mTc makes focus on 99Mo, its radioactive mother of 65.976 h half-life (National Nuclear Data Center, 2016), and

Experimental

The fact that led to the identification of 95 mTc as by-product of 99 mTc was accidental: a nuclear medicine service informed that solutions of methylene diphosphonic acid labelled with 99 mTc supplied by IPEN, showed a pale bluish coloration. The Laboratory of Radioisotope Production carried out different tests on the solutions and demonstrated the chemical nature of the problem, caused by minimal quantities of molybdenum. The limit of concentration necessary to avoid the coloration was

Results and discussion

Table 1 shows the peak analysis of one of the control samples, measured after the decay of 99 mTc. The results are coherent with the chemical evidences: only technetium and rhenium isotopes were identified in the samples. Rhenium is frequently found as impurity of molybdenum compounds, even those of analytical grade (Samadi et al., 1975, Lee et al., 1985). The chemical behaviour of technetium and rhenium is very similar; thus, a reasonable expectation is that small activities of rhenium

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