Standardization of Ca-45 radioactive solution by tracing method

Abstract

The procedure followed by the Laboratório de Metrologia Nuclear (LMN) at the IPEN, in São Paulo, for the standardization of the 45Ca is described. The activity measurement was carried out in a 4<FONT FACE=Symbol>pb</FONT>-gamma coincidence system, by the tracing method. The radionuclide chosen as the beta-gamma emitting tracer nuclide was 60Co because of its end-point beta-ray energy which is close to 45Ca. Six sources were prepared using a 1:1 ratio (beta-pure and beta-gamma) dropped directly on the Collodion film, and other two solutions of 45Ca + 60Co were mixed previously using a 1:1 and 1:2 ratio before making the radioactive sources. The activity of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer.

Standardization of Ca-45 radioactive solution by tracing method

Cláudia Regina Ponte Ponge-Ferreira; Marina Fallone Koskinas; Mauro da Silva Dias

Instituto de Pesquisas Energéticas e Nucleares, Caixa Postal 11049, 05422-970, São Paulo, SP, Brazil

ABSTRACT

The procedure followed by the Laboratório de Metrologia Nuclear (LMN) at the IPEN, in São Paulo, for the standardization of the ⁴⁵Ca is described. The activity measurement was carried out in a 4pb-g coincidence system, by the tracing method. The radionuclide chosen as the b-g emitting tracer nuclide was ⁶⁰Co because of its end-point beta-ray energy which is close to ⁴⁵Ca. Six sources were prepared using a 1:1 ratio (b-pure and b-g) dropped directly on the Collodion film, and other two solutions of ⁴⁵Ca + ⁶⁰Co were mixed previously using a 1:1 and 1:2 ratio before making the radioactive sources. The activity of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer.

1 Introduction

This paper describes the procedure followed by the Laboratório de Metrologia Nuclear (LMN) at the IPEN - CNEN/SP, in São Paulo, for the standardization of ⁴⁵Ca radioactive solution by tracing method.

This method consists of using 4pb-g coincidence method [1,2] for the standardization of a pure b-emitter mixed with another radionuclide which decays by simultaneous emission of two radiations such as b-g, a-g to be used as tracer. The tracer is standardized separately by means of conventional 4pb-g coincidence method.

In the tracing method [3,4] a series of sources containing aliquots of the pure b-emitter and a suitable b-g emitter are prepared. The observed disintegration rate of b-emitter and the tracer b-efficiency Î_bt are measured within a range of Î_bt by using external absorbers.

The results are plotted against (1-Î_bt) and the intercept corresponds to the disintegration rate of the pure b-emitter.

Radionuclide ⁴⁵Ca decays with half life of (163 ± 1) days [8] by beta transition, 0.0017% populating the excited state of ⁴⁵Sc and 99.9983% to the ground state with maximum beta energy of 256 keV. Due to the low gamma ray emission probability per decay it may be considered a pure beta emitter radionuclide. ⁴⁵Ca decay is presented in Fig. 1.

Radionuclide ⁶⁰Co was chosen as tracer because of its end-point b-ray energy (317.89 keV) which is close to ⁴⁵Ca. It decays with half-life of (5.271±0.002) years, by b^– emission populating the excited levels of ⁶⁰Ni and proceeds to ground state by emission of two main gamma rays (1173.24 and 1332.51 keV)[8].

2 Experimental Method

2.1 Source Preparation

⁴⁵Ca solution was obtained by means of ⁴⁴Ca (n,g) ⁴⁵Ca reaction in a thermal neutron flux at the IPEN 2 MW research reactor. The sources were prepared by dropping known aliquots of the solutions on a 20 mg/cm² thick Collodion film. Six sources were prepared using a 1:1 ratio (b-pure and b-g) dropped directly on the Collodion film and other two solutions of ⁴⁵Ca + ⁶⁰Co were mixed previously using a 1:1 and 1:2 ratio before making the radioactive sources.

The Collodion film was previously coated with a 10 mg/cm² gold layer in order to turn the film conductive. A seeding agent (Cyastat SM) was used to improve the deposit uniformity and the sources were dried in a warm (45 degrees Celsius) nitrogen jet. The accurate source mass determination was performed using the picnometer technique.[5] The b-g tracer was standardized previously by measuring several sources prepared by the same procedure.

2.2 4pb-g coincidence measurement

A conventional 4pb-g coincidence system was used, consisting of a 4p proportional counter filled with 0.1 MPa P-10 gas mixture, coupled to a pair of 3" x 3" NaI(Tl) crystals. The events were registered by a method developed at LMN which makes use of a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer.[7] The gamma window was set by gating the gamma-rays of tracer (1173 keV + 1332 keV).

The number of detected events in the proportional counter is given by:

where:

Î_bCo is the tracer efficiency in the mixed source;

N_0(Ca+Co) is the counting rate of proportional counter due to the mixed source;

N_0Co is the activity of ⁶⁰Co tracer of the mixed source;

N_0Ca is the ⁴⁵Ca beta-branch disintegration rate;

Î_bCa is the ⁴⁵Ca beta efficiency.

When the b-emitter and the b-g tracer are combined in a single source, a functional relationship exists between the detection efficiencies. This relation can be defined by a polynomial function G where:

Since the tracer efficiency, Î_bCo may not always be accurately obtainable from coincidence counting data, is convenient to use the expression involving only observed b-g and coincidence counting rates.

The expression can be rewritten as:

The function G' was fitted by weighted least squares using code LINFIT [9] and the extrapolation (1 – N_c/N_g)/N_c/N_g = 0 gave the expected N_0Ca value. Suitable corrections for background, decay, dead time and accidental coincidences were included in calculation.

3 Results and Discussion

Figure 2 shows the extrapolation curves obtained for the three different methods of preparing sources: mixing solutions with ratios 1:1 and 1:2 and by drops with 1:1 ratio. The b efficiency was varied using external absorbers.

The extrapolated value for the two mixing solutions were in agreement with each other, namely (154.3 ± 1.9) kBq/g and (154.3 ± 2.5) kBq/g, respectively. However, for the other preparation method (drops 1:1), the extrapolated value was (150.3 ± 1.3) kBq/g, 3% lower. The possible causes for this difference are being investigated.

Received on 8 October, 2003

• [1] A. P. Baerg, Metrologia, 3, 105 (1967).
• [2] P.J. Campion, Int. J. Appl. Radiat. Iso. 4, 232 (1959).
• [3] A. P. Baerg, S. Meghir, and G. C. Bowes, Int. Journ. Appl. Radiat. Isot. 15, 279 (1964).
• [4] A. Williams, Int. Journ. Appl. Radiat. Isot. 15, 709 (1964).
• [5] P. J. Campion, Procedures for accurately diluting and dispensing radioactive solutions. Bureau International des Poids et Mesures, Monographie BIPM - 1, 1975
• [6] A. M. Baccarelli, M. S. Dias, and M. F. Koskinas, Appl. Radiat. Isot. 58, 239 (2003).
• [7] W. O. Lavras, M. F. Koskinas, M. S. Dias, and K. A. Fonseca, Primary Standardization of ⁵¹Cr Radioactive Solution. IRPA, 2000 (CDROM).
• [8] F. Lagoutine, N. Coursol, and J. Legrand, Table de radionucléides. Laboratoire de Métrologie des Rayonnements Ionisants. Bureau National de Métrologie., 1984.
• [9] M. S. Dias, Polynomial least square fitting codes with covariance analysis. Internal Report of the Nuclear Metrology Laboratory, IPEN, 1998

Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Accepted
  08 Oct 2003
• Received
  08 Oct 2003