A stereo magnetic tape system for radioactive tracer field experiments

International

Journal

of Applied

Radiation

and Isotopes,

1972,

Vol.

23. pp. 271-277.

Pergamon

Press.

Printed

in Northern

Ireland

A Stereo Magnetic Tape System for Radio, active Tracer Field Experiments C. A. R. BAIN, Southern

and

W. R. McMURRAY

Universities

Nuclear

Institute,

Fame,

J.

F.

Cape,

RAJDL

South

Africa

(Received 26 January 1972) A portable detection apparatus is described which makes it possible to undertake radioisotope experiments where transient changes in count rate must be observed even in difficult environmental conditions. In addition to conventional scaler, ratemeter and chart recorder facilities the apparatus includes a commercially available stereo tape recorder. The output pulses from either one or two scintillation detectors are recorded on one stereo track and time pulses from a crystal oscillator are recorded on the other. The data are read out at the laboratory into a multi-channel analyser operated in multiscaler mode using the time pulses to actuate the address advance. This system accurately reproduces the count rate as a function of time in the original data. The operating performance of the system is discussed and illustrated by means of results obtained from radioisotope tracer studies of sand movement on the sea floor near harbour installations. UN

SYSTEME STEREO A INDICATEUR

A RUBAN MAGNETIQUE RADIOACTIF CONDUITES

POUR DES EXPERIENCES A L’EXTERIEUR

On decrit un appareil detecteur transportable qui rend possible les experiences radioisotopiques oh des variations transitoires doivent Ctre observtes mCme sous des conditions difficiles d’environnement. En plus des facilitts conventionnelles d’tchelle de dix, mesure de taux et enregistrement sur graphique, l’appareil comporte un enregistreur a ruban stereo disponible dans le commerce. Les pulsations Cmises par un ou par deux dttecteurs de scintillation sont enregistrees sur un sillon stereo et les pulsations de temps d’un oscillateur a cristal sur l’autre. Au laboratoire les don&es sont remises a un analysateur a multicanaux fonctionnant en la mode de multi-Cchelles et utilisant les pulsations de temps pour l’actuation de l’avancement d’adresse. Ce systtme reproduit exactement le taux de compte comme fonction de temps dans les don&es originales. On discute le fonctionnement du systtme et on presente comme illustration des resultats qu’ont rendus des recherches B indicateur radio-isotopique sur le mouvement de sable sur le fond de la mer aupres des installations de port. CHCTEMA

C CTEPEO-MAI’HBTHOH HOJIH

OIIHCaHO

IIOpTaTklBHOe

pa~HOH30TOIIHbIe YYllTbIBaTb CXeMy

06bFIHOI’O

OAHOrO

M3

ABYX

WCKOti

AOpOkKKe,

B

pa6oTamnmti IIpOABIWeHWI BpeMeHM

C~I4HTHJIJWII(HOHHbIX a

BCe

B

IIOCpeACTBOM $(JIH I13YWHHH

ElCXOAHbIMH pe3yJIbTaTOB, ABM?KKeH&W

HMIIyJIbCbI

Ha

AHOM

OT

IIpll

271

CTepeO.

KBaprlOBaHHOrO

CWiTbIBaHYlt?

MOPR

B03MOXtHOCTb

B

B

CKOpOCTb

IIpHMt?HeHIW OKOJIO

ce6e

AHarpaMM, BbIXOAHbIe Ha

a

BTOPOl;t

CBCTeMbI,

aKTbIBaqIil4

SBBHCHMOCTM

WIJIIOCTpHpOBaHHaFI

pajZ(I4Om30TOIIHbIX

IIOpTOBbIX

aHaJIH3aTOp, AJIR

B

COOPY%!HM&.

OT

CTept?OCKOIIH-

I’eHepaTOpa-Ha

CYeTa

TaKHCe

llMIIyJIbCb1

OAHOZt

IIMIIyJIbCbI

HYH(HO

IIepecGTHyIO

MHOFOKaHaJIbHbIti

BpeMeHHbIe

pa6oTa

IIpOBOAkITb

YCJIOBMHX

BKxKmaeT

pE!lWCTpHpyIOTCX

BOCIIPOEI3BOAIIT

IIOJIyWHHbIX IIeCKa

THIIa

06cyxrAeHa

AaHHbIMI4.

AaeT

OKP)‘H(aIOIZII?X OHO

HCIIOJIb3yU

TOqHO

3KCHEPBMEHTOB

pWHCTpHpOBaHHZ4

AeTeKTOpOB

PeWIMe,

CllCTeMa

C+Ta.

pE!KOp~Iep

IIpOllCXOAIlT

MHOrOCYt?THOM aApWa.

B

BPeMt?HHbIe

KOTOpOe

TPYAHbIX

CpeACTBa

JIeHTOYHbIti

naboparoprm

B

CKOPOCTH

mHTeHCHMt?Tp,

BbIIIy~eHHbIti

I4

AJIH

HHjH4HATOPA

YCTPOtiCTBO,

KOrAa

M3MeHeHHH

THIIa,

IIpOMbIILIJIt?HHO

AOpO?KKt?.

HeTeKTOpHOe

3KCIIepHMeHTbI,

IIepeXO~HbIe

JIEHTOH

PAfiBOAKTHBHOPO

SIHAMKaTOpOB

OT

272

C. A. R. Bain, W. R. McMurray and J. F. Rajdl EIN STEREOMAGNETBANDSYSTEM FUR RADIOAKTIVE VERSUCHE AN ORT UND STELLE

INDIKATOREN-

Eine tragbare Detektorapparatur wird beschrieben, die Radioisotopenversuche ermaglicht, wo laufende Veranderungen in der Zahlgeschwindigkeit selbst unter schwierigen Umweltbedingungen beobachtet werden mtissen. ZusPtzlich zu den herkijmmlichen Einrichtungen wie Zahler, Zahlgeschwindigkeitsmesser und registrierendem Messgerat enthalt die Apparatur ein im Handel erhaltliches Stereobandgerat. Die Ausgangsimpulse von einem oder zwei Szintillationsdetektoren werden auf der einen Stereospur registriert und Zeitimpulse von einem Schwingkristall auf der anderen. Die Datem werden im Laboratorium gelesen unter Verwendung eines Vielkanal-Analysators in einer multiskalaren Arbeitsweise, wobei die Zeitimpulse das Fortschreiten der Adresse tibernehmen. Dieses System gibt genau die ZLhlgeschwindigkeit als Funktion der Zeit in den urspriinglichen Daten wieder. Das Arbeitsverhalten des Systems wird besprochen und illustriert durch Ergebnisse von mit Radioisotopenindikatoren ausgeftihrten Studien der Sandbewegung auf dem Meeresgrunde in der N5he von Hafenanlagen. 1. INTRODUCTION USE of radioactive

tracers in industry and hydrology is widespread. In the measurement of the tracer material, environmental factors often necessitate a high degree of portability for the detector and associated electronic recording apparatus. This requirement and the hazards of use in certain environments limits the sophistication of equipment that can be taken into the field, and hence the full utilization of all potential information available from the tracer test may not be realized. This is particularly true for field studies of dynamic systems involving parameters that are changing rapidly. In such field measurements the radioactive events are usually collected in simple form on scalers and printers. Time variations are recorded with rate meters and chart recorders which are often adequate for slowly varying systems. Rapid changes in count rate cannot be recorded on such equipment. It is usually impractical to take multi-channel analysers into the field(l) to make full use of the data presented by the tracer test. Ilowever, the recording of primary radioactivity events on magnetic tape in the fieldc2) with subsequent analysis by playback through more sophisticated apparatus in the laboratory makes it possible to obtain full information from the original data without the limitations imposed by the slowness of response of rate meters and chart recorders or the non-availability of pulse height spectrometers. In several tracer applications rapid variations of count rate must be recorded (e.g. measurements of THE

flow rates in fast flowing systems using the pulse methodt3)). When faced with this requirement we developed a stereo magnetic tape system to record the radioactive events in the field and analyse them in the laboratory where the full analyser and computer facilities are available. As a first stage in this development the original radioactive events are pulse height selected in a window discriminator and recorded on the magnetic tape without further spectral information. A further development is envisaged to record the analogue pulse heights in digital from on the tape. 2. APPARATUS 2.1

General description

The apparatus used in the field consists of one or two radiation detectors, the “Portable Detection Apparatus”, tape recorder and a chart recorder, all battery operated. An ordinary commercial stereo tape recorder (Uher 4200) was chosen in order to make the portable apparatus inexpensive. If their input circuitry were redesigned even cheaper tape units would function quite adequately. The detectors are connected to a coaxial cable up to 100 m long. The remaining equipment is mounted in a special rack as shown in Fig. 1. A separate 12-V car battery supplies power to the equipment. The output of either one or both radiation detectors is fed via a discriminator to one track of the stereo recorder. The other track records pulses from a crystal controlled clock which operates as a time reference for the detector events.

_ _-.. .-

FIG. 1. A photograph

of the portable ing on a small

;--.

electronic equipment fixed in a rack suitable boat used in sand tracing studies.

for mount-

272

A stereo magnetic tape system for radioactive

In the laboratory the recorded radioactivity events are played back through a pulse shaper unit into a multi-channel analyser used in multiscaler mode. The clock track provides address advance signals to the analyser which ensures that each channel in the analyser memory corresponds to the equivalent time intervals during which field data were collected. The read out of the analyser is therefore a record of the count rate as a function of time. The time interval per channel can be selected to suit the measurement being made. The multi-channel record can be read out to a printer, magnetic tape or plotter or transferred to a computer for further analysis. 2.2

tracerjeld

I

Stereo Magnetic

I

tape

unit

Tape-to-Analyser Converter

4

4 Multi-channel

analyser

( in multi-scaler

Circuitry

mode)

I

Figure 2 is a simplified block diagram of the electronic circuitry for both the write and read

Printer/ Plotter/ Punch/ Mag. tape Computer

(b)

-__________t______-+---

273

experiments

PLAYBACK

OF DATA

Detection Apparatus” systems. The “Portable supplies power along the coaxial cable to the preamplifier and E.H.T. converter built into the detector head. The output from the detector is fed back along the same coaxial cable to a mixer and a d.c. restorer and into an adjustable window discriminator. The data pulses are then fed to (i) the data track of the smeez tape recorder; (ii) an internal rate ; (111) an external mini-chart recorder Chart ((ii) and (iii) have linear and logarithmic ranges) ; and (iv) an AND gate to a 6-decimal decade scaler with nixie display. The AND gate is controlled by a timing unit which allows the counting period to be preselected. A lOO-kHz crystal clock supplies pulses for the timing unit and also pulses at selected frequen(a) BLOCK DIAGRAM OF PORTABLE DETECTION cies for the timing channel of the stereo tape APPARATUS recorder. The timing channel can also be used to record speech. FIG. 2. A simplified block diagram of the Both the tape recorder and chart recorder can electronic circuitry of the portable detection be switched on or off from the “Portable apparatus. The connection of the stereo magnetic tape unit to either the write or read Detection Apparatus” control panel. A push systems is shown in (a) and (b) respectively. button switch on the latter connects to an

7 Mixer

El

r

2

274

C. A. R. Bain, W. R. McMurray and J. F. Rajdl

event marker on the chart recorder. The rate meter and mini-chart recorder are available as monitors of the count rate variations to assist in the planning of further measurements and their subsequent evaluation. The main record is contained on the data channel of the stereo magnetic tape which is stored for subsequent playback into a multi-channel analyser. The playback (read) interface (Fig. 2 (b)) consists principally of shaping circuits to match the output pulses of the tape recorder to the multi-channel inputs. 2.3 Count rate response of the tape recorder system Figure 3 shows the count rate response curve for the tape recorder at a tape speed of 19.05 cm/set. It is seen that for random pulses a dead-time loss of 20 per cent occurs at 2 x 1 O4 counts/set. For a pulse generator input (nonrandom) the losses at 2 x lo4 counts/set are negligible, and there is an instrumental cut-off at about 3 x lo4 countslsec. Such high count rates are seldom encountered in field tracer experiments and the losses at such frequencies are not a serious limitation of the apparatus. There is, however, a divider on the data input to the tape recorder to allow known high count rates to be recorded accurately.

Ik

COUNTS

FIG. 3. Count rate losses in the magnetic recorder

tOk

PER SECOND

tape system for random spaced pulses.

ZOk

3. OPERATIONS 3.1

IN

THE

FIELD

Various applications

The apparatus has been designed to satisfy various field requirements and numerous modes of operation can be used. In the simplest case it can be used without magnetic tape as a conventional scaler with nixie tube display to count pulses for preselected times between 15 set By using two and 5 min (or manually). detectors simultaneously with the magnetic tape flow rate measurement can be made using the pulse velocity method. The crystal clock can provide time reference pulses for the clock track of the magnetic tape with timing intervals from 500 ,usec to 50 set, thus enabling the flow rate of high velocity gases or slow moving systems to be measured. 3.2

Use in sand tracing on the seajoor

The apparatus was originally designed to be used in conjunction with a radioactive tracer technique developed for the in situ determination of sediment movement on the seafloor.(4) The development of this technique in South Africa has been undertaken by the Southern Universities Nuclear Institute together with the Fisheries Development Corporation of South Africa and the South African Atomic Energy Board. The technique depends on the attachment of a radioactive label to a sample of sandt5) which is then deposited on the sea floor. The radioactive tracer, which has been used in numerous such investigations around the coast of South Africa is gold-198 (half-life 2.7 days) produced in the SAFARI I reactor at Pelindaba by the South African Atomic Energy Board. Gold-l 96 produced by the C.S.I.R. cyclotron at Pretoria@) has also been used as a tracer in these studies and other A NaI(T1) tracers have been developed. scintillation detector contained in a waterproof housing is used for in situ measurement of the tracer activity on the sea floor. In the simplest Detection Apparatus” is cases the “Portable merely used as a scaler and the accumulated counts for a set time (usually 15 set) at a discrete point on the sea floor are recorded. The full capabilities of the apparatus are used in the dynamic mode when the detector is towed from a ski boat along the sea floor over

A stereo magnetic tape system for radioactive tracerjeld

experiments

275

N2CfE

BOAT WSITICN

(motors)

FIG. 4. An isometric display of multi-channel readouts from a set of tracking runs covering an area where radioactive tracer had been dispersed.

55m.

CONTOUR

MAP

OF

TRACER

DISTRIBUTION

ON

THE

SEA-FLOOR Richard’s

Boy

24 3.71

FIG. 5. An isoactivity contour map derived from the data presented in Fig. 4. the region of interest. Tn this case the detector is contained in a weighted sledge”) and dragged over the sea floor in a predetermined pattern with positions fixed at regular intervals by theodolite bearings from two shore based points or by using telemetry equipment. One stereo

track records the detector events while the other records timing pulses from the electronic clock. For the towing speeds used and the distances traversed the timing interval found to be most suitable has been 1 sec. The time pulses going to the tape are also recorded on the

276

C. A. R. Bain, W. R. McMurray and J. F. Rajdl

Sediment tracerdispersion d Water

depth

Deposition

Lambert’s

Bay

7metres date

8:12:1970

9:12.1970

nixie scaler to give readings of total time from the start of the measurements. In a typical tracking run the detector is towed along the sea floor and at a predetermined starting point the tape unit, nixie scaler and chart recorder are simultaneously switched on. At set intervals during the run, at 30-60 set, the boat position is determined and logged. At these times the nixie display reading is also logged and thus the boat position is correlated with the timing pulses and the data recorded on the magnetic tape. Also at these times the event marker on the chart recorder is activated. A number of such tracking runs are done to cover the area where the radioactive tracer has been dispersed and to track its movement over a period of days. After returning to the laboratory, the primary data collected on the magnetic tape is played back to produce multi-channel “spectra”. A series of such “spectra” can be viewed isometrically as shown in Fig. 4 which was prepared from data collected in a sand movement study conducted at Richards Bay. From such results isoactivity contour maps are plotted (as in Fig. 5.) to provide a picture of the dispersion of the tracer after a certain elapsed time. A series of isoactivity contour maps in turn provide detailed information of sediment movement as a function of time. This is illustrated in Fig. 6 using data collected at Lamberts Bay. These studies were undertaken in collaboration with the C.S.I.R. Hydraulics Research Unit and the Fisheries Development Corporation respectively in connection with harbour development investigations. The same portable electronics is employed to determine the radioactive content of sediment sample collected by rocket core samplers or by In this case the samples are skin divers. counted by one of the same NaI(T1) detectors placed in a lead castle. Counts for a preset time are recorded in the nixie-tube scaler. 4. CONCLUSION

6. A set of isoactivity contour maps showing the time variation of sediment tracer dispersion in a study at Lamberts Bay.

FIG.

The versatility of the apparatus described here has been proved in extensive field experiments over the past 2 yr. It has withstood the harsh sea environment and we have been able to collect useful data in adverse weather conditions even in a small boat exposed to winds of up to force 4 and 5.

A stereo magnetic tape system for radioactive tracerjeld

The chief innovations of our apparatus are (a) the use without modifications of a conventional commercial tape recorder to record the primary information; (b) the use of stereo channels to give precise time related results; and (c) playback through a multi-channel analyser used in multiscaler mode. The time constant limitations of ratemeter circuits for recording data are eliminated in using magnetic tape so that even transient fluctuations in the count rate can be recorded. This is essential in measurements involving rapid changes with time. The apparatus can be used in virtually any type of industrial or hydrological radioisotope tracer experiments. It has recently been used in the measurement of radioactive gas tracers at very low concentrations to investigate leaks from a gas main. Several modifications are envisaged in the future. The whole apparatus can be further miniaturized to fit into a small case by using a casette tape recorder, light emitting diodes for scaler display, and power pack batteries. The ultimate development of this kind of apparatus is to use analog to digital converters (converters with adequate resolution are now available in cheap integrated-circuit form) to record the primary spectral data in digital form onto the magnetic tape. A cheap casette tape unit can be employed and although the maximum count rate which can be recorded

experiments

277

will be more limited, the record on the tape will contain all the information available in the original measurements in the field. is a pleasure to thank F. WILLIAMSfor his part in designing and constructing the circuitry used in the apparatus described in this report. The authors also acknowledge their indebtedness to all those with whom they have collaborated in the investigations cited in this report.

Acknowledgements-It

REFERENCES 1. DUANED. B. and JUDGEC. W. Radioisotopic Sand Tracer Study. Point Conception, California, U.S. Army, Corps of Engineers Coastal Engineering Research Centre Report No. MP2-69 (1969). 2. EDENG. E. and BRIGGSR. Radioisotope Techniques Developed in Water-pollution Studies. p. 191. IAEA (1967). 3. HUNTLEYA. R., GLASSW. and HEIGLJ. J. Ind. Eng. Chem. 63, 381 (1961). 4. BAIN C. A. R., MCMURRAY W. R., RETIEP G. DEV., VONKA. P. M. and VAN As D. Radioisotopes as a tracer for studies of sediment transport in the sea. SANCOR Symposium Oceanography in South Africa (1970). 5. CAMPBELLB. L. Int. J. appl. Radiat. Isotopes 5, 286 (1963). 6. NEIRINCKXR. D. and VAN DER MERWE M. J. Radio them. Radio anal. Letts 8, 41 (1971). R. A. and SOMERE. Ingenbren Denmark 7. ALLINGHAM 4, 91 (1960).