A 12-channel magnetic beta-ray spectrometer for quick measurements

Technical notes

Referqmces

37

that the sensitive layer of no detector presently available is thick enough to dissipate the full energy of an electron in case it exceeds a certain limit ( t ) something like 3 M e V . Actually, the m a x i m u m energy of #~+- or ~---radlations from short-lived nuclides is sometimes very high, and this method is not usable there. In such cases, the method (II) has often been used, but ithas obvious disadvantages; that is, its resolution is rather poor and the response function is not simple. Also, both (I) and (If) arc incapable of distinguishing positons from ncgatons, and they have an undesirably high sensitivity toward gamma-rays. In respect to these points, the method (III) has m a n y advantageous characteristics--itscapability of segregating positons from ncgatons, its low gamma-ray background and its simple response function, etc. Moreover, it is rather easy to expand the measurable range of electron energy. A high resolution is obtainable when a high detection efficiency is not important. However, one serious drawback of this method is that the instruments so far have been designed on the single-channel principle;{s} a full spectrum must, therefore, be measured by a point-by-point scanning methods. As a result, this method cannot bc easily used when a sample contains short-lived nuclides, because not only the mensurcmcnt is timeconsuming, but alsothe obtained spectrum isdistorted as the source intensity or sample composition changes during a scan. It has been thought, therefore, that a "multiInternational Journal of AppliedRadiationand Isotopes,1975, Vol. 26, pp. 37-40. Pergamon Press. Printed in Northern Ireland channel" magnetic spectrometer would be very convenient for the measurements of #~+- or ~--rays from short-lived nuclides. A conventional spectrograph is a sort of multichanncl instrument, but a A 1 2 - o h = - - e l Magnetic Beta-Ray photographic emulsion detector is not at all satisSpectrometer l'or O u/clc Measurefactory with respect to sensitivity and accuracy. ments A n improvement in these respectscan bc expected by replacing the photoplate by an array of small (Received 22 April 1974; in revisedform GM-countcrs. A difficulty encountered here is 22 May 1974) that the size of a counter cannot hc made infinitesimal, and some bufflc boards and exit slitsmust bc Introduction installed for each electron channel in order to FOR THE study of short-lived nuclides, some con- eliminate strayelectronsand obtain a good resolution. vcnicnt instrument is often wanted for obtaining As a result, an entire spectrum range cannot be quickly undistorted spectra of electrons. Usually, covered fully with counters; a certain "shaded three different methods have bccn used in electron intervals", within which electrons are not counted, spectroscopy; Si-SSD(I), an organic scintillator are unavoidable. Nevertheless, such a "multi(If), and a magnetic beta-ray spcctrometcr(III). interval", discontinuous, measurement should give However, none of these existing methods seems enough information for one to estimate the full to bc satisfactory: The method (1) has a high spectrum shape in case the spectrum is of the type resolution and good detection efficiency and is a with broad, continuous beta-rays. In the present powerful tool for measurements of electrons, par- work, 12 small GM-countcrs have been instaUcd ticularly of conversion electrons,the kinetic energies at intervals along the focal plane; the resulting of which can be expected not to be very high. spectrometer has proved to bc convenient and However, the application is limited by the fact powerful for quick electron measurements. 1. HmGx~s H. P., BALL D. and EASTHAM S. J. nucl. Med. 14, 907 (1973). 2. ANDROSG., HARPER P. V. and LATHROP K. A. J. din. Endocrlnol. Metab. 25, 1067 (1965). 3. GoouzN A. W. G., GLASS H. I. and WXLLIAMS E. C. Sere. nud. Med. 1,345 (1971). 4. SURPRENANT E. L . , WEBBER M . M. and BENNETT L. R. Int. J. appl. Radiat. Isotopes 20, 77 (1969). 5. CH.~aZLWOND. E. Radiat. Res. 15, 455 (1972). 6. I-IENDESW. R. Phys. Med. Biol. 14, 491 (1969). 7. WAY K. et al. Nuclear Data Sheets 8, 58 (1969). 8. LEDERER C. M., HGLLANDER J . M . and PSRLMA~ I. Table of Isotopes, p. 445. Wiley, New York (1967). 9. EVANS R. D. Radiation Dosimetry (Edited by F. Aa-rL~ and W. RosscH), p. 135. Academic Press, New York (1967). 10. HAGER R. S. and SELTZER F,. Table of Internal Conversion Coeffwient Nuclear Data, Vol. 9, p. 62 (1967). I I. BRGWNELL G. L., ELI.~T XtN.H. and I ~ D D Y A. R. J. nucl.Med. Suppl. 1, 28 (1968). 12. SMITH E. M., B R O W ~ L L G. L. and ELLST W . H. Prindples of Nuclear Afeclidtw (Edited by H. N. WAGNER, JR.), p. 864. Saundcrs, Philadelphia (1968),

$8

Tedmical rioter

DescrJpt/oB o f the Spectrometer The present ~ectrometer is of a flat type with semicircular electron orbits with radii of from 40 to 106 m m in a homogeneous magnetic field with a flux density of from 0 to 4 kG; the energy range covered is approximately from 0"1 to 10 MeV. I n Fig. 1 the layout of the designed electron paths and the related spectrometer components are shown. The entrance- and exit-slit openings are changeable. The sample is put on a thin Mylar (Du Pont) ribbon and usually placed right outside a 8 p m Mylar window of the vacuum chamber for quick sample introduction. The energy loss at the air layer(,~0"3mg/cmI) and the window film is corrected whenever necessary. The passage between the source and the entrance-slit is magnetically shielded by an iron cover. I n order to eliminate scattered stray electrons, linings and buffles made of Neoprene or Acrylite are used. The counter array consists of 12 gas-flow GM-counters, 10 mm wide and 30 mm long, arranged side by side at l l-ram intervals. The whole counter block is mounted on a rack-pinion system so that counters can be slid within a 15-ram limit along the focal plane. The signal handling system is shown in Fig. 2. Each one of the counters has its own PH(pulse height) fixer unit which assigns a characteristic PH to signals from the attached counter. Thus, any one of the signals can be identified by means of a PH analyzer as to the counter which has sent

it out. When more than two counters emit signals within the pulse duration, there will be a piling-up of pulses to result in an erroneous PH. Pulses from the PH fixer units are thus shaped to have a 4~psec duration so that such pile-ups may be minimized. Moreover, in order to prevent such pile-up pulses from being registered, an antl-pile-up gating system is used.

CAlibrations and Standard/zat/ons The magnetic flux density B is measured by means of a calibrated Gaussmeter. The radius of the electron path p is determined from the distance between entrance- and exit-slits. The electron energy E6(keV ) is obtained from the B . p(G cm) value using a published table. (s) For obtaining an electron spectrum in the present system, those counts are combined that have been taken with many different counters and at different p-positions. In order to prevent the resulting spectrum from being distorted, "standardizations" must be made with the detection efficiencies of individual counters and also the transmission efficiencies at all p-positions. The relative detection efficiencies of any two counters are compared by bringing each counter, one by one, to the same position so as to take counts with all the other conditions unchanged. The relative transmission efficiencies at any two p-positions are compared by measuring an adequate sample spectrum (a fiat

FIG. I. Layout of the beta-ray spectrometer components and the electron paths.

39

T~Jmie~l aot~s

GM Counters

"Fi:!: P ~ ~ .

PI|s~M':~. '1

I Pile'upDetectorI

~.

Gate -I , PHA Fro. 2. Signal handling system. plateau is desirable) at each pposition, choosing the B-value so as to keep the B • p-value properly. Resolution and Transmittance I n order to test the attainable resolution, the pure conversion electrons from m ~ I n (E 7 = 892 keV) were measured with 2 - m m slit openings; the results are given in Fig. 8. I n this experiment, the whole spectrum region was measured in a series of five runs in this way: A single run gave 12 points, one at every 1 l-ram interval of 2 • p; the next run was carried out with the counter block slid 2 m m sideways, and so on, all with a constant B-value. Counts stored in the P H analyzer were read-out for each run. Spectrum (a) was taken with the

= 2 mm B = 243G

Sits 3

(a)

source placed inside the v a c u u m chamber to demonstrate the best resolution. Spectrum (b) was taken with an ordinary external source, where the shih and broadening of the lines m a y be seen. T h e resolution thus obtained ( F W H M ) , expressed as A(B • p)/B • p, is 1"3 per cent with Spectrum (a) and 1"6 per cent with Spectrum (b). W i t h 4-ram slit openings and an external source the resolution is 2"8 per cent. W h e n the same conversion lines were measured with neither buffles nor linings, the lines were apparently broadened and the base line was lifted considerably upward, showing the effect of the stray electrons. This shows that the buttes and linings are important for a good resolution. T h e absolute transmittance measured at O = 98 m m with 2-ram slits is 1"3 x 10-4, while that measured at p = 91 m m with 4 - m m slits is 3.3 x 10- l .

Exanaple o f Beta-Ray M e a s u r e m e n t s I n Fig. 4 the results of a practical experiment {i} are shown. T h e carrier-free zz~rnIn ( T l l s = 18 rain) was produced by the 12°Sn(y,p) reaction and was purified by oxine extraction. T h e beta-ray measurements were carried out with two position settings of the counter block (open circles and black dots in Fig. 4), one being 5"5 m m slid from the other. While a sample was decaying with a half-life of 18 min, the above-mentioned two positions were set by turns repeatedly at constant time intervals of 2 rain, with an accumulation of counts in two m e m o r y sections, one for each group, of the multichannel P H analyzer. T h e accumulated counts of one group were corrected with the decay for 2 rain. A F e r m i - K u r i e plotting of the beta-spectrum thus obtained shows a reasonable straight line, giving 2"65 M e V as the m a x i m u m energy.

(b),', II II II II fl It

eM

@O

"o

i

1 I

0'

1.8

65 MeV

i

I

2.2 2.4 By x 10-s (G'cm)

2.0

~ i 2 .

0 2.6

FIO. 3. Spectra of zzs'nIn conversion lines measured with (a) an internal source, and (b) an external source.

~

i ,~" o.._<;1 2 3 Ele (MeV)

FIG. 4. F e r m i - K u r l e plottings of beta-rays from z Z ~ I n (T1/B = 18rain), an example o f practical experiments.

Technical notes

40

Acknowledgeraents--The help of Mr. T. SAITOwith the preliminary work, the Group on OULNS, available to us, and with the Oaussmeter acknowledged.

kindness of who made the help of calibration

the Cyclotron their facilities Dr. G. SODA are gratefully

Cheraistty Department, and Saouzow FtmvsmMa Laboratoryfor NuclearStudy A. Mrro Faculty of Sdence, Osaka University, Toyonaka, Osaka Japan Radiology Department K. MORIKAWA College of BiG-Medical Technology, Osaka University Toyonaka, Osaka, Japan References 1. MA'~zR J. W. I. R. Trans. nud. Sd. NS..9 124 (1962).

2. Alpha-, Beta- and Gamma-Ray @ectroscopy (Edited by K. SmosArm), Vol. 1, Chapter 3, p. 79. North-Holland, Amsterdam (1965). 3. Alpha-, Beta- and Gamma-Ray Spectroscopy (Edited by K. SmosxHr~), Vol. 1, Appendix 2(B), p. 856. 4. FtmosmMX S., Mrro A., Mvl~o Y., IWAa'AS. and SASAJIMAK. (to be published).

International Journal of Appfied Radiation and Isotopes, 1975, Vol. 26, 40-41. Pergamon Pre~. Printed in Northern Ireland |

pp.

An Underwater Varlable-Geometry Irra,4is.tor (Received 18 May 1974; in revisedform 23 July 1974) NOTWITHSTANDING the relatively large number of g a m m a irradiation facilities available at the South African Atomic Encrgy Board for research and development studies,(1) the pressure of work has necessitated a further extension of the available e°Co irradiation rigs. Based on our experience with a number of commercially available irradiation facilities, the main criteria laid down for the design of an additional irradiation source were the following: (i) the rig had to bc housed in the already existing Pool Irradiation Facility;~2) (ii) it had to accommodate cobalt pencils similar to the AECL type C188 since pencils of the same dimensions are used in the other existing facilities and are manufactured on site by the Atomic Energy Board;

(iii) maintenance had to be simple and any potential mechanical malfunction had to be easily rectifiable. The first facility manufactured was of a very basic design with a simple fixed-geometry annular arrangement of pencils, with provision for placing various sizes of watertight sample containers at the centre of the array. Experience with this facility led to the design of a somewhat more sophisticated rig, the major modifications being that the individual pencils are separately contained and the array diameter is variable between 10 and 60 era. This underwater variable-geometry irradiator, illustrated in Fig. 1, is constructed from austenitic stainless steel to allow for continuous operation in the demineralized water of the Pool Irradiation Facility. The design incorporates 12 vertical concentric source tubes, each affording containment for one a°Co pencil of the AE(3L (3188 type. The source tubes are fitted with funnel-like openings to facilitate the underwater positioning of the pencils in the source tubes. Each source tube is attached by a cranked arm to a vertical shaft. The shafts are interconnected by an endless wire rope. Rotation of the radius control shaft imparts rotational motion to the vertical shafts, thus moving all source tubes concentrically to the desired pitch diameter. The source tubes can thus be set concentrically from a maximum of 60 cm to a minimum of I0 cm pitch circle diameter, with 10 positive-stop intermediate positions. The pitch circle diameter is clearly indicated by a pointer connected to the radius control shaft. The sample container, also of austenitic stainless steel, incorporates a screwed plug with piston-type O-ring to afford a watertight assembly. The lid of the sample container is fitted with a ring for fixing the rope by which the container is lowered into the pool. The e°Co pencils are 45.2 cm long, have a body diameter of 0"96 cm and are fitted with solid stainless steel end caps which are slotted to enable underwater remote-control handling of the pencils. With a specific activity of 13 Ci/g, each pencil has an activity of some 1500 (3i. With all 12 pencils positioned, the irradiator has a maximum activity of about 18 k(3i. Dose rates along the centre axis of the sample container can be varied between 3"0 × 104rad/h (four pencils with source pitch diameter of 60 cm) and 2-4 × 10s rad/h (twelve pencils with source pitch diameter of 10 cm) by altering the source pitch diameter and/or the number of pencils in the array. A minimum of four pencils are used to achieve the lower dose rates and still retain a reasonable dose uniformity throughout the sample container. The simplicity and versatility combined with the