Determination of the ¹²⁹I transmutation rate and ¹²⁹I(n, 2n)¹²⁸I correction using gas mass spectrometry

Abstract

Two groups of ¹²⁹I and ¹²⁷I targets were analyzed using a gas quadrupole mass spectrometer (QMS) to determine the transmutation rates via the melting method. Sodium iodide was chosen to make the target. The iodine composition in the ¹²⁹I targets is 82.7% ¹²⁹I and 17.3% ¹²⁷I. The transmutation rate of the ¹²⁹I(n, γ)¹³⁰I reaction was determined by measuring the ¹³⁰Xe with QMS. An equivalent corrective method was brought out to correct the ¹²⁹I(n, 2n)¹²⁸I branch which is interfered with by the ¹²⁷I(n, γ)¹²⁸I reaction. And the correction formula was deduced in theory. For very little ¹²⁸Xe from the ¹²⁹I(n, 2n)¹²⁸I reaction, the equivalent corrective method could not be suitable here. However, it is suitable for the mass of ¹²⁸Xe from ¹²⁹I(n,2n)¹²⁸I reaction that reaches the accurately detective level of the mass spectrometry.

Keywords:

• ¹²⁹I
• transmutation rate
• mass spectrum
• xenon
• melting
• ¹²⁷I

1. Introduction

Artificial transmutation of ¹²⁹I is an alternative way to reduce its radiotoxic risk [1–4]. Stable xenon isotopes are the main products of this artificial work. ¹²⁷I is the stable iodine isotope of ¹²⁹I and usually chosen instead of ¹²⁹I in relevant transmutation experiments in early researches[5,6]. In 1997, researchers from the Netherlands Energy Research Foundation (ECN) who gave their long-duration (192.95 days) transmutation results of ¹²⁷I which had been irradiated in the high flux reactor at Petten[5]. In 2004, Japanese researchers and their Dutch co-workers used ¹²⁷I in another long-duration (271.23 days) transmutation experiment at Petten[6]. However, in the two experiments above, the xenon isotopes releasing action was not carefully researched. In 2010, a target-melting method was brought forward and adopted by our team where it was observed that 99.9% of the ¹²⁸Xe had been released from the target and thus an accurate measurement result of the transmutation rate was obtained[7]. And this method was adopted again to measure the ¹²⁹I transmutation rate this time.

Additionally, the iodine isotopic composition corresponds to 85–87% ¹²⁹I and 13–15% ¹²⁷I, depending on the sample. Moreover, the ¹²⁹I(n,2n)¹²⁸I reaction and the ¹²⁷I(n,γ)¹²⁸I reaction have the same product: ¹²⁸I. Thus when taking the double isotopic nature of the iodine samples into account, the relevant question arises: what proportion do the two reactions form the ¹²⁸I, respectively? In 2002, using the ¹²⁷I(n,γ)¹²⁸I and ¹²⁹I(n,γ)¹³⁰I cross sections from the JENDL-3 and EXFOR libraries, Henzl et al. calculated the ¹²⁷I(n,γ)¹²⁸I reaction product on the basis of the mass of¹²⁷I and ¹²⁹I under spallation neutron conditions[8]. They concluded that 93–95% of the total ¹²⁸I had been produced from the reaction ¹²⁹I(n, 2n)¹²⁸I. However, their calculation method contained a lot of parameters and the result had been large error inevitably.

In this work, an equivalent corrective method was derived to deduct the ¹²⁸I from ¹²⁷I(n,γ)¹²⁸I reaction on the basis of the accurate measurement of ¹²⁸Xe using QMS.

2. Experimental

2.1. Iodine targets preparation

As shown in Figure 1 , the target core was sealed by 6061 Al alloy and eligible in leakage check. The core included three parts: 6061 Al bowl, NaI powder, and Al powder (99.99%). The granule diameter of NaI and Al powder was about 50 μm. The Na¹²⁷I crystal was produced by General Research Institute for Nonferrous Metals. It was a national standard material. The purity of Na¹²⁷I was 99.99%. The mixture powder of Na¹²⁷I and Na¹²⁹I was obtained from the solution, which had been heated at 353 K until changed to powder.

Figure 1 Illustration of the target

The Na¹²⁷I used in ¹²⁷I target was directly weighted. When preparing the ¹²⁹I target, two piles of NaI (including Na¹²⁷I and Na¹²⁹I) powder were weighted at the same time. One pile was used to prepare the target and the other was used to measure the mass of ¹²⁷I and ¹²⁹I with the ICP-MS (Inductively Coupled Plasma – Mass Spectrometer) by using isotope dilution method. The Nu Plasma MC-ICP-MS produced by Nu Instruments Company was employed to do this work and two ion receiving channel was used to measure the ions of ¹²⁷I and ¹²⁹I at the same time. To lessen the interference from Xe, the high pure Ar gas was used and the ¹²⁹I concentration of the solution was set to a high level. At the same time, decreasing the plasma power was a good method to decrease the interference for the high ionization energy of xenon. The mass discrimination factor was obtained from another national standard material Te. The factor of ¹²⁸Te/¹³⁰Te was used as that of ¹²⁷I/¹²⁹I for very close mass number. To get fine data, the ratio of the sample signal and the background signal is more than 300. Moreover, the ¹³¹Xe signal was used to deduct the 129 amu background signal. The probability of producing H¹²⁶Xe⁺ or H¹²⁸Xe⁺ in the ion source was at 10⁻⁵ grade and could be neglected. The total determination error was about 0.5%. The measured iodine isotopic composition in ¹²⁹I targets was 82.7 % of I-129 and 17.3% of I-127 in mass. The mass of ¹²⁷I and ¹²⁹I in the ¹²⁹I target was obtained from two weighted values and the measured mass ratio.

Two groups of targets were made in this experiment. Each group includes one ¹²⁷I target and one ¹²⁹I target. All targets had the same structure as shown in Figure 1. The ¹²⁷I target was used to be the equivalent target of the ¹²⁹I target. All target cores were pressed under 400 MPa pressure. Table 1 lists the details of four targets in this experiment.

Table 1 Details of four targets.

Number	^127I/g	^129I/g	Target type	Pressure/MPa	Irradiation position
7-1	0.02132	–	^127I	400	Center water cavity
9-1	0.00449	0.02153	^129I	400	Center water cavity
7-2	0.04751	–	^127I	400	Center water cavity
9-2	0.00429	0.02055	^129I	400	Center water cavity

2.2. Irradiation

All targets were irradiated in No. 8 channel of the center water cavity of the Xi’an pulsed reactor[9]. As shown in Figure 2 , four targets were separated to two groups. Group 1 included 9-1 and 7-1. Group 2 included 9-2 and 7-2.

Figure 2 Targets location in the irradiation tube

The ¹²⁹I target was above the ¹²⁷I target. Two groups of targets lied in the middle of irradiation tube where there was steady neutron flux. This arrangement ensured that the transmutation rates of ¹²⁷I in two kinds of targets could be as same as possible. That was the equivalent correction condition.

Neutron conditions in center water cavity are: thermal neutron flux is about 5.0 × 10¹³n/(cm²·s) and epithermal neutron flux is about 7.7 × 10¹¹n/(cm²·s). All targets were irradiated for 6 hours.

2.3. Postirradiation analysis

2.3.1. Online isotope dilution mass spectrum analysis

GAM400 QMS (produced by IPI Corp. in 2003 in Germany) was used to measure the xenon isotopes. The lower detection limit of GAM400 QMS for xenon is 2 × 10⁻¹² mol. The error of measuring 10⁻¹⁰ mol Xenon is 1%[10]. Two standard gases are adopted in our experiment. One is the xenon isotopes standard gas which was bought from IRMM (Institute of Reference Materials and Measurement, Belgium). The other is the dilution gas calibration standard gas which was produced by the IPI Corp. allying with NINT.

All targets were measured with melting method after 30 cooling days. The details of melting method were described in [7]. Online isotope dilution mass spectrum method was used to analyze the ¹²⁸Xe and the ¹³⁰Xe. The ¹²⁹Xe was selected as the xenon dilution isotope. The isotope ratio correction coefficients of ¹²⁹Xe/¹²⁸Xe, ¹²⁹Xe/¹³⁰Xe, and ¹²⁹Xe/¹³²Xe were measured with the xenon isotopes standard gas before measurement. Then the dilution isotope ¹²⁹Xe was calibrated by the calibration gas at first. When measuring, sample a part of the released xenon gas and the ¹²⁹Xe dilution gas, respectively. And mix two kinds of gases and get rid of non-noble gas by a gas getter pump which was an alloy of Zr-V-Fe and worked at 923 K to adsorb over 99% non-noble gas including hydrogen. For the 1000 Pa pressure limitation of the inlet channel, this purification process could remarkably increase the sensitivity and decrease the interference including from the hydrogen. After that, the leaving mixed gas was inlet in the analyzer to measure the ratios of ¹²⁹Xe/¹²⁸Xe and ¹²⁹Xe/¹³⁰Xe. At last, the number of ¹²⁸Xe atoms in the released xenon can be calculated by formula (1).

¹²⁸ N _S: the number of atoms of ¹²⁸Xe in sample gas, unit, mol.

¹²⁹ N _D: the number of atoms of ¹²⁹Xe in dilution gas, unit, mol.

η _D: sampling proportion of dilution gas.

η _S: sampling proportion of sample gas.

^129/128 R _D: isotopic ratio of ¹²⁹Xe and ¹²⁸Xe in dilution gas.

^129/128 R _S: isotopic ratio of ¹²⁹Xe and ¹²⁸Xe in sample gas.

^129/128 R _MS: isotopic ratio of ¹²⁹Xe and ¹²⁸Xe in mixed gas.

^129/128 ρ: the isotopic ratio correction coefficients of ¹²⁹Xe/¹²⁸Xe.

^129/128 R _S was very small compared with ^129/128 R _MS because of the very little ¹²⁹Xe in the background gas in the target. So ^129/128 R _S can be neglected. Formula (1) changes to a brief one:

At the same time, the number of ¹³⁰Xe atoms could be also obtained by formula (2) after change of the up-marker “128” with “130”. That was:

In order to validate what percent of the released xenon proportioned in melting-cooling process, we pumped the stainless steel container to 10 Pa after measuring the released xenon and melted the target 9-2 again. The second released gas from the target 9-2 was measured again by QMS. Detail data were shown in Table 2 .

Table 2 Determination results of four targets after the irradiation.

				Release	(n,2n) Correction	Transmutation	Transmutation rate
Number	Release mode	^128Xe/mol	^130Xe/mol	rate	^128Xe/mol	rate	determination error
7-1	700°C, melting	7.94 × 10^−10	–	99.9%	–	5.04 × 10^−6	1.0%
9-1	700°C, melting	1.96 × 10^−10	2.78 × 10^−9	99.9%	3.05 × 10^−11	1.68 × 10^−5	1.5%
7-2	700°C, melting	1.74 × 10^−9	–	99.9%	–	4.95 × 10^−6	1.0%
9-2	700°C, melting (first)	1.75 × 10^−10	2.65 × 10^−9	99.9%	1.91 × 10^−11	1.67 × 10^−5	1.5%
9-2	700°C, melting (second)	<2 × 10^−12	2.98 × 10^−12	0.1%	–	–	5%^a

Deducting the background, the targets were made in air and the pressure was 96 ± 1 kPa. The air volume in each target was evaluated about 0.8 ml. The number of ¹³⁰Xe atoms in each target was calculated as 1 × 10⁻¹³ mol. The number of ¹²⁸Xe atoms in each target was calculated as 4.6 × 10⁻¹⁴ mol. The values above were too little compared with the transmutation product ¹²⁸Xe and ¹³⁰Xe. So they could be neglected.

2.3.2. ¹²⁹I(n,2n)¹²⁸I reaction rate correction with equivalent target

The ¹²⁹I transmutation rate ρ _T9 included two parts: ¹²⁹I(n,γ)¹³⁰I reaction rate ρ _T9-0 and ¹²⁹I(n,2n)¹²⁸I reaction rate ρ _T9-8. So

ρ _T9-8 can be obtained from formula (5),

¹²⁸ N _S9: the total number of atoms of ¹²⁸Xe in irradiated ¹²⁹I target, unit, mol.

¹²⁸ N _S9-7: the number of atoms of ¹²⁸Xe produced by ¹²⁷I(n,γ)¹²⁸I reaction in irradiated ¹²⁹I target, unit, mol.

ρ _T9-7: the transmutation rate of ¹²⁷I(n, γ)¹²⁸I reaction in irradiated ¹²⁹I target.

P_{β 8}: the β ⁻ emission probability of ¹²⁸I, that is, 0.931.

N _I-7: the total number of atoms of ¹²⁷I in ¹²⁹I target, unit, mol.

N _I-9: the total number of atoms of ¹²⁹I in ¹²⁹I target, unit, mol.

Define that ρ _T7 was the transmutation rate of ¹²⁷I(n,γ)¹²⁸I reaction in the irradiated ¹²⁷I target. For the equivalent transmutation rates of ¹²⁷I in two kinds of targets, there was a proximate formula: ρ _T7 = ρ _T9-7. ρ _T7 could be obtained from the ¹²⁷I target by analyzing ¹²⁸Xe.

So formula (5) could be proximately changed to formula (6):

Additionally, ¹²⁹I(n,γ)¹³⁰I reaction rate ρ _T9-0 can be calculated as:

¹³⁰ N _S9: the number of atoms of ¹³⁰Xe in irradiated ¹²⁹I target, unit, mol.

So formula (4) was changed to formula (8):

This was the corrected formula for the total transmutation rate of ¹²⁹I.

3. Results and discussions

Our measurement results are listed in Table 2.

As shown in Table 2, the proportion of release ¹³⁰Xe in the second melting process was only 0.1% (last row, column 5). So 99.9% of the xenon isotopes had been released from the target in the first melting and cooling process. This result accorded with the data of [7] very well. The data of column 3 and 4 could be directly used to calculate the transmutation rates. This calculation contributes only 0.1% for the total error.

The data of column 6 were the ¹²⁹I(n,2n)¹²⁸I reaction correction ¹²⁸Xe. We could see that the correction values were evidently larger than the theory value. Because there were very few high energy neutrons in Xi’an pulse reactor which could induce the (n, 2n) reaction, ¹²⁸Xe from ¹²⁹I(n,2n)¹²⁸I reaction was very small in theory. Therefore, a very small correction value would be obtained by using formula (8). However, the data of column 6 were not small. It was not reasonable. Why? A critical reason was ρ _T7≠ ρ _T9-7. The distance of Target 7-1 and 9-1 was about 15 mm. They were not in a plane. So the neutron conditions could not be regarded as same. Maybe this was the main reason. At the same time, Target 7-1 and 7-2 were in the same plane and their reaction rates were in accordance with 2%. So do Target 9-1 and 9-2. The neutron conditions could be therefore regarded as the same when targets were in the same plane and their distances to center line were equal.

Although we could not achieve the correction results this time, the equivalent correction method was really useful to the practice of ¹²⁹I(n,2n)¹²⁸I correction for the fast neutron transmutation experiment in which much more ¹²⁹I(n,2n)¹²⁸I reaction could be happened to produce much more ¹²⁸Xe.

4. Conclusion

The sodium iodide was used to make four targets which had been irradiated in the Xi’an pulse reactor. The transmutation rates of four targets were determined by the xenon isotope analysis with a QMS. From the experiment, we could conclude as follows:

1.	99.9% of all produced xenon in the pressed target could be released by using the melting method.
2.	Good error of the transmutation rates could be obtained by using the gas mass spectrum method.
3.	The equivalent method was brought forward firstly. But the correction results were invalid in this experiment. However, it was a feasible method to the practice of 129I(n,2n)128I correction for the fast neutron transmutation experiment.

Notes

^aThis error is for the determination of ¹³⁰Xe from the second melting.