Absolute measurement of 126Sn radionuclide concentration with AMS

N.H

Nuclear Instruments and Methodsin Physics Research B 114 ( 1996)125- 130

ham Interactions with Materials A Atoms

ELSEVIER

Absolute measurement of 12%nradionuclide concentration with AMS P. Gartmmann a’* , R. Golser b, P. Haas b, W. Kutschera b, M. Suter a, H.-A. Synal ‘, M.J.M. Wagner a, E. Wild b a Institutfiir Teilchenphysik, ETH-Hiinggerberg, CH-8093 Ziirich, Switzerland b Institutfir Radiumforschung und Kernphysik der Unioersitiit Wien, Boltzmanngasse 3, A-1090 Vienna, Austria ’ Paul Scherrer Institut, c/o ETH Hijnggerberg, CH-8093 Ziirich, Switzerland Received 8 December 1995 Abstract A new attempt has been made at the Zurich AMS facility for absolute measurements of isotopic ratios in connection with

a project for the determination of the half-life of ‘%n [P. Haas et al., this issue, following paper]. A ‘*%n/Sn ratio of (9.23 k 0.87) x 10e6 was measured in material extracted from spent fuel rods of a nuclear power plant. Several specific problems had to be solved. For the separation of the isobaric interference of ‘26Te the method of projectile X-ray detection was applied. A gas ionization chamber was used to determine ‘*“(Sn + Te) in a second independent way. To study mass fractionation effects, several stable tin isotopes were measured. A detailed description of the experimental setup and the measuring procedure is given. The results and the various sources of uncertainties are discussed.

1. Introduction

The direct determination of long half-lives can be done by the measurement of both the absolute activity and atomic concentration of the radionuclide in a given sample material. In general, the atomic concentration is determined by mass spectrometry. However, in the determination of low radionuclide abundances, isobaric ions and molecules can disturb the measurement with a mass spectrometer. In accelerator mass spectromehy CAMS), molecular interferences can be eliminated through stripping processes which break up the molecules. In addition, the high energy of the ion beam enables single particle detection techniques, and isobars can be se arated to a certain & extend. The half-lives of 32Si [l], Ti [2] and 6oFe [3] were measured with this method, i.e. combining activity and AMS measurements in suitable sample materials. In the present project ‘*%n was studied, one of the few long-lived radionuclides whose half-life is still poorly known (t,,* = 10’ a [4]). Being a fission product, the main natural production mechanism of ‘%n is through sponta-

* Corresponding author. Tel. +41 1 633 6505, fax +41 1 633 1067, e-mail [email protected].

neous fission of 238U. In a nuclear reactor it is produced abundantly by neutron induced fission of 235U and 239Pu. Therefore, the sample material was extracted from spent fuel rods of a nuclear power reactor. The final sample preparation and the activity measurements were performed at the University of Vienna [S]. The primary difficulty in the determination of the ‘*%n concentmtion is the stable isobar ‘26Te with the atomic number Z = 52, which is two units above ‘*%n (Z - SO). In between, the short-lived daughter of ‘?Sn, ‘26m+‘26Sb (Z- 51, t,,, = 19.0 min + 12.4 d), is virtually absent. First AMS measurements with the sample material mentioned above were performed at the Argonne ATLAS facility [6]. They showed that ‘26Te can be separated from ‘*%n at high energies (400 MeV) using a gas-filled magnet and a gas ionization chamber (GIG). However, the complexity of the measuring procedure did not allow absolute ratio measurements, and only an approximate ‘*%n/Sn ratio of 10e6 was derived. Therefore, a new attempt was started at the Zurich AMS facility. At the energies available (30 MeV for ‘*%n5+ ions), no reasonable separation is possible with GIC and/or a gas-filled magnet. Therefore, the newly developed method of projectile X-ray detection [7] was used. Several AMS laboratories are now engaged in this method of Z-identification for heavy radionuclides [8-lo]. To determine the absolute radionuclide concentration of ‘*%n in Sn, several stable

0168-583X/%/$15.00 8 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01570-1

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retractable: Faraday- attenuation

injection magnet 90 (pulsing)

EJ

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analyzing

EN-tandem accelerator electrostatic deflector 90”

Cs source with samples

pro$t;FtioX;ray

Fig. 1. Schematic view of the experimental setup for the Sn isotope ratio measurements at the Zurich AMS facility.

Sn isotopes were measured simultaneously with the ‘26Sn From this, mass fractionation effects can be estimated, which had to be evaluated for the final result of the 129n/Sn ratio. detection.

2. Instrumentation The measurements were performed at the Zurich AMS facility [ 1I, 121. A schematic view of the setup for the present experiment is shown in Fig. I. At the low energy side the heavy ion injector [ 131 provided a good separation of Sn isotopes (see Fig. 2) due to the combination of energy and momentum analysis, which efficiently removed energy tails from the sputter process in the negative ion source. The vacuum chamber of the injection magnet is insulated and connected to acceleration gaps on the entrance and exit side. This allows for a fast sequential injection of different isotopes into the tandem by applying high voltage pulses to the magnet chamber [14]. In this way, the energy of ions with different mass can be adjusted resulting in the same magnetic rigidity inside the magnet. The associated power supply is capable to provide two independent voltage pulses with durations of 100 ks up to several seconds. This arrangement allows for a quasi simultaneous measurement of three different isotopes. Hence, effects resulting from temporal changes in the beam intensity are minimized. Radionuclides are usually injected with zero voltage on the magnet chamber (PO), and stable isotopes are injected with voltage pulses applied (PI, P2). Compared to more common AMS conditions, a relatively high “‘Sn concentration was expected. Therefore the beam pulses were enlarged to 25 ms. Between these pulses there were two intervals of 15 and 55 ms for the radionuclide beam. The sequence Pl-PO-P2-PO was repeated with a frequency of 8f Hz. At the high energy side the analyzing system consisted of the following elements. An electrostatic quadrupole lens placed at the exit of the tandem focused the beam to the

object slits of the analyzing magnet. With a 15-degree electrostatic deflector, beam components with a particular energy-to-charge ratio were selected. In the focal plane of the analyzing magnet various isotopes could be measured. Due to the small separation of neighboring Sn isotopes (18 mm/amu), new shielded Faraday cups (FC) with narrow dimensions (length X diameter - I!90 X 18 mm), and a new CIC were built. Also new current integrators [14] were built, with components having small leakage and switching currents. An integration time of 20 ms was chosen. This eliminates currents induced by the 50 Hz line frequency. Because this time is shorter than the pulse duration, switching effects were cut out, selecting only the constant part of the ion beam. In the case of ‘26Sn, there exist two stable isobars, 126Te and ‘26Xe. Since noble gases do not form stable negative ions, 126Xe is not extracted from the ion source. The Te background can be reduced by radiochemical purification of the sample material, which however is limited around the 100 ppb level. Two metallic Sn samples

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Mass [amu] Fig. 2. Negative-ion mass spectrum of the “6Sn sample showing the ten stable isotopes of Sn, as measured after the injection magnet (see Fig. 1). The peak at mass I29 was absent in blank Sn samples and possibly indicates some residue of lz91 from the original spent fuel rod material.

P. Gartenmann et al./ Nucl. 1n.w.

and Meth.

(A,B) were prepared independently [S]. To avoid further background from the ion source, the sample holders were made from pure aluminum. Beam tests showed a significant reduction compared to other holder materials. It should be noted that the formation probability of Te- ions in the cesium-beam sputter source is much higher than that of Sn-, perhaps by as much as a factor of 100 (matrix effects may also be important, but are not yet investigated). In any case, this enhances the problem of isobar interference from Te impurities in the Sn samples. For the determination of ‘26Te a GIC cannot be used, because the difference in the energy losses relative to ‘*‘?Sn is too small compared to the width of the energy straggling. So, the only practicable method is PXD. The potential of this new isobar identification technique has been previously studied at the Zurich AMS facility for 59Ni and 6oFe detection [9]. For this measurement an X-ray detector system was installed at the very end of the AMS beam line, about 6 m from the focal plane of the analyzing magnet. With a magnetic quadrupole doublet lens the beam could be refocussed without any beam losses to a 6 p,m thick Ti stopping foil mounted 2 mm in front of the X-ray detector. The X-rays emitted during the stopping process in the foil were analyzed with a Si(Li) detector with the following characteristics: diameter = 10.1 mm, active area = 80 mm’, active thickness = 5 mm, window thickness = 80 nm, Be window thickness = 25 pm, energy resolution = 178 eV FWHM for 5.9 keV with a shaping time of 24 p,s. This foil-detector arrangement covered a solid angle of about 4% of 4n. The arrangement had the advantage that it did not interfere with the set-up used for routine AMS measurements. In contrast to other PXD experiments where usually K X-rays are measured, L X-rays gave much higher yields for the stopping of Sn ions at 30 MeV. The energies of L X-rays are sufficiently high (3.5-4 keV), so self-absorption in the stopping foil is manageable (47%). Based on the separation of the L, lines of Sn and Te (AZ = 2) a reasonable suppression of Te with respect to Sn by about a factor of 10 can be achieved (see Fig. 3). Compared to the normal line pattern of characteristic X-rays, the centroids are shifted to higher energies by amounts in the order of 100 eV due to Doppler shifts and a reduced shielding of inner shells of the highly ionized projectiles. In addition, LJL, intensity ratios are different 1151.The material of the stopping foil was chosen to provide maximum yield for L X-rays in Sn and Te. During collisions molecular orbitals are formed, and similar target and projectile energy levels lead to enhanced excitation [16]. A stopping element where the energies of K X-rays match those of the L X-rays from Sn and Te has a lower mass, resulting in less self-absorption. Therefore, a 6 km thick Ti foil was used as the stopping material providing the desired X-ray excitation properties and the mechanical stability. The operating conditions of the EN-tandem were chosen based on the following considerations: high stripping

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2.5

3

3.5 Energy

4.5

5

55

[keV]

Fig. 3. X-ray spectrum of mass-126 ions from sample B (heavy line), containing 74% ‘26Sn and 26% ‘26Te.The two conuibutions were determined from the line shapes of pure elements measuredwith stable isotope beams of ‘24Sn(medium heavy line) and ‘26Te(light line). yield, high transmission, stable operating conditions and high X-ray yield. Therefore, the accelerator was operated at 5 MV and the 5+ charge state was selected leading to a beam energy of 30 MeV. For beam particles around mass 130, this results in the maximum magnetic rigidity which can be handled with the analyzing magnet. The best transmission was obtained by shipping in the residual gas with the recirculating tubomolecular pump in the terminal running 1171.This corresponds to a stripping gas thickness of about 0.3 kg/cm*, which is well below the thickness needed for charge state equilibrium conditions. With these settings 3% of the injected Sn - ions were transmitted as Sn5+ ions to the detectors after the analyzing magnet. To obtain the correct time for the measurements, two different methods were used. On-line, a I-kHz clock including dead time signals from the GIC-ADC and the PXD preamplifier was used. For off-line data analysis of pile-up effects resulting from high counting rates, two pulser signals (GIG 52 Hz, PXD 100 Hz) were recorded together with the respective energy spectra.

3. Measurements Two independent detecting methods for 12?Sn, using PXD and GIC respectively, were combined in the present experiment. The radionuclide was always measured relative to several stable Sn isotopes with known abundances [18]. The experiment was devided into phases of simultaneous measurements of three different isotopes. Four different phases (described below), each lasting about 200 s, were measured for one run. To test reproducibility, eight runs were performed. A schematic presentation of the different measuring phases is shown in Fig. 4. In phase I, ion beams of the Sn isotopes 120, 122 and 126 were analyzed. The PXD technique was used to measure the ‘26Sn/ “‘Te ratio. For

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GIC, an attenuation grid was used. It was placed in front of the tandem (see Fig. I) and allowed to adjust the beam intensities to acceptable counting rates in the respective detection systems. Besides this attenuator, only the pulsing voltage and the analyzing magnet were changed between the different phases. All other beam optical elements were only tuned at the beginning of a run.

PXD (m=126)

GIC (m =

126) PXD (m = 122)

grid

\

FCl (m = 120)

4. Data analysis phase II

1

phase III 1

I

GiC (m = 124)

PXD (m = 120)

m grid phaseiv

1

Fig. 4. Schematic view of the different measuring phases described in the text.

later mass fractionation estimates, the currents of the stable isotopes “‘Sn and “*Sn were measured. The 12’?Sncounting rate was normalized to the current of ‘*‘Sn. But for a determination of the absolute isotope ratio, the PXD detection efficiency had to be known. It was measured in phase II, using the same isotopes but directing the lz2Sn beam to the PXD. In this way, the detection efficiency could be calculated from the counting rate of the PXD and the measured current of the 12’Sn ion beam, assuming that the analyzed beams have intensities given by the natural ‘22Sn/ “‘Sn ratio. In addition, it was possible to normalize the counting rate of the 12%n + Te) events in the GIC to the measured current of the 12’Sn ion beam. Since isobar separation in the GIC is impossible at the available beam energies, the ‘26Sn contribution to the GIC signal was calculated using the ‘26Sn/ ‘26Te ratios measured in phase I. In phase III, an ion beam of ’ '*Sn was measured in a FC instead of measuring X-rays of the 122Sn beam. Again, the 12%n counting rate in the GIC was normalized to the measured beam current of 12’Sn to obtain a ‘26Sn/Sn ratio. In this phase, additional information on mass fractionation was obtained from the ’ ‘*Sn/ “‘Sn beam current ratio. In phase IV, the detection efficiency of the PXD was measured independently using the stable ion beams of “‘Sn and ‘24Sn. While 12’Sn was measured in the PXD system, ‘24Sn ions were counted in the GIC. Using nominal beam intensities according to the natural isotopic ratios, the detection efficiency was calculated. In order to measure stable isotopes in the PXD and

The L X-ray peak intensities of ‘26(Sn + Te) were measured with the PXD system in phase I. In order to obtain line shapes for Sn and Te separately, two spectra with pure ‘24Sn and ‘26Te beams, respectively, were measured (see Fig. 3). From this, the ‘26Sn/ ‘26Te ratio in the mass-126 beam can be calculated. However, the two isobars have different X-ray production yields. In addition, the X-ray energies and therefore the self-absotptions are not the same. The combined effect was measured in an independent experiment and resulted in an efficiency ratio of ~(‘~~Sn)/c(‘~~Te) = 0.9 f 0.2. The relatively large uncertainty had, however, little effect on the final ‘26Sn/Sn ratio since most measurements were made with sample A, which had a ‘“Sn/ ‘26Te beam composition of 19/I. For sample B, a ratio of 3/l was measured. The X-ray detection efficiency relative to the FC and GIC measurements was determined with stable Sn isoto s. In phase II, ‘22Sn was detected in the PXD and ,2r Sn in a FC. The efficiency for X-rays from 12’Sn was calculated using the natural ratio of Sn isotopes. In the same way the efficiency for X-rays from 12’Sn was determined in phase IV measuring 12’Sn in the PXD and ‘24Sn in the GIC. Since the atomic excitation process is a function of the collision velocity and in the experiment all ions were measured at the same energy, a correction is

I....) 0

,.S..i....I

..,I.

5

10

15

20

25

MeasuramentNumber

Fig. 5. Chronological display of ‘26Sn/Sn ratios determined from ‘*%I/ “‘Sn ratio measurements. Data are shown according to measuring phase and method: phase I/PXD (=I, phase II/GE CO), phase III/GIG (0). Measurements 1-7 and 1l-24 were performed with sample A, 8-10 with sample B. The weighted mean with its uncertainty is indicated with horizontal lines.

P. Gartenmann et al./ Nucl. Instr. and Meth. in Phys. Res. B 114 (1996) 125-130

necessary to calculate the X-ray efficiency for ‘*?n. In the used energy range the X-ray yield is proportional to a power law of the ion velocity [161 and a simple extrapolation with 1.3%/amu is possible. The final X-ray efficiency of approximately 4.8 X 10e4 was calculated for each run using the weighted mean of the results from the two phases. Using the ‘26Te background subtraction for the GIC measurements in phase II and III, and the X-ray efficiency for the PXD measurement in phase I, an absolute number of ‘*‘?Sn counts could be calculated. The radionuclide concentration relative to the total number of Sn atoms was determined with the measured current and the natural abundance of ‘*‘Sn. For the uncertainty estimate of the individual data points in Fig. 5 the following effects were considered. The current integrators were calibrated over a range of 5 pA to 2 nA with a current source relative to a Keithley type 485 picoamperemeter. A non-linearity could not be observed. So, currents down to the range of the offset currents could be measured. These were stable within a few percent, so that a good accuracy was reached even at the pA level. Then, the DC beam currents measured with the integrator system were compared with the picoamperemeter. Also the collection efficiency of the new FCs were compared with longer and larger FCs. Both types have a small opening angle and the latter have been proven to be very reliable in other AMS measurements. Statistical uncertainties were calculated for the counts in the GIC and PXD. Dead-time and pile-ups were corrected. The uncertainties of the natural abundances were also taken into account. All these uncertainties were propagated to the results used in other phases (i.e. X-ray efficiency and ‘26Te background), and to the radionuclide concentration. The weighted mean of all measurements with a standard deviation of 1.3% and x * = 1.32 shows the consistency of the different measuring methods. Only the very first PXD measurement and the GIC measurements of sample B (measurement 9 and 10) show larger deviations. The latter may indicate a systematic error in the correction of the ‘26Te background subtraction with the Sn/Te X-ray efficiency ratio. But this has no consequence for the accuracy of the other measurements. In addition, no significant differences between the two measuring days (measurements l-10 and 1l-24, respectively) nor any drifts due to cratering effects in the source samples were observed. Mass fractionation effects can affect the measured isotopic ratios. It is assumed that the Sn carrier used for the sample preparation had natural isotopic composition [18]. A comparison of the nominal ratios of stable isotopes with the measured ones gives an indication of the mass fractionation. Because the radionuclide was normalized with ‘*‘Sn, the fractionation is calculated relative to this isotope. The results from phases I (‘**Sn/ ‘*‘Sn) and III (“*Sn/ ‘*‘Sn) shown in Fig. 6 give a fractionation of a different sign.

116

120

129

122

124

126

Mass Number A

Fig. 6. Mass fractionation of Sn isotopes. The %n fractionation is determined from a linear tit ( -_) through the measured isotope ratios. The external uncertainty is indicated (- - -).

Assuming that the fractionation to be linear in mass, a correction can be extrapolated for the ‘26Sn/ ‘*‘Sn ratio, which leads to a correction factor of 1.053 f 0.030 ( x2 = 2.83). On the other hand, fractionation effects of individual processes in the accelerator system can be estimated. From the mass spectra measured at the low energy side (see Fig. 2), a linear mass fractionation with a resulting correction factor of 1.10 can be extracted. This fractionation is probably due to a velocity dependence in the negative-ion formation process. For the stripping in the terminal a fractionation can be estimated only for equilibrium conditions based on the semi-empirical formula of Sayer [19], giving a correction factor of 1.07. Comparing these two values with the total fractionation, there must be unknown fractionation processes (e.g. beam optics) and/or non-linear effects. For this reason, the standard deviation of the linear fit of 0.030 underestimates the fractionation uncertainty. For fits with x2 greater than one, but not largely so, it is usual to scale the standard deviation with the factor x. Having additional unknown effects we add another factor of 2 to the uncertainty. We then obtain for the total fractionation correction factor for the ‘*?Sn/ ‘*‘Sn ratio the value of 1.053 + 0.099. The resulting ‘26Sn/Sn ratio is (9.23 + 0.87) x 10e6, where the main uncertainty comes from the mass fractionation. Using the atomic weight of 118.71 g Sn/mole, we convert the ‘*?&@n atomic ratio into a ‘*?Sn concentration of (4.54 f 0.33) X lOI atoms 12?n/mg Sn. Together with the activity concentration of 4.97 + 0.15 Bq ‘*‘?Sn/mg Sn [5], the half-life is calculated to be t,,,(12”Sn)

= (2.07 + 0.21) X IO5 a.

5. Conclusions

This paper presents the first quantitative measurement of ‘26Sn with AMS, which was only possible due to the application of the new PXD technique. The value for the

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half-life of “?n listed in most tables of isotopes is = lo5 yr, which is the result of the estimate of Ref. [4]. It has now been improved by a direct determination to a value with only 10% uncertainty. An improved half-life value would be feasible with a detailed study of mass fractionation effects in the AMS measurement. Due to the fact that the concentration of 12’Sn in the sample is significantly higher than expected, and that the Te impurity in the sample may be lower by several orders of magnitude (including the very different negative ion formation probabilities), conventional mass spectrometry could probably also be considered for a ‘26Sn/Sn ratio measurement. The developed technique for absolute isotope ratio measurements can be applied for the measurement of other uncertain half-lives as discussed in Ref. IS].

Acknowledgement

This work was supported in part by the Swiss National Science Foundation.

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