Spurious pulses in proportional counters filled with tissue equivalent gas

355

Technical notes International Journal o f A p p l i e d R a d i a t i o n a n d I s o t o p e s , 1 9 7 3 , V o h 2 4 , pp. 8 5 5 - - 3 5 7 . P e r g a m o n Press. P r i n t e d i n N o r t h e r n I r e l a n d

Spurious Pulses in Proportional Counters Filled w i t h Tissue Equivalent Gas (Received 3 January 1973) IN ~a~.v'xous papers the time distributions of spurious pulses in proportional counters filled with methane (t) and with a 9:1 mixture of argon and methane (z) have been described. T h e present note reports the results obtained using a gas filling comprising 64"4 Yo methane, 32.4 % carbon dioxide and 3-2 ~ nitrogen, a mixture which is frequently referred to as a tissue equivalent gas. T h e techniques used to record the time distributions were essentially the same as those used previously and will not be described again. Figure 1 shows time distributions obtained with a counter, having a 3.8-cm diameter cathode and a 5 × 1 0 - 3 c m diameter anode, at three different pressures of tissue equivalent gas. I n each case the counter was actuated by a source of Fe-55. These distributions were obtained at points 100-150 V beyond the end of the plateau and correspond to total spurious counts of 8-20 per cent of the plateau counting rate. This is considerably higher than for the m e t h a n e case (1) for which the total spurious counts were of the order 2 per cent. A t the higher pressures there are two m a i n features of these distributions, viz. a monotonic decrease in spurious pulses as the time interval increases and then, at longer time intervals, a broad distribution appears, having a peak at a b o u t 100-200 #s and a tail which extends out to more than a millisecond. (The " p e a k " at about 3/~s is due to the finite dead time of the recording system; there is at present no reason to suppose that the distribution does not continue to increase for intervals less than 3/ts.) These general features persist as the pressure is reduced but below 300 torr the peak disappears from the time distribution which is then d o m i n a t e d by the pulses at short time intervals. T h e lowest pressure at which time distributions were recorded was 20 tort and the shape o f t h e distribution at this pressure was very similar to that shown for 50 tort. T h e counter used to obtain the data of Fig. I had an a l u m i n i u m alloy cathode but some further experiments were m a d e using this counter lined with a 0.5-ram layer of electrically conducting tissue equivalent plastic in order to see if a significant ioncathode effect existed with this material. H o w e v e r time distributions taken at 600 and 50 tort with this liner had the same general shape as the corresponding distributions shown in Fig. 1.

~F

,:

= .

.,;

300 torr

i

z~:- ~

.....

'

J

I

'

• ...... o

T

...

,

ii:i

~ =,,

I

. . . . . . . .. o,, o

,

I

t

71- I;U!

MJCrOS4K:Oflcls

F~o. 1. T i m e distributions for three pressures of tissue equivalent gas. T h e main graphs represent the initial parts of the corresponding inset graphs, the abscissae of which are in milliseconds. T h e counter voltages were 3.35, 2-65 and 1.625 kV for the 600, 300 and 50 tort data respectively. It should be noted that, at 600 tort at least, the shape of the time distribution changes markedly if a higher gas gain is used (with a corresponding decrease in the amplifier gain). Thus, using a voltage of 3.9 kV across the above counter filled to 600 torr, a marked cathode photoelectric effect was observed which gave rise to a sharp peak just after the dead time of the system. T h e intensity of this peak was such that it contributed a major fraction of the total spurious pulses. Although this operating potential was 550 V higher than that used to obtain the corresponding data of Fig. 1, the gas gain is only some four times greater; this is due to the fact that the gas gain curve in this region is distorted due to space charge effects.(n) C,m~Pzoz~ and K r N G ~ (~) have reported an anomalous effect occurring in some gases including tissue equivalent gas. This effect manifests itself as a double

356

Technical notes

shows a typical pulse height spectrum due to Fe-55 and the anomalous second peak on the high energy side of the main peak is clearly seen; this spectrum was obtained just beyond the end of the counting rate plateau at a pressure of 720 tort. A single channel pulse height analyser enabled the initiating pulse for subsequent time analyses to be selected. The resulting distributions, for the analyser gate settings shown in Fig. 2, are given in Fig. 3 in which the ordinate represents the probability per pulse selected by the appropriate gate setting, per unit time. These data show that the anomalous peak is responsible for the spurious pulses at short time intervals together with some at longer times but, in the main, the latter are associated with the normal peak in the pulse height spectrum. The cathode photoelectric effect, observed at higher counter voltages, was found to be associated mainly with the anomalous type of discharge; this is consistent with the fact that much more light is emitted in such discharges as compared with normal discharges. (3~ Although there are indications that the anomalous effect may be due to a streamer mechanism no detailed explanation has yet been proposed and it is therefore unwise to speculate on the mechanism by which spurious pulses may be generated in anomalous discharges. The position and shape of the peak in the time distribution at about 200 #s, due mainly to normal discharges, precludes an ioncathode effect as the mechanism responsible for it (an ion mobility of about 20 cm/s per V/cm at

t

oil< C-~,t¢ B (&)

~

I

Pulse height

Gate A {e) , ==

Fio. 2. The pulse height spectrum due to 5.9keV X-rays in a counter filled with tissue equivalent gas at 720 tort.' The counter dimensions were 3-8 cm diameter cathode, 5 × 10-3cm diameter anode and the counter voltage was 3-725 kV. peak in a spectrum in which only one peak would normally be expected. The effect is only observed at relatively high pressures; in the present counter it disappeared below about 200-300 tort. Figure 2

10 (20O

2001

i

,ooo \

.

.

.

.

.

.

.

.

.

.

o

.

.

.

.

.

.

,o

i \ ~

.

.

.

,

L "x

N

.

2o

""""°°°"

r I

~-, 0

"

" 2O

~0

60

80

Micro=¢conds

FIG. 3. The time distributions obtained by selecting the initiating pulse according to the gate settings shown in Fig. 9. Gate A circle data, gate B triangle data.

Technffal notes N T P would be necessary), but suggests the existence of a two stage process involving the growth of certain species which subsequently decay by the production of single electrons. At low pressures the distribution of spurious pulses at short time intervals is similar to that due to the anomalous peak but, since the latter does not occur at low pressures, the mechanism may well be different. P. J. CAMPION

M. BURKE National Physical Laboratory Teddington, England

References 1. CAMPIONP. J. and K1NGHAMM. W. J . Int. J. appl. Radiat. Isotopes 20, 479 (1969). 2. C~PION P. J. and K m o H . ~ M. W. J. Int. J. appl. Radiat. Isotopes 22, 55 (1971). 3. CAMPIO~ P. J. and KSNOHAMM. W. J. Xth Inter-

national Conference on Phenomena in Ionized Gases, Vol. 1, p. 88. Parsons, Oxford (1971).

International Journal of Applied Radiation and Isotopes, 1973, Vol. 24,

pp. 357-359. Pergamon Press. Printed in Northern Ireland

The Preparation of Indinm-lU Labelled Bleomycin for Tnrnour Localisatlon

(Received 19 December 1972) Bta~omvcaN (BLM) was first obtained from a culture filtrate of a strain of Streptomyces certicuillus, by Umezawa in J a p a n in 1962. ~1) It is a mixture of 13 peptides which has been shown to have antitumour activity. (2) The 13 peptides can be separated in two main groups, A (A1-AT) and B (BI-Be) and form complexes with bivalent elements such as copper. (3) The effect of BLM against squamous cell carcinoma and its mechanism of action has been recently reported, (4) several other centres (s.G) have investigated its use as an antitumour chemotherapeutic agent. The use of BLM labelled with cobalt-57 as a tumour localizing agent has recently been reported by Notr~L etal. (~) Cobalt-57 has a very long half-life (270 days) and hence is not regarded as an ideal radionuclide to be used clinically. We have, therefore, studied ways by which BLM may be labelled with other, shorterlived, radionuclides of both bivalent and trivalent elements. The nuclides studies are shown in Table 1. Of these, Indium-111 is the nuclide of choice. It has almost ideal gamma ray energies for external detection with existing rectilinear scanners and gamma

357

TABLE 1. Radionuclides suitable for labelling bleomycin Principal y radiation energies Radionuclide

Half-life

Cobah-57 Zinc-62

270 d 9.3 h

Lead-203 Gallium-67

52 h 78.3 h

Indium-Ill

67 h

(keV) 122 511 593 279 93 184 296 179 247

(87~) (47 ~ ) (22~) (89~) (40 ~ ) (24~) (22~) (89~) (94~o)

cameras, and it can be produced in a relatively large quantity even with a small cyclotron. ~s) Using i n d i u m - I l l BLM encouraging results consistently staging both primary and secondary tumours have been obtained so far on more than 40 patients with various carcinomas. ~9'10) Indium-111 is produced carrier free in our laboratory by 30-MeV alpha particle bombardment of natural silver. (s) After the chemical separation indium-111 is obtained "carrier free" in 0.05M HCI solution. The product is then checked for radiochemical and chemical contamination due to bivalent and trivalent elements that may possibly be present in the solution. The radionuclides of bivalent elements (e.g. STCo, 6~Zn and s°3Pb) are chelated by BLM, or by a component thereof, simply by mixing together the two solutions at room temperature. The reaction can take place in neutral or acidic solution but a neutral, preferably isotonic saline, solution is desirable for an injectable product. The radionuclides of trivalent elements are not however chelated under these conditions, and a different approach is necessary in order to label BLM with n l I n (or 67Ga). The solution of hydrochloric acid containing In-111 is boiled to dryness under a stream of nitrogen and the residue is dissolved in a known volume of 0-9 ~ NaC1. Sufficient bleomycin is weighed in a clean beaker to yield 1 nag of BLM per mCi indium-111 at the time of preparation. The weighed quantity of BLM is dissolved in a known volume of 0.9 ~ NaC1 and transferred to the indium-111 solution in 0'9 ~ NaCl. The mixture is heated in a water bath at a temperature of 55o-60°0 for about 25 min. Higher temperature, up to 100°(2, may be used though no specific advantage has been observed. The solution is then allowed to cool and the pH is measured with a micro pH meter and found between pH 4 and 6. The solution is then sterilized by filtration through a 0.22-,u Millipore filter, alternatively the sterilization