Choudhury, Ajoy K

Roy, Bijon

Sandell, Probir K

English

IACS Institutional Repository

8INVESTIGATIONS INTO THE LOW ENERGY GAMMA RAY BACKGROUND AND ITS VARIATIONSBIJON BOY, PliOBIR K. SANDELL AJOY K. OHOUDHURYDkpabtment of Experimental Mi<|>ical Scienceb,Institute of Post-gkaduatk Medical Educa:^on & Research, Calcutta(Received, July 7, 19f0)ABSTRACT. The gamma-ray background of CMoutia as determined by scintillation spectrometry using a Nal (thallium activated) cuystaltonsistB of two broad energy bands being contributed by the radioactive substances present i and identified as those from the radium and thorium fai indicates that there are two possible regions of 1 ow level oi an optimum channel level and channel width. The varij is restricted mostly to lower energy region of the backg investigation.the soil, air and building materials ily. The background so determined iting which is achieved by choosing dion in intensity of the background mnd spectrum and is under furtherI N T R O D U C T I O N111 low level counting using scintillation (jotmtcrs the background (!Ount is alw a^ys a limiting factor. On the other hand in certain low level counting, as in bio-medieal tracer tecdiiiique different authorities, (Johnston, 1955; Anderson and Libby, 1957) Medical Research Council (Report 1956) have stressed the need of low doses of radioactive tracers in diagnostic and investigative works. Pos­sibly it is safe to use a dose which is of the order of the natural background in its activity. Furthermore, in in vivo nieasuromonts during bio-medical tracer works, an effective shielding against the background is often not possible. Thus, in all the above cases an attempt at low level scintillation counting must essentially be preceded by a survey of the nature of the background at the particular place. This means the determining of the energy distribution in tJie background spectrum, the position of its main jieaks, if any, and their relative intensities. As the gam rna- emitters usually used in low level counting as in bio-imdieal work, like I (131), Fe (55), Cr(51), Au(198) etc., have characteristic energies below 700 KeV it is of interest to analyse spectromctrically the region below this.Recently, many excellent review works on low level counting have been published (Hayes, Anderson and Langham, 1955; Anderson and Hayes, 1956). But hardly any attempt has been made in this country towards a spectrometric study of the background of the place before undertaking such studies. In this laboratory, this spectrometric work has been carried on for the last one year. In the following paper it is desired to report the results of the one-year investigation77on the subject, the variations observed and the possible importance o f the results obtained in low-level counting, for instance, in bio-medical tracer technique.M E T H O D S  A N D  M A T E R I A L SThe studies on the radioactive background o f Calcutta were carried out with the help of RIDL Scientillation Spectrometer consisting of a Scaler model no. 16D, B-162; Ratemeter/Electronic Sweep Meter t5rpe 114-B, serial B-79; and Scintillation counter No. 43A with interchangeable Nal (thallium activated) crystal of thickness. The gamma energy distribution was determined under the following conditions: crystal used =  Nal thick; counter voltage ^  700 volts; period of sweep — J hour; ratemeter scale multiplying factor — 0.5; channel width == 2.0; percentage standard deviation 10%. A full automatic sweep (without any attenuation) was taken in every case in differential position from 100—0 channel volts, the reset level heliopot being kept at ‘O’ . The graphic record was made with the help of a Honeywell Brown Kleetronik Reciorder automatically.At the above setting of the instrument, graphic records of the background spectrum was taken throughout the whole year. At the same time, keeping the same instrument settings, scans of I (131) and Or (51) spectrum were made and their peaks, 364 KeV and 330 KeV respectively were used to calibrate the back­ground spectrum so obtained.In order to eliminate possible influence of circuit oscillations in the discri­minator or elsewhere, and also of photomultiplier noise on the background spectrum, the crystal of the counter was removed from the top of photomultiplier tube and the rest o f the counter (consisting of photomultiplier tube, cathode follower, etc.) was wrapped light-tight completely in black paper and a full scan from 100—0 channel level volts was taken keeping the settings of the spec?trometer as before.To study the effect of shielding of the counter on the background spectrum, the counter was inserted in a lead castle with walls IJ" thick and a full scan from 100-0 channel level volts was made under same instrumental settings. Observa­tions were also made on the variation in background spectra with the changes in atmospheric conditions. Thus an attempt was made to note the variation of intensity below 75 KeV at different rimes of the day at two hours iixterval.After calibration, the energies corresponding to the different peaks in the average spectra obtained were found to tall3’^  with those of radium and thorium emanations. To examine this fully the spectra o f the important naturally occurring isotopes, viz., Ra (226) in the form of needle and Th (230) as crystal sample were recorded in the same graph containing the background spectrum under same instrumental settings. An attempt was made to trace the sources contributing to the background and the following materials were analysed: (1)78 Bijon Roy, Probir K . Sandell and A joy K . Chotidhurysand from Mogra, (usually used for the building materials here), (2) the silt from the banks of the Gauges, (3) soil samples from the northern and southern parts of the city and (4) the soil from around the building housing the spectrometer. To examine these, a lead chamber having 1 cu. ft. (approximately) o f internal volume was constructed with walls thick. A background tracing with only the counter inside was taken initially and then c^ch of the above samples was packed in the space with the counter placeii in l||ie middle. Care was taken to preserve the relative geometry of the counter withwespect to the surroundings the same in every case. |R E S U L T S  IThe background spectrum consists of two 1#oa(i energy bauds. One su(»h graph is given in Fig. 1. The whole energy spcctr|m extends from 50 to 275 KeV (Table I). On removing the crystal, a graph wm (detained as shown in Fig. 2. This therefore gives the contribution due to circuit oscillations and phok>multi- plfer noise. Comparing Figs. 1 and 2 one can ca ^ y  recognise the resultant back­ground intensities in this low energy region. Sileldijig of the counter v ith leaf! \Y thick cuts o ff completely the broader higher energy band and attenuates the lower energy region t(- about J of its unshielded value (Fig. 3). P>om Fig. 4, whore the Ra (226) and Th (230) graphs are superposed on the background, one finds that the energy peaks corresponding to values round about 230— 270 KeV and 105— 120 Kev could be associated with the gamma-rays of Ha (226) and Th (230). These elements have been tracer! in the sand of the building materials and the soil of the city (Figs. 5-9; Table II). From these figures the relative contributions of the sand or of the soil may be clearly seen. Scanning for whole day ai two hours intervals showed a variation in the intensity under 55 KeV. The maximum increase o f the intensity in this r egion occurred round about afternoon. Attempts have also been made to indicate this variation together with that of atmospheric temperature and humidity as obtained from the Meteorologiijal office, Alipore.TABI.K 1Results o f the gamma ray spectrometric analysis of the background ofCalcutta ‘Investigations into the Low Energy Oamma Ray, etc. 79M * Energy range inKeVPeak values inKeVMaximumintensityrangeNatural radio­active isotope with same energy rangeHigher energy band27a-105*276, 246, 230, 186, 175, 170, 160, 135, 126, 106.160-136 Radium and Thorium familyTABLE II80 Bijon Roy, Probir K , Sandell and A joy K . GhoudhuryM axim umSamplesGamma ray intensityenergy range in KeVrange in KeV1. Sand(from Mogra) 216-60 130-1262. Soil(from the Ganges) 170-50 146-1263. Soil(from Lake area) 265-60 140-1264. Soil(from Science College compound) 175-60 140-1305. Soil(from S.S.K.M. Hospital compound) 220-60 136-126Fig, 1. Shows the radiation background of Calcutta. The scanning was done with the help of HIDL H(5intillatioii spectrometer from right to left. The energy of the gamma rays e^ ttends from 275-60 KeV.V>* ■■■; ; ;t >■................. ' '■■■■ ^ i:.F ig , 2. Shows the contribution  due to  circu it oscillation and |diotoxnultipliar noise. This was taken w ith the crystal (K a l— thallium activated) rem oved.Investigations into the Low Energy Gamma Ray, etc. 81Fig. 3. Indicates the effect of shielding the prol ith lead, thick. It shovs ibocomplete cutting off of the broader higher energy ban d attenuation of the lower energyregion to about 1/3 of its unshielded value.Fig. 4. Shows the superim})Ositiou o f  the tracings due to Ka (226) and Th (230) on the background tracing. N ote that the energy peaks o f  the background could be asBOciated with the gamma rays o f  radium and thorium fam ily.Pig. 5. Shows the gamma ray contribution of soil, collected from lake area (Suii« Calcutta), to the background of the place. The gamma ray energy range due to this exter«d> from 265-4K1 KeV.S2 Bijon Hoff, Probir K . SandeU and A joy K . Ohavdhnry\  - ' 1 ''A 'Fj> . 6. Shows the «amma my ooniribution of soij, follocted from Science College (O^ ilcutta) oom-)ouncl, to the bacfkground of the place. The gamma ray energy range due to this oxt^ ends from 175-50 KeV.Pig. 7. Shows (,ho gamma ray contribution of sand, collected from Mogra, to the back- ground of the )>lace. The gamma ray energy range due to this extends from 2J(5-60 KeV..rV:AFig. a. Shows the gamma ray contribution of soil, collected from the Ganges, to the background o( the place. The gamma ray energy range due to this extends from 170-40 KW,Investigations into the' Low Energy Oamma Ray, etc. 83Fig, 9. Shows the gamma ray contribution of bo] ’ollect<?d from the S.S.K.M Hospital compound, to the background of the place. The gamm|| ray energy range due to this extends from 220-50 KeV.Fig. 10. Shows the superimposition of the three gamma ray spectra due to the back­ground {——), radio-iodine (1-131, — .—) and radio-chromium (Cr-51,----------- ). Hshows that 50% of the count <liie to the two isotO|>es for a particular sample lie fibove the highest energy limits of the background.Fig. 11. Shows the background scan carried out at Eindhoven, Holland, with the help of Philips Scintillation Spectrometer. Note the important peaks round about 90 and 220 KeV with possible peaks at 50 and 29 KeV.Scanning with samples of I  (131) and O  (&1) showed that nearly 50% of the counts resulting from particular sample were above the highest energy limits of the background (Fig. 10). This, more or Jess, was experienced with the other isotopes mentioned above except Fe (55) in which case the radiations were found to be concentrated in the region 90-105 KeV.A similar background scan was carried out at Eindhoven, Holland, by one of the authors with the help of Philips scintillation spectrometer. The graph thus obtained has important peaks round about 90 and 220 KeV with possible peaks at 28 and 50 KeV, and is similar to the ones obtained here. The intensity of the energy round about 80 to 90 KeV was greatest. The energies in the spectrum would correspond to those found in the members of the uranium and radium family (Fig, 11),84 Bijon Roy^ Prcbir K . Sandell and A joy K . GhoudhuryD I S C U S S I O NThe background of Calcutta consists of 2 broad energy bands being con­tributed by the radioactive substances present in the soil, air and the building materials. The radioactive elements present are those of the Radium and Thorium family.From the background spectra obtained it is evident that most of the background counts are concentrated in 2 regions— the first between 260 to 135 KeV after which on both sides the counts fall off rajiidly and the second region is found to be below 65 KeV. Therefore, in low^  level counting as with the isotopes for bio-medical tracer work it is necessary only to adjust the channel level of the pulse-height analyser in such a way as to eliminate these regions. The background counts automatically will then fall o ff without going into the problem of shielding. The best procedure would be to sot the channel level potentiometer to a value which corresponds to energies higher than 270 KeV and then expanding the channel window to include the characteristic peak of the isotope being used within the channel window which lies above 275 KeV. The window width prefer­ably should be greater than 2W where ‘W ’ is the recorded half width o f the photo­peak of the isotope used. In the case of Fe (55) the window  ^ should be adjusted near about the trough found between the higher and lower energy bands, i.e., between 65 and 105 KeV. Hence there are 2 regions of low level counting— one above 270 KeV and the other in trough between the higher and lower energy band, i.e, between 60—105 KeV. Thyroid uptake measmements with low doses using the above procedure yielded more reproducible results*The effect o f shielding on the 2 energy bands of the baickground may at first sight seem to be anomalous as because the lower energy region is expected to be out o ff more easily than the higher energy rays on shielding. Possibly, low energy secondary radiations produced by cpsmic rays in the material o f the shield is responsible for this.Th© reason for the variation in the intensity of the low energy region as obtained during certain period of the day cannot be explained satisfactorily right now without further investigation. According to Wait (1937) there is some fluctuation in the gamma-ray content of the atmosphere with a maximum at about noon. This is in agreement with our observation. Further, others (Wilkening, 1952) have reported the variation in the radon and tlipron content of the atmosphere with the time of the day. The present investigftion points out that this varia­tion in background is restricted mostly to low 4^crgy region of the background spectrum. This is a subject matter of further |jfivestigation.A C K N O W L E B G ]  JNTGrateful thanks aro due to Prof. S. D. Cha  ^ srjee, I). Sc., F. N. I. and Prof. S. N. Bose, P.R.S., for their helpful (liscussio| and constant encouragement. Authors also like to acknowledge indebtedness Major General A. K. Gupta, Director, Institute of Post-graduate Medical Edication and Research, for kindly permitting them to carry on the work and also the facilities obtained.Investigations into the Ijow Energy Gamma Ray^ etc. 85R E F E K E N C E 6Anderson, E. 0. and Kayos, F. N.. 1955, Ann. Rev, Nuclear Sc,, 6. 303.Anderson, E. and Libby, W. F ., 1957, Adv. Biol, Med. FIujh., 5, 385.Blifford, r. H., Friodrnan, H., Loc*khart, L. B. and Bans, K. A ,, 1955, J, AHn^npheric and Terreftt. Phys,, 9. 1.HayoH, F. N.. Andorson, E. C. and LauKham, W. H., 195“>. Pror, Intern, Conf.Peaceful uneft of Atomic energy,, 14, 182..[ohriftton, W. H ., 1955, reaoefdl uses o f Atomic Enoi'Ky Tnternaiioiial Conf., 14, 149-54. W ait, 1937, Terrestrial Magnetism & Atmospheric Electricity., 42, 1.Wilkening, M. K ., 1952, Nucleonics., 10, 5, 36.

Investigations into the Low Energy Gamma Ray Background and its Variations

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/2409/1/Investigations%20into%20the%20low%20energy_Bijon%20Roy%20Probir%20K%20Sandell.pdf

Investigations into the Low Energy Gamma Ray Background and its Variations

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository