Nuclear Instruments and Methods in Physics Research B 269 (2011) 2745–2749

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Nuclear Instruments and Methods in Physics Research B

journal homepage: www.elsevier .com/locate /n imb

32Si AMS measurement with DE-Q3D method

Jie Gong a,b,⇑, Chaoli Li a, Wei Wang a, Guowen Zheng a, Hao Hu a, Ming He a, Shan Jiang a

a China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, Chinab China Nuclear National Company Everclean Co., Ltd., P.O. Box 2102, Beijing 100037, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 April 2011Received in revised form 12 August 2011Available online 8 September 2011

Keywords:32SiIsobaric suppressionAccelerator mass spectrometry (AMS)DE-Q3DRadiationUltra-sensitive measurement

0168-583X/$ - see front matter � 2011 Elsevier B.V.doi:10.1016/j.nimb.2011.08.026

⇑ Corresponding author. at: China Institute of AtomBeijing 102413, China. Tel.: +86 13 810750824; fax: +

E-mail address: gongjies@hotmail.com (J. Gong).

Accelerator mass spectrometry (AMS) is one of the most promising methods for the measurement oftrace amount of 32Si for its advantages of small sample size, short measurement time and extremely highsensitivity. However, the isobaric interference from 32S often badly hinders the AMS measurement of 32Si.The DE-Q3D detection technique established in this work brought about an overall suppression factor oflarger than 1012 for 32S. As a result, a sensitivity of better than 1 � 10�14 (32Si/Si) has been achieved, basedon the measurement of a blank sample.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The cosmogenic nuclide 32Si with a half-life of about 140a is theonly long lived radio isotope in the 23 known isotopes of silicon. Ithas very important application value in geochronometry in therange of 100–1000 years [1–3] and the research of global biogeo-chemical silica cycle [4,5]. 32Si in nature is generated in smallamount through primary or secondary cosmic-ray induced spalla-tion of Ar such as 40Ar(n,4p5n)32Si, and 40Ar(p,2ap)32Si in the atmo-sphere. Two third of 32Si is generated in stratosphere and the other1/3 in troposphere. It falls with rain and snow to the ground. Theatmospheric production rate of 32Si is 0.72 atoms/m2s and the bal-anced amount on the earth is about 2 kg. 32Si can be used in manyresearch areas, such as measurement of the age of groundwaterand glacier, studies on the mixing and flow process of groundwaterand seawater, and biochemical cycle of Si in the atmospheric circu-lation, measurement of the age of siliceous sediments in ocean,determination of the deposition rate of shallow sediments, estimateof the cosmic rays exposure age of meteorites and so on.

Because of the limitation in detection sensitivity, the half-lifevalues of 32Si reported so far vary widely [1,6], which hamperedthe use of radiometry for the determination of 32Si. Acceleratormass spectrometry (AMS) method can measure 32Si in samples aslittle as several milligrams with ultra-high sensitivity rapidly. It isone of the most promising methods for the measurement of 32Si.

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ic Energy, P.O. Box 275(50),86 10 68011703.

AMS of 32Si must, however, solve the problem of the very high fluxof 32S ions that inevitably accompany the 32Si ions. The highest sen-sitivity to date,�10�15 in the 32Si/Si ratio, has been achieved using agas-filled magnet to separate 32Si from 32S [7,8]. Fifield andMorgenstern have applied this method to the measurement of32Si in glacier ice [9,10]. In this work a new 32Si detection method,AMS-DE-Q3D, was developed by taking the advantages of a highvoltage (13 MV) tandem accelerator and a large scale Q3D magneticspectrometer of CIAE-AMS system. As a preliminary result, a sensi-tivity of better than 10�14 (32Si/Si) has been achieved.

2. Experimental

2.1. Production of 32Si

32Si can be produced by some nuclear reactions such as36S(p,ap)32Si, 37Cl(p,a2p)32Si [11,12], 31P(n,c)32P(n,p)32Si [12,13],30Si(n,c)31Si(n,c)32Si [14]. Based on an analysis of all possible nucle-ar reaction channels for producing 32Si and the experiment condi-tions available in our country, the reactor neutron induced31P(n,c)32P(n,p)32Si reaction chain was finally chosen for producing32Si; taking the advantages of the high fluxes of both thermal andfast neutrons in the swimming pool reactor of CIAE. According to cal-culation, 31P in the form of Mg2P2O7 was irradiated for 250 h in theswimming pool reactor. The resulting atomic number ratio of32Si/31P was estimated to be about 1.3 � 10�12. Relevant parametersfor the estimate of 32Si production are given in Table 1. A calculatedgrowth curve of 32Si during reactor neutron irradiation is illustratedin Fig. 1.

Table 1Parameters relevant to 32Si production calculation.

Sample Thermal neutron flux Fast neutron flux Cross section (mb) Predicted yield

Mg2P2O7 2.65 � 1013 n/cm2s 6 � 1012 n/cm2s 190a, 120b 32Si /31P = 1.3 � 10�12

a 31P(n,c)32P cross section for thermal neutron.b 32P(n,p)32Si cross section for fast neutron.

Fig. 1. Calculated growth curve of 32Si during irradiation of Mg2P2O7.

Fig. 2. Schematic diagram of Beijing tandem accelerator-Q3D facilities.

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2.2. The preparation of 32Si sample

From our previous experience, the best ion form to be extractedfrom the ion source was Si� and the best sample form was SiO2 + Fe(with a weight ratio of 1:5) in measuring 32Si in terms of maximizing28Si� beam current (1.7 lA) and minimizing 32S interference [15].After 13 months decay the following chemical procedures wereused to obtain the 32Si-containing SiO2 sample. 1.284 g irradiatedMg2P2O7 powder was first dissolved in HNO3, a drop of HF was thenadded to unify the produced 32Si to the chemical form of SiF2�

6 . After

the addition of 1.80 ml Na2SiO3 solution (203.5 mg/ml, as Si carrier),Si(OH)4 and Mg(OH)2 were co-precipitated with aqua ammonia.Mg(OH)2 was dissolved when the pH value was adjusted to 6 byHNO3. After centrifugal separation, the Si(OH)4 precipitate waswashed with 1 N HNO3, de-ionized water and absolute ethyl alcoholin sequence. Then the Si(OH)4 was ignited in a muffle furnace at1200 �C to obtain the 32Si-containing SiO2 powder and remove mostS. The chemical yield of the procedure is about 80–90%.

The nominal value (atomic number ratio) of 32Si/Si in the 32Sisample is 5.00 � 10�12 calculated based on the radiation conditions,

Fig. 3. Position spectra of 32S and simulation 30Si along the focal plane.

Fig. 4. Dimensions of ionization chamber.

Fig. 5. Dimensions of anode planes.

J. Gong et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2745–2749 2747

Fig. 6. Two-dimensional spectra of a 32Si sample and a commercial blank sample.

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J. Gong et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2745–2749 2749

relevant nuclear parameters, and the chemical procedure. Four sub-samples were taken from the 32Si sample for AMS measurements of32Si/Si.

2.3. AMS measurement

The measurement of 32Si by AMS is challenging due to a stronginterference from the isobaric nuclide 32S (natural isotope abun-dance 95.02%). The detection system was improved in many as-pects to get a high suppression factor of 32S.

Ion beam with �1 charge state was extracted from MC-SNICSion source. After passing through the electrostatic analyzer andinjection magnet, the ion beam of M = 32, charge = �1 was injectedto the HI-13 Beijing Tandem Accelerator. There was a carbon foilstripper in the terminal and the terminal voltage was set at10.5 MV. Molecular ions such as O�2 would be dissociated due tothe coulomb explosion effect. The resulting atomic ions would beexcluded by the choice of an odd charge state (7+) around the ana-lyzing magnet. 32S7+ ions were also transmitted because of thesame electric rigidity and magnetic rigidity as 32Si7+. Both 32Si7+

and 32S7+ ions were then injected into Beijing Q3D magnetic spec-trometer after passing through the high energy analyzing systemof the HI-13 tandem accelerator. The system is shown in Fig. 2.

A 3 lm Si3N4 foil stripper was set up at the entrance of the Q3D.After passing through the stripper, 32Si and 32S would have differ-ent charge state distributions and residual energy. The Si3N4 foilhas very good homogeneity, which minimizes energy straggling.The 11 + charge state of 32M ions, which offered a stripping proba-bility ratio of 7 for 32Si/32S after passing through the Si3N4 foil, waschosen for transmitting to the focal plane of Q3D magnetic spec-trometry. Beijing Q3D magnetic spectrometer has advantages ofhigh resolution, large dispersion, large spatial angle and large kine-matic compensating ability. In the experiment, a 32S suppressionfactor of 106 was achieved by Si3N4 foil and Q3D magnetic spec-trometer estimated from the scattered 32S background count rateat the focal plane position of 32Si and the 32S beam current mea-sured before the Si3N4 foil.

According to calculation based on TRIM, the energy loss differ-ence between 32Si and 32S ions was 3.2 MeV. The peak distance be-tween 32Si11+ and 32S11+ at the focal plane is 264 mm whilebetween 32Si11+ and 32S10+ it is 442 mm. Fig. 3 shows the positionspectrum of 32S counting rate along the focal plane, which wasmeasured with the movable silicon surface barrier detector(SBD). The counting rate of 32S at the peak was so strong that itcannot be directly measured by the SBD. 30Si ions with the samemagnetic rigidity as that for 32Si ions were used for optimizingthe magnetic field for 32Si transmission in the Q3D magnet.

In the experiment, the peak position of 32Si was determined by30Si simulation: The terminal voltage was first set at 11.2 MV for30Si simulation of 32Si at 10.5 MV. The energy loss in Si3N4 foilwas determined as 15.2 MeV by the SBD. 32Si at 85.2 MeV shouldlose 0.27 MeV more energy than 30Si at 89.6 MeV in Si3N4 foilbased on ENLOSS calculation. So setting the terminal voltage to10.65 MV would produce 32Si ions which had the same magneticrigidity after Si3N4 foil and the same peak position on focal planeas 30Si at 11.2 MV terminal voltage. Before 32Si measurement, itwas necessary to set up the beam transport system with 30Si at11.36 MV terminal voltage (the same magnetic rigidity as 32Si at10.65 MV).

A four-anode gas ionization chamber was then mounted in thesimulated 30Si position of focal plane to record 32Si, where it pro-vided excellent discrimination between 32Si and 32S ions. Fig. 4and Fig. 5 shows the dimensions of the ionization chamber and

dimensions of anode planes. 1 lm Mylar foil and 42 mbar C4H10

gas was chosen for work.Fig. 6 shows the two-dimensional spectra for a 32Si sample. It

can be seen that 32S was clearly identified with the ionizationchamber. There were no counts on the 4th anode. If the gas pres-sure is adjusted appropriately, only 32Si ions can reach the 4th an-ode. At the gas pressure employed, there were no counts of either32S or 32Si on the 4th anode, although in principle it would havebeen possible to adjust the gas pressure so that only 32Si couldreach this electrode.

In order to determine the 32Si/Si ratio of the standard material,the 30Si negative ion beam current was measured continuously inan off-axis Faraday cup after the injection magnet at the low-energyend of the accelerator while 32Si was injected into the accelerator.All four sub-samples were measured in sequence several times.From the number of 32Si counts at the focal plane of the Q3D, andthe measured transmission efficiency for 32Si as simulated with a30Si beam, an average 32Si/Si value of 5.17 � 10�12 was obtainedwhich differs by only 3.4% from the nominal value 5.00 � 10�12. ASiO2 commercial blank gave no 32Si counts in 20 min, which indi-cated that the detection sensitivity for 32Si/Si was better than1 � 10�14.

3. Conclusion

The DE-Q3D method for the measurement of 32Si showed astrong ability in suppressing interference of 32S. The overall sup-pression factor for 32S was higher than 1012 and the sensitivityfor 32Si/Si was better than 1 � 10�14. Compared with GFM method,DE-Q3D method will cause a larger peak distance between 32Si and32S and stronger 32S suppression ability because of the higheraccelerator terminal voltage and large scale magnet spectrometer.This work provided a new detection technique for AMS measure-ment of 32Si and laid a foundation for accurate determination ofthe half-life of 32Si. And this method can also be used to measuresome other nuclides such as 36Cl and 53Mn.

Acknowledgements

Thanks for the help from Prof. Tian Weizhi in the establishmentof chemical procedure for 32Si sample preparation and Prof. YuanJian in 32Si production. This work was supported by the NationalScience Foundation of China, under Grant No. 10875176.

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