Small animal and infant liquid scintillation counter

International Journal of Applied Radiation and Isotopes, 1969, Vol. 20, pp. 249-254. Pergamon Pre~. Printed in Northern Ireland

Small Animal and Infant Liquid Scintillation Counter** E. R . G R A H A M Missouri Agricultural Experiment Station, University of Missouri, Columbia, Missouri, U.S.A. Performance and calibration of a small animal or infant liquid scintillation counter is described. With this instrument it is possible to measure the total amount of potassium in an animal or an infant which contains 10-12 g of potassium with an accuracy of 4% at 1 standard deviation or at 8% for 2 standard deviations for a counting time of 15 min. U N C O M P T E U R DE S C I N T I L L A T I O N LIQ.UIDE P O U R LES PETITS A N I M A U X ET LES ENFANTS On d ~ r i t le fonctionnement et l'~talonnage d'un compteur de scintillation liquide pour les petits animaux ou les enfants. Avec cet instrument il est possible de mesurer la quantit~ totale de potassium d a m un animal ou dans un enfant contenant 10 ~ 12 grammes de potassium avec une pr6gision de 4% ~, une d~viation normale ou de 8 ~ h deux d~viations normales pour une p~riode de compte de quinze minutes. H~H,~HOfiTHBIIYl CI4HHTHJIJIH~HOHHblIYl C q E T q H H ~JIH OIIPE,~E.TIEHHFI HAJIHFI B HEBOJIbIIIHX HiHBOTHblX H ~ET.flX B cTaTbe on~cuBaeTCA pa6oTa x Ka~a6poBKa HtHRHOCTHOFOCH~IHTH~IaHI~HOHHOFOCqOTqKKa. 9T~K npH6opoM MOmHO ~sMepFrrb o6n~ee HOJIHtIeCTBO Ha.TIHFI B HtHBOTHOM HHH pe6enHe, eoRepman~Hx OT 10 RO 12 rp Karma. TOqHOTb HSMepoHHR paBHa 4 ~o Ha 10TRJIOHeHHe OT cTaHRapTa, H~IH 8 ~o Ha 20TR~IOHeHHH ~JIH BpeMeFIH Ho~cqeTa paBHOr0 15 MRHyTaM. FL1DSSIGKEITS-SZINTILLATIONSZAJ-ILER F O R K L E I N T I E R E UND S&UGLINGE Leistung und Eichung eines Flfissigkeits-Szintillationsz~ihlersffir Kleintiere mad Siiuglinge werden beschrieben. Mit diesem Instrument ist es m6glich, den gesamten Kalinmbetrag in einem Tier oder Siiugling zu messen bei einem Kalinmbetrag von 10-12 g mit einer Genauigkeit yon 4% bei 1 Standard-Ahweichung oder yon 8% bei 2 Standard-Abweichungen ffir eine Z~ihlzeit yon 15 rain. STUDIES on h u m a n a n d a n i m a l n u t r i t i o n h a v e b e e n e n h a n c e d b y the use o f sensitive g a m m a r a y d e t e c t i o n counters. I t has b e e n f o u n d t h a t the d e t e r m i n a t i o n o f t h e n a t u r a l * Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 5472. Approved by the Director. 1" Research supported in part by the AEC Contract No. A T (11-1)(1064) and by the Universi W of Missouri Research Council. The counter was designed by E. R. Graham, made by the University Science Instrument Shop, and assembled by Ben Schmidtke of the Packard Instrument Company.

o c c u r r i n g potassium-40 isotope in infants c a n b e r e l a t e d to the n o u r i s h m e n t o f t h e infantt¢). M a n y workers h a v e expressed a desire to d e t e r m i n e t h e p o t a s s i u m c o n t e n t o f small a n i m a l s , such a s chickens, rats, m i n i a t u r e pigs, etc. S o p h i s t i c a t e d a n d e x t r e m e l y sensitive l i q u i d scintillation counters h a v e b e e n d e v e l o p e d a n d used w i t h g r e a t success for a n i m a l s a n d p e o p l e in the r a n g e o f 4 0 - 1 0 0 0 lb. H o w e v e r , suitable sensitive l i q u i d scintillation counters for weights in t h e 10 l b r a n g e a r e m o r e difficult to design a n d to d e v e l o p . I t is t h e p u r p o s e o f this p a p e r to describe 249

250

E.R. Graham

the design, operation characteristics, and the calibrations of a small counter which can be used with either small animals or infants.

D E S I G N OF THE C O U N T E R The counter is a two tank arrangement in which the tanks hold 87 1. of scintillation liquid. T h e y are 26 in. in length and represent of a 24-in. dia. circle. T h e tanks are coated inside and out with white epoxy paint. T h e y were constructed of 16gauge ( 1 . 5 m m or 0.0598 in.) stainless steel. O n the back of each detector tank is mounted a 40.6 cm (16 in.) dia. D u m o n t K-2128 photomultiplier tube and a pre-amplifier. T h e tanks are arranged so that they face each other. T h e diameter is adjustable with a m i n i m u m of 7 in. at the center of the tanks. (See Fig. 1.) Both detector tanks can be moved in a vertical direction. T h e bottom tank can also be m o v e d in the horizontal, as well as vertical direction. It is also possible to adjust the top tank a small a m o u n t in the horizontal direction. The detectors are shielded by an 80 metric ton steel c h a m b e r whose walls are 1 0 c m thick and whose floor and ceilings are 15 cm thick. T h e dimensions of the steel room are 2"7 m high, 2 . 7 m wide, and 5 . 5 m deep. Additional shielding is provided by 44 tons of pure quartz sand which was placed over the top of the room to a thickness of 61 cm and 31 cm thickness on the north and south walls. T h e end walls and doors are i0 cm thick steel and are not shielded with sand. INSTRUMENTATION T h e photomultiplier tubes and the preamplifiers are connected to a Packard dual channel liquid scintillation body monitor system and in this system each tank is separately switched to the control box; therefore, each tank can be looked at individually or both used in pairs. Regulation of high voltage control to each photomultiplier is regulated by a coarse and fine control and a very fine control switch. T h e coarse control advances in steps of 100 to a m a x i m u m of 1900V. T h e fine control advances in steps of 10, and the vernier is graduated in units of 1 V, to a m a x i m u m of 10 V. T h e m a x i m u m capable voltage would

be 2010. T h e high voltage to each photomultiplier tube is regulated by a panel. Although the basic operating voltage is 1300, each photomultiplier tube has a different set of high voltage controls and must be standardized so that when the two tanks are operated together both photomultiplier tubes will be balanced and function as one detector.

BALANCE AND STANDARDIZATION A cesium-137 source was used for standardization and balancing of the pair of photomultiplicr tubes. The discriminatorswcrc adjusted in such a way that the cesium energy peak appeared at 0.480 M c V . These adjustmcnts wcrc made with the high voltage control panel and with the focus and gain adjustments of the prc-amplificr and other amplifiers. The gain controls for each tank will nccd to bc adjusted so that the Cesium energy peak will appear at 0.480 M c V . At the cesium standard sourcc energy peak, approximately half that gain will result in the proper setting for potassium-40 measurements. It is advisable, however, to use a 4°K standard (KCI as a dry salt point source is preferred) to adjust the final voltage and amplifier gain settings for m a x i m u m resolution and efficiency on potassium-40, when potassium-dO is to bc measured. This procedure is also recommended for any other isotopes which arc to bc counted by this system. CALIBRATION PROCEDURE FOR POTASSIUM-40 A source of potassium-dO, such as a polyethylene packet containing 4000 mcquivs (298"2 g) of KCl, is placed in the center of the counter. The curve of count rate vs. energy will bc shown in the middle column of Table I and presented graphically in Fig. 4. Both background and net counting rate are shown. Window settings for o p t i m u m potassium counting m a y be obtained by examining the u p p e r and lower levels of pulse energy which will be recorded in the counting window so that the most efficient ratio for potassium-dO comp a r e d to background m a y be determined. T h e highest ratio is not necessarily the most desirable. T h e best window setting is sometimes referred

FIG. 1. The right tank holding the baby is pushed under left tank. 7 in. minimum dia.

The 14 lb baby may be counted with

250

FIG. 3. Tanks in the blocked position, container holding chickens has a dia. of 16.5 cm.

Small animal and infant liquid scintillation counter

~251

TAeLZ 1. Counting rate and efficiency in relation to window setting for potassium (point source KCI, 4000 me). For lS¢Cs (point source 2.16 x 105 gamma d/n'tin)

B.G.

Spectrometer window

(c/m)

Gross (c/m)

Net (elm)

Efficiency* (%)

080-130 130-180 180-230 230-280 280-330 330-380 380---430 430-.480 460-530 530-580 580---630 630-680 680-730 730-780 780--830 830-880 880-930 930-980

6092 2213 1485 1032 758 640 568 482 440 404 364 332 286 258 239 219 198 168

8599 3351 2324 1831 1510 1388 1308 1267 1305 1354 1326 1275 1157 992 864 699 534 395

2507 1138 838 799 752 748 740 785 865 950 962 943 871 734 625 480 336 227

8.10 3.67 2.70 2.58 2.42 2.41 2.39 2.53 2.79 3.06 3.10 3.04 2-81 2.37 2.01 1.55 1"08 0.73

* The sum of efficiency percentages arrived at in the above manner equaled 49.34%. window of 080-1000 frequently results in an observed efficiency of 51%.

Spectrometer window 080-130 130-180

180-230 230-280 280-330 330-380 380---430 430---480 480-530 530-580 580-630

(c/m)

Cesium-137 Gross (c/m)

5694 2208 1427 1011 833 654 582 491 445 408 377

20,456 15,090 15,833 16,308 14,299 9,990 5,934 3,033 1,561 979 719

B.G.

Net (c/m)

Efficiencyt (% )

14,771 12,882 14,406 15,297 13,466 9,336 5,352 2,542 1,116 571 342

6.82 5.95 6.66 7.07 6.22 4.31 2.47 1.17 0.51 0.26 0.15

A

~"The sum of efficiency percentages arrived at in the above manner equaled 41.59%. With double gain and a window of 100-1000 efficiency of 49.3% was observed. to as the "figure of merit": E2/BG; however, in counting both infants and animals, the shortest counting time resulting in reasonable accuracy becomes more important than the figure of merit. The plot of counting rate against threshold at 5 per cent window, as presented in Fig. 4, shows the resolution of the gamma-ray spectra

for ~°K and xS~Cs to be the same as found by Kessler tS). This is to be expected since our photo-tubes are the same as Kesslers and the liquid scintillator* nearly the same. The spectra was different than the one observed * Scintillation solution purchased from the Packard Inst. Co.

252

E.R. C,ra/~m 5O

40

a0

:~ 20

10

0

12 '

;o'

8

6'

4'

2'

0'

2'

4'

6'

8'

;0

1'2

inches from center of counter Fio. 2. The relative counting efficiency for a point source to the distance of the source along the central axis of the well. (point source 4000 me K) (window 075-1000) (Eft. calculated from 198 gamma d/m/g K). by FORBEStm when plastic scintillators were used. This figure shows excellent efficiency for 4°K and ]S¢Cs and good resolution for lSTCs. T o be able to get the best accuracy for 4°K with the shortest counting time, it appears advisable to use a window of 080 to 1000 (see T a b l e 3) and make corrections for xS¢Cs or other isotopes which emit g a m m a radiation in the 0.3-0.7 M e V energy region. It m a y be that for infants and very young animals, corrections will be unnecessary. I f corrections are needed, they m a y be m a d e by channel ratio method or by solving a simultaneous equation for la~Cs and 4oK contributions to the net counting rate.

LOSSES DUE TO GEOMETRY I t is obvious that a point source of radiation placed at the end of the well of the counter will result in fewer pulses being recorded than the same source in the center of the counter. This is referred to as the "end effect". T h e results of an " e n d effect" study on this particular counter are shown in Fig. 2. T h e results show that the counting efficiency does not drop off rapidly as the end is approached.

This is probably due to the fact that the counter is equipped with only two photomultipliers and that the effective distance between tanks is only 17.8 cm resulting in greater efficiency than was observed b y GARROWt8).

LOSSES DUE TO SELF-ABSORPTION IN THE SOURCE An approximate estimation can be m a d e of losses due to self-absorption b y measuring potassium salt and polyethylene containers TABLE 2. The effect of tank distance on potassium efficiency. (4000 m.e. source of KCI centered)

"Yanks blocked Tanks separated Tanks separated Tanks separated Tanks separated

Well dia. minimum (in.)

K efficiency % 080-1000 window

7 8 9 10 11

51 "9 48"7 45 "3 42"7 40"9

Small ~imal and infant liquid sdntillation counter TABLE 3. P o t ~ u m

253

efficiency as related to source composition and configuration Window 080--1000

Window 400-1000

Efficiency*

BG

Efficiency

BG

Source

(%)

(c/m)

(%)

(c/m)

KCI (point source) Solution KCI 6 I., 7 in. dia. (16 in. long) Plasticdoll Sugar and KCI filled(4 Ib) 15 in. long, av. dia. 3.5 in.

51.0

16,311

28-6

3925

45.6

16,311

25.4

3925

48.5

16,311

27.2

3925

* Tanks adjusted to blocked position. (distance between tanks 7 in.). Efficiency calculations are based on 198 gamma d/rain/1 gram potassium.

22 20

18

g

14(

10

4

2 |

080

180

180

230

280

330

380

430

480

530

580

630

680

730

780

830

Fxo. 4. Differential gamma-ray spectra with xaTCs and leK sources. • 4eK,

880

930

@ ~TCs.

254

E.R. Gr~'mm

containing potassium chloride dissolved i n water. Simulating the size and shape of various parts of the animal body presents a problem. This subject has been discussed in detail by DEANc1~. Probably the best calibration is Obtained by giving the animal a known amount o f the artificial isotope ~ K . This will be done i n the future to complete the calibration. Table 3 shows results of a study made by KCI salt, KC1 salt in solution, and KC1 mixed with sugar and placed inside a hollow plastic doll.

DESIGN CHARACTERISTICS T h e two tank system allows for a modification of the diameter of the inside well. This works out to be a definite advantage when small animals or small infants are to be counted. When both tanks are blocked (see Fig. 3), the

dia. between the detecting surfaces would be 17.8 cm (7in.). The tanks may be moved apart allowing for larger diameters which would accommodate larger animals or larger samples. T h e y may be moved at one inch intervals without a great loss in efficiency (see Table 2 for efficiency loss when tanks are separated.)

REFERENCES 1. DEAN P. N. Ex~rim~.al Techniquefor High Predsion Calibrationof Whole Body Counters, Ch. 1O, Tho~a.~ PubliAhlng, Springfield, Ill. (1963). 2. FORBESG. B. J. appl.Radiat. lsot~s 19, 535 (1968). 3. Gxsaow J. S. W. Iad/an med. J. 14, 73 (1965). 4. GARROWJ. S. Lancet, September 4, page 455--458 (1965). 5. KESSLERW. V., Zxxuza P. L., C ~ J . E. and SHAW S. M. Int. J. appl. Radiat. Isotopes 19, 287 (1968).