Journal o f Radioanalytical Chemistry, Voi. 58 (1980) 161-164

MASS SPECTROMETER WITH A MULTICHANNEL ION DETECTOR BASED ON MICROCHANNEL PLATES V. I. DUKItANOV, A. G. ZELENKOV, A. A. KURASHOV, I. B. MAZUROV, Yu. F. RODIONOV, I. N. SERIKOV, V. P. TARASEVICH 1. V. Kurchatov Institute o f Atomic Energy, Moscow (USSR)

(Received March 15, 1980)

Position sensitive detectors based on microchannel plates were introduced in a mass spectrometer. With the help of this multichannel ion detector, the measurement time and errors are reduced significantly.

Introduction In magnetic spectrometers, widely used for isotope composition analysis, single-channel detection o f an ion-mass-spectrum is usually carried out by ion beam scanning over the detector entrance slit. 1 Until recently electric pulse multichannel detection was difficult to carry out because a position-sensitive detector (PSI)) for 3--6 keV ions was not available. At present such PSD's can be made by applying modern electronic microchannel plates (MCP), enabling to locate radiation incident on MCP with high resolution.

Experiment Multichannel system o f ion detection

We have designed and made an ion PSD based on MCP and RC line, whose performance and main parameters are described elsewhere. 2 The detector was installed at the output o f a mass spectrometer analyzer instead of a conventional electrometer so as to make the MCP front face coincidence with the spectrometer focal plane (Fig. 1). Focussed ion beams give rise to localized electron avalanches whose bombardment o f the resistive layer o f the collector RC line induces current pulses in its circuit. The shape o f these pulses depends on the location o f electron impact on the resistive layer surface. Electric signals taken from the two opposite ends o f the RC line with the electronics shown in Fig. 1 are transformed into a pulse spectrum whose height is a linear function o f electron J. Radioanal. Chem. 58 (1980) 11

161

V. I. DUKHANOV et aL: MASS SPECTROMETERWITH A MULTICHANNEL 1

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i

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3

', L---_.2-~ Fig. I. Scheme of the system. 1 -

Mass s p e c t r o m e t e r v a c u u m c h a m b e r , 2 -

i o n b e a m s w i t h 5 k e V e n e r g y a n d m a s s e s M1, M 2 a n d M3, 4 separated ion beams, 6,7 t o r : R C line, S F -

- microchannel plates, 8 - electron avalanches, 9 -

source follower, ZCD - zero-cross discriminator, SA -

c o p y a m p l i f i e r , P S F - p u l s e s h a p i n g filter, D time-to-amplitude converter, MCA -

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ion source, 3 -

electromagnet, 5 collec-

spectros-

d e l a y line, O - o s c i l l o s c o p e , T A C -

multichannel analyzer

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100 Chonnel number

Fig. 2. Neodymium mass spectrum

ocation at the collector. Thus it is linearly dependent on the ion mass, the number of appropriate height pulses being proportional to the number of ions of the corresponding mass.

Results Performance tests of PSD were carried out by measuring the mass spectra of sodium, potassium, barium, neodymium and uranium. The neodymium mass spectrum is shown in Fig. 2. It illustrates the possibility of simultaneous multichannel detection o f all seven stable isotopes of neodymium with mass numbers 142, 143, 144, 145, 146, 148 and 150. This spectrum toca162

Y. Radioanal. Chem. 58 (1980)

V. I. DUKHANOV et al.: MASS SPECTROMETER WITH A MULTICHANNEL

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150

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2(30 Chonnel number

Fig. 3. Uranium mass spectrum (fragment)

tion distribution along PSD is 11 mm (the distance between the 142 and 150 peaks). Position nonlinearity does not exceed Y_5% over the working range. The observed difference in peak widths can be accounted for by noncoincidence of the MCP front face with the mass spectrometer focal plane. The instrument resolution was determined in an experiment A = C(M1 + M2)/(a + bXM2 --M1) where

a - full peak width at 10% height for mass number M 1; b - that for mass number M2; and C - center to center distance between the peaks. The value of A = l l 0 0 was obtained from the measurement of the 233U mass spectrum (Fig. 3). Experience shows that the PSD using MCP does not at least deteriorate resolution of the mass spectrometer used while substantially increasing its sensitivity. The latter was confirmed by uranium mass spectrum measurements, where 1 - 2 nanogram of uranium was put on a three tape ion source evaporator. Information on relative isotopic sensitivity can be obtained from the mass spectrum (Fig. 2),where the mass concentration ratio of uranium-235 and uranium-233 is 5.2- 10 -3 . The background count rate is about 3 p/s" cm 2. This may be accounted for by microdischarges in some MCP channels and depends on their quality.

Conclusions The realization of ion mass spectrum detection with PSD using MCP essentially involves the development of a multichannel instrument possessing the advantages both of a mass spectrometer and a mass spectrograph. J. ~ d i o a ~ l . Chem. 58 (1980) 11"

163

V. I. DUKHANOV et aL: MASS SPECTROMETER WITH A MULTICHANNEL Using PSD decreases the measurement errors due to the instability of ion-beam source intensity in time, saves analysis time, and permits to apply conventional nuclear electronics with computer data acquisition and analysis systems. Two year's experience with the above detection system gives every reason to consider it reliable and promising.

References 1. J. D.~WALDRON, Advances in Mass Spectrometry, Pergamon Press, London, 1959. 2. W. PARKER, K. D. EVANS, E. MATHIENSON,Nucl. Instr. Methods, 121 (1974) 151.

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