High sensitivity alpha-particle and electron spectroscopy

Nuclear Instruments and Methods in Physics Research A242 0986) 395 398 North-Holland, Amsterdam

HIGH SENSITIVITY ALPHA-PARTICLE AND ELECTRON SPECTROSCOPY

395

*

Irshad AHMAD Chemistt~v Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA

Thin massless sources of 238pu and 249Cf, prepared in an electromagnetic isotope separator, have been used to determine the characteristics of newly developed passivated ion implanted silicon detectors. These detectors are found to be superior to the usual Au Si surface barrier detectors, both in resolution and peak tailing. For 238pu c~-groups we have measured a resolution (fwhm) of (9.2+0.2) keV and find the tail to be 1.0×10 ~ of the peak height. This tailing is more than an order of magnitude better than that obtained with the best surface barrier detectors. Relative intensities of 2~'~Cf c~-groups have also been measured with high precision.

1. Introduction

2. Experimental methods

Alpha-particle spectrometry is a convenient and powerful tool for the qualitative and quantitative analysis of heavy elements, in particular, spent reactor fuels and environmental samples. Unlike ),-ray and X-ray spectroscopy this technique does not suffer from efficiency calibration problems. All peaks in an a-spectrum have the same efficiency-geometry product. Also, this technique can be used for low background counting: counters with practically zero counts per day background are routinely used for low level counting. In a complex a-spectrum, the errors in intensities d e p e n d on the system resolution and the peak tailing. Both of these properties are governed by the source quality (thickness and uniformity) and the detector characteristics. The state of the art A u - S i surface barrier detectors have a resolution (full width at half maximum (fwhm)) of 12.0 keV for 6.0 MeV a-particles. In these spectra the tail at - 300 keV below the peak is 2 × 10 4 of the peak height [1]. Recently, the fabrication of passivated ion implanted silicon detectors have been reported [2], which give resolutions of better than 10.0 keV [3]. Although high resolution spectra measured with these detectors have been reported, no detailed investigation of the characteristics of such detectors have been performed. Using thin massless sources we have measured the resolution and tailing of these detectors. We find that these detectors are capable of providing 9.0 keV resolution and a tailing which is an order of magnitude smaller than the tailing obtained with the usual surface barrier detectors. In the present paper we report the results of our investigation.

2.1. Sources

* Work performed under the auspices of the Office of High Energy and Nuclear Physics, Division of Nuclear Physics, US Department of Energy under contract number W-31-109ENG-38. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

The quality of an a-particle spectrum depends on the quality of the source and the detector properties. In order to investigate the characteristics of a detector a thin massless source is needed. Such sources can be prepared either by an isotope separator or by the collection of recoil atoms during the decay of an a-emitting nuclide. Sources prepared by other techniques, like electroplating or vacuum sublimation, are usually not mass free. In the present investigation we have used thin, massless sources of 23Spu and 24'~Cf prepared in the Argonne electromagnetic isotope separator. The sources were prepared [4] by depositing 200 eV ions from the decelerated beam of the separator onto a 15 m g / c m 2 A1 foil for 23Spu and a 200 ~tg/cm 2 AI foil for 249Cf. The activity was confined to a spot of 4 mm diameter and had a total mass of less than 1 /tg. The sources were of high isotopic purity and were invisible. 2.2. Alpha spectra A passivated ion implanted silicon detector was purchased from E N E R T E C [5]. The detector had an area of 20 m m 2 and was 2 5 0 / t i n thick. The detector and the source were placed in a small vacuum chamber which was then evacuated to 10 2 Torr pressure. Signals were obtained by connecting the detector to a low-noise preamplifier: the load resistor in the preamplifier was 100 M~2. The preamplifier output was connected to a combination of amplifier and biased amplifier whose output was fed to a multichannel analyzer. A test pulse was injected into the preamplifier in order to measure the system electronic noise. The detector was operated at 110 V and at this bias it had a leakage current of 7 nA. The test pulse gave a resolution II. DETECTORS

1. Ahmad / High sensitivity spectroscopy

396

of 3.7 keV, which is then the noise in the system. Fig. 1 shows the resolution of the system measured with a 23Xpu source. The spectrum was obtained by c o u n t i n g the source for a short time (1 h) in order to reduce possible peak b r o a d e n i n g due to gain shift. A resolution of (9.2 _+ 0.2) keV was obtained. A n o t h e r spectrum of the 238pu source measured with the passivated ion implanted Si detector, and showing a larger region is displayed in fig. 2. Also, included for comparison, is a spectrum measured with a 25 m m 2 area and 100/~m thick surface-barrier detector. Both spectra were measured at the same detection geometry and with the same electronic set-up. As can be seen, the tailing in the spectrum taken with the passivated ion implanted detector is more than one order of magnitude lower than that measured with the surface-barrier detector. The a-particle spectrum of a 249Cf source measured with this detector is shown in fig. 3. Again, the tailing is very low; the tailing at a channel 600 keV below the main 249Cf peak is 1.0 × 10-5 of the peak height of the main a-group. In b o t h spectra, it is possible to measure intensities as low as 1.0 × 10 3%_ 2.3. Electron spectroscopy K e m m e r et al. [6] have used passivated ion implanted Si detectors with very low leakage currents ( < 1 nA) to measure X-ray and low-energy y-ray spectra.

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They were able to obtain resolutions of 1.5 keV for the 122 keV y-ray of 57Co. It seems natural to use these detectors for low energy electron spectroscopy; resolutions similar to that for X-rays are expected. Although much better resolutions can be obtained with a cooled Si(Li) detector [7], the passivated ion implanted Si detectors, because they operate at room temperature, provide much more convenience and versatility. We have measured the electron spectra of 238pu and 249Cf with the passivated ion i m p l a n t e d Si detector a n d these are displayed in figs. 4 and 5. In the case of 238pu, we can easily identify the L 2, L 3, M and N lines from the 43.5

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Fig. l. A small portion of the 2:~SPu a-spectrum measured with a 20 mm 2 area and 250 p.m thick passivated ion implanted silicon detector. Energy scale is 1.0 keV per channel.

Fig. 3. The a-particle spectrum of a 249Cf source measured with the passivated ion implanted Si detector. Detector geometry was 0.13%, and energy scale is 0.73 keV per channel. This spectrum displays the best tail that we have obtained with any source and silicon detector combination.

1. Ahmad / High sensitivity spectroscopy

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keV transition. The noteworthy thing is that there is very little background due to the tailing of the a-peak. Thus, one can measure the intensities of very weak transitions. For example, the intensity of the 99.9 M + N line is known [8] to be only 0.03% per 23Spu o¢-decay and this transition can very clearly be identified in the spectrum. Similarly K, L, M, and N lines of the main transitions in 245Cm are well resolved in fig. 5.

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3. Results

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Fig. 4. Conversion electron spectrum of a 238pu source measured with a passivated ion implanted Si detector. The intensity of the 99.9 M + N line is known to be 0.03% per a-decay. The detector geometry was 0.13% and energy scale is 0.245 keV per channel.

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The o~-particle spectra of 23Spu and 249Cf measured with the passivated ion implanted Si detector were analyzed both using hand-plotted graphs and also by a c o m p u t e r code F I T E K [9]. The intensities of a-groups determined by the two methods agreed with each other and also with the previous measurement [10]. In table 1 we compare the intensities of 249Cf a-groups determined in the present work using the F I T E K program with the previously reported intensities obtained by hand-plotted graphs [10]. We find that the two sets of values are in excellent agreement with each other attesting to the adequacy of the computer code. In the comparison we have not included the intensities measured by the magnetic spectrographs [8] because it is known that these intensities suffer from systematic errors [1 ]. We present two examples of the use of the highsensitivity c~-particle spectroscopy. In the case of 23Spu,

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Fig. 5. Conversion electron spectrum of a 249Cf source measured with a passivated ion implanted silicon detector. The peak resolution is 3.8 keV and the energy scale is 0.245 keV per channel.

Table 1 Intensities of strong 249Cf alpha groups Energy a) [MeV]

Excited state [keV]

Intensity in % per alpha decay

6.194 6.140 6.072 5.946 5.903 5.850 5.812 5.784 5.760 5.694 5.559 5.501

0 55 122 253 296 350 388 ) 418 443 509 644 702

2.46 -+0.02 1.33 _+0.01 0.346 ± 0.007 3.33 + 0.03 3.21 +_0.03 1.43 -+0.02

2.4+_0.1 1.3_+0.1 0.40 + 0.06 3.4+0.1 3.2_+0.1 1.4+0.1

82.5 -+0.5

82.6+_0.3

4.69 _+0.05 0.30_+0.01 0.113_+0.005 0.044 _+0.002

4.8+0.2 0.30 + 0.05 0.11 ±0.01 0.044 _+0.004

Present work using FITEK program

Ahmad [101

~) Ref. [8]. II. DETECTORS

398

1. Ahmad / High sensitivity spectroscopy

two quite different values are quoted [8] for the intensity of the ~2%-group feeding the 6 + state in 234U. Baranov et al. [11] have obtained an intensity of 0.0018~ for this a-group, whereas K o n d r a t ' e v et al. [12] report value of 0.005%. Thus the previous values differ by a factor of 3. We have therefore measured this intensity using the passivated ion implanted Si detector and obtain a value of (3.6 _+ 0.5) x 10- 3%. A n o t h e r use of the high-sensitivity a-spectroscopy is the determination of the half-life of a nuclide when the half-life of its parent or daughter is known. The 249Cf source used for the m e a s u r e m e n t of the spectrum shown in fig. 3 was prepared 13 years ago in the isotope separator. Since we know the time interval during which the daughter 245Cm grew and the half-life for the decay of 249Cf, we can calculate the half-life of 245Cm from the a-activities of the two nuclides in the a-particle spectrum. Using a half-life of 351 y [13] for 249Cf decay we calculate a half-life of (8.7_+ 0 . 2 ) × 10 3 y for the a-decay of 245Cm. This is in good agreement with the more precise value of (8537_+ 45) y determined by specific activity and mass spectrometry technique [14].

ties, can be determined with high precision. 2) Measurement of very weak alpha branching. Because of low tailing very weak peaks can be identified a n d their intensities determined. 3) Determination of alpha activity when higher eneceA' alpha groups are present. For example, the a m o u n t of 229Th can be quite accurately determined despite the fact that the peaks due to its daughters have higher energies and produce tails which lie under the 229Th a l p h a peaks. 4) Determination of absolute alpha disintegration rates. Because all counts are concentrated in the peaks these detectors can be used as low-geometry c~-counters. This can be done by defining the physical geometry, i.e., by carefully measuring the diameter of the aperture in front of the detector and the distance between the source and the aperture. 5) Electron spectroscopy with moderate resolution. Because these detectors are operated at room temperature, electron spectroscopy seems to be the most promising application of the passivated ion implanted silicon detectors.

4. Discussion

Acknowledgments

The detector used in the present investigation was not the best selected detector. It had a leakage current of 7 nA. Detectors with leakage currents of less than 1 nA have been used in X-ray spectroscopy [6]. Using such detectors it would be possible to obtain a resolution (fwhm) close to 2.0 keV for electron lines. At such resolution, these detectors become competitive with cooled Si(Li) detectors. However, because they can be operated at room temperature with a preamplifier with room temperature FET, they can be used in m a n y applications. For example in accelerator-based research, they can easily be placed in any location. Also, in most accelerator based y-ray and electron spectroscopy, a thick target is used. Thus the resolution and tailing in such experiments will be primarily controlled by the target thickness. In such applications, room temperature detectors will be as good as the cooled detectors. We have only tested a 250 /~m thick detector (which stops electrons up to 260 keV energy) but, in principle, thicker detectors can be fabricated.

The author wishes to thank K.E. R e h m for providing the c o m p u t e r code used for the spectrum analysis and Tetsuro Ishii for his assistance in data analysis.

5. Conclusion The passivated ion implanted silicon detectors, because of their superior resolution (9.0 keV for 6.0 MeV a-particles) and very low tailing (1.0 × 10 5) are ideally suited for the following applications. 1) High-precision measurement of relative intensities of alpha groups. The high resolution and low tailing make it easier to resolve peaks in a complex alpha spectrum. Thus peak areas, and hence, relative intensi-

References [1] 1. Ahmad, Nucl. Instr. and Meth. 223 (1984) 319. [2] J. Kemmer, Nucl. Instr. and Meth. 169 (1980) 499. [31 F. Amoudry and P. Burger, Nucl. Instr. and Meth. 223 (1984) 360. [4] J. Lerner, Nucl. Instr. and Meth. 102 (1972) 373. [5] ENERTEC-Schlumberge, Lingolsheim, France. [6] J. Kemmer, P. Burger, R. Henck and E. Heijne, IEEE Trans. Nucl. Sci. NS-29 (1982) 733. [7] I. Ahmad and F. Wagner, Nucl. Instr. and Meth. 116 (1974) 465. [8] C.M. Lederer and V.M. Shirley, Table of Isotopes, 7th ed. (Wiley, New York, 1978). [9] This computer code was written by W. Stoeffl of Technical University, Munich and was modified by K.E. Rehm for the analysis of heavy-ion spectra measured at the Argonne Tandem-Linac accelerator. [10] I. Ahmad, University of California report UCRL-16888 (1966). [11] S.A. Baranov, V.M. Kulakov, V.M. Shatinskii and Z.S. Gladkikh, Yad. Fiz. 12 (1970) 1105, Soy. J. Nucl. Phys. 12 (1971) 604. [12] L.N. Kondrat'ev, G.I. Novikova, V.D. Dedov and L.L. Gol'din, Izvest. Akad. Nauk SSSR Ser, Fiz. 21 (1957) 907. [13] J.R. Stokely, C.E. Bemis Jr, R.D. Baybarz and R. Eby, J. Inorg. Nucl. Chem. 35 (1973) 3369. [14] K.W. MacMurdo, R.M. Harbour and R.W. Benjamin, J. lnorg. Nucl. Chem. 33 (1971) 1241.