Two inexpensive automatic sample changers for use with an MCA

N U C L E A R INSTRUMENTS AND METHODS

II2

(I973) 5 9 7 - 5 9 9 ;

©

NORTH-HOLLAND

PUBLISHING

CO.

T W O I N E X P E N S I V E A U T O M A T I C S A M P L E C H A N G E R S F O R USE W I T H AN M C A JON J. E R I C K S O N , ERIC L. E A S T W O L D , J U D S O N F. P A R K E R , R O B E R T J. B A G L A N , K E N T H. LARSEN and A. B E R T R A N D BRILL

Division of Nuclear Medicine, Vanderbilt Hospital, Nashville, Tenn., U.S.A. Received 5 April 1973 Two inexpensive automatic sample changers for use with automatic-cycling multichannel analyzers are described. Both small dry samples and small liquid samples can be used.

Most multichannel analyzers (MCA) presently available contain some means of automatic cycling between the storage mode and the read-out mode. When used in conjunction with an automatic samplechanging mechanism, it provides the user with the capability of counting multiple samples without having to manually change the samples. We have designed and constructed two inexpensive sample changers which are used with an M C A and a lithiumdrifted germanium detector for y-ray spectroscopy studies. Fig. 1 illustrates a mechanism which allows us to count small samples of freeze-dried tissue with an accurately reproducible geometry. The samples are enclosed in small paper envelopes, which in turn are fastened at specified intervals on a length of paper tape of the type used in any paper tape punch. The paper tape with the samples attached is placed on a tape reel and then threaded through a lead enclosure, a

position sensor and finally wound onto a take-up reel, which is driven by a 1 rpm motor. The position sensor is simply a metal post and a wire brush between which the tape passes. The circuit shown in fig. 2 controls the changing of the sample in the following manner: When the sample is in front of the detector and the M C A is storing a spectrum the relay controlling the power to the motor is de-energized and the motor does not run. When the M C A begins to read out the spectrum data it produces a negative going signal on the control lead to the electronics. This switches the state of the flip-flop (FF) which energizes the relay allowing the m o t o r to run and so move the paper tape through the counting chamber. At some distance in front of each sample a row of holes is punched across the tape. When this row of holes passes through the position sensor, it allows the wire brush to contact the metal rod producing a signal which opens the relay and stops the motor. The

UN COUNTED MPLES

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POSITION

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dAMPL OPATIONS C0NT Fig. I. Drawing of mechanical system for paper-tape sample changer. Inset illustrates a sample mounted on the tape with the control perforations which are used to stop the tape advance when the sample is in front of the detector.

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J. J. ERICKSON

next sample is then in position for counting and will remain there until the MCA again switches from STORE to READOUT. This sample changing system was satisfactory for small samples, but the need to count larger liquid samples prompted the development of the second system shown schematically in fig. 3. The liquid samples when obtained by our laboratory are in small bottles of various sizes. In order to handle the varioussize bottles a carrier consisting of a 3 ounce plastic pill jar with a large metal washer attached to the top is used. The carriers containing the samples are placed in a slide at one end of the changing mechanism. The changing mechanism consists of a bidirectional motor driving a long belt, to which is attached an electro-

et al.

magnet. To obtain a sample the magnet is moved to the slide containing the carriers. Upon encountering a micro-switch the motor direction is reversed and the magnet is energized. This causes the sample to be picked up and moved to a position over the detector where a second micro-switch causes the motor to stop. The magnet remains energized and the sample is held in this position until the MCA switches to READOUT. The negative going signal from the MCA causes the sample to be carried to the far end of the mechanism where the magnet is de-energized and the sample is dropped into a box. The motor then reverses and returns to pick up a new sample, the switch at the detector being ignored during the return trip. The electronic controls for this changer are shown

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Fig. 2. Schematic

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for the paper-tape

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SAMPLE

HOLDING MECHANISM FOR

Fig. 3. Drawing from the pick-up

of mechanical system for automatic changing of small liquid samples using an electromagnet to move the sample point to the discard point and to hold it over the detector for counting. Inset drawing shows the motorized method used to move the new samples to the pick-up point.

TWO

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AUTOMATIC

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CHANGERS

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Fig. 4. Schematic of the control electronics for the liquid-sample changing mechanism. in fig. 4. It contains the START and STOP pushbutton as does the simpler paper tape changer described above, and in addition provides the option of manually running the mechanism by means of a bidirectional switch. The 5 s delay in the starting circuitry allows the motor to coast to a complete stop before attempting to reverse direction during the turn around at the ends of travel. The samples are moved into position at the pick-up point by a string fastened to a motor shaft, as

shown in fig. 4. When a sample is removed the motor winds the string until the next sample activates the micro-switch at the end of the slide. Both of these sample changers have significantly reduced the time and labor involved in studies requiring the counting of a large number of samples. They are relatively simple and inexpensive to construct and require no specialized equipment or supplies.