Revista Mexicana de Fisica 19 (1970) FA122- l'A133

DOSIMETRY FOR SEEDS OF DAHLlA AND DIOSCOREA

IRRADIATED WITH 1.0 MeV ELECTRONS

}.ReyesL.*

Instituto dr Física, Universidad Nacional de México

O. de la Teja and R. Yillalobos- Piettini

Laboratorio de Genética y Radiobiología.

Instituto de Biologia, Universidad Nacional de México

FA 122

ABSTRACT: A :echnique is presented foc calculating che absorbed dose foc

seeds of Dab/ia and Dio~corea irradiated with 1.0 MeV elecuons.

An irradiation design is explained in crder to obtain a uniformdepth- dose disrribution through rhe samples.

INTRODUCTION

In conducting research in (he field ol radiobiology, irradiaríon by ac-celerated monoenergetic electrons, with energies between 0.5 and 2.0 MeV,is frequently used. The maio advantages of such irradiaríon as comparedwüh X- and garnrna- irradiadon lie in the possibility oí obtaining very highdose rates, (at the level al 105 to 107 rads per second), as well as in the

•Programa de Aplicaciones Industriales de la Radiación, Comisión Nacional deEnergía Nuclear.

Dosimelry lar Seeds F A123

comparatively simple and accurate determination of the integral dose, entirelyor partially absorbed, in the sample.

In addition to the integral dose, it is also nece .••••ary to know the dosedistribution in the samples, mainly in the depth direction, because the lowpenetration of electrons at these energies can give diHerent depth- dose distti ...butions along the sample and introducesome indetermination or diHerenteHects of radiation in the biological properties under study. In particular, ir'"radiation of very thick seeds (figure 1) introduceseveral problems if the inte-gral dose is not uniforrnly distributed in the sample.

Because the radiobiologist is intecested in changes induced in thetesta, in the embryos oc elsewhere, the purpose of this note is to give a tech ...nique for dosimetry in seeds of DahIía coccinea and DioscoTea composita, forwhich studies of the changes induced by irradiation are in process.

EXPERIMENTAL

Irradiation designo - Seeds in small glass containers (figure 2) placed on arotating aluminium table (described elsewhere 1), were irradiated with LO MeVelectrons delivered by the Van de GraaH accelerator* at the Instituto de Físi ...ca, UNAM. In this arrangement the samples pass during 0.5 'seconds under abeam that is 'scanning at a frequency of 25 Hz.

Materials. - Polyvynil chloride (PVC) foils (Bekelite CO. QCA 5960, clear151) suitable for high level dosimetry2 were used. Cobalto activated borosili.cate glasses, BSAC (type F-0621, Bausch and Lomb Co. Rochester, N. Y.)were also used for dosimetry, with the appropriate dimensions (l5x6x 1.5mm)to allow reading to be taken in aspectrophotometer (Spectronic 20, Bauschand Lomb) with aspecial adaptor.

Spectrometry.- Electron spin resonance (ESR) measurements, in air at roomtemperature, were performed with a spectrometer (Varian V - 4502 -15) at theInstituto de Física, operating at a microwave frequency in the X - band bear9.5 GHz with 100 KHz field modulation.

Transmittance measurements of the irradiated PVC foils were donewith a spectrophotometer (Hitachi Perkin- Elmer, EPS- 3T) at a wavelength

•Programa de Aplicaciones Industriales de la Radiación, CNEN and Instituto deFísica, UNAM.

FAl24

(a) !777I I/II1ZZ7ZmZZ7mm2J!:a:jr- 11 m.. --~I

Reyes et al

( b) - --

•(d) • •

Fig_ l. Photographs o£ the seeds 01 DahUa coccirr.,a b) and Dioscofrd compositad) used in the experiments. Cross section schemes ofthe seeds oi Dahlia(a) and Dioscorra (e) show the average sizes measuted in many sampJes.

Dosimet,y jor Seeds F ~ 125

o

N

••••

••'"

=¡;¡

¡;;

••~NO..<"

N •• •• •• •• ..• •• '" OO O O O O O O O O O

~3 _

esO

T••

12 ••••

f

Fig. 2. Beam profiJe determined by (he concentration of (he free radicals in pveinduced by irradiation. The PVC foil was placed 00 rhe aluminium rabie(6 cm below ,he acceleratoc exit window) and irradiared Wilh 1.0 ~ieV e.lecrcons during 3 minutes; lhe conccmcation was measured by ESR immedi.arely after. Here (h/w) and (h/w)m represent lhe peak ro peak height of[he fiesr dcrivativc oí [he ESR absorption curve per mass unü {or aoy sampleand foc thar which shows (he maximum value, respectiveI)'; a indicares rhecnly uniform parr, abaur LO cm wide.

FA126

DE 345 rnj.1, using as reference a non- irradiated foil.

RESUL TS

Reyes et al

The size oí [he scanning beam was carefully determined using (heconcentration of free radicals in (he PVC foils induced by irradiation. A2.6 x 8.0 cm foil was placed 00 (he aluminium rabIe (6 cm below (he acceler-atoc exit window) and irradiated with 1.0 \JeV electrons; ir was then cut in20 srnall parts (2 x 11 mm) aloog (he axis perpendicular to (he scanning di-recrion and (he concentraríon oí (he free radicals in each pan was irnmediate-Jy measured by ESR. The beam profiJe along rhis axis is shown in figure 2.From (his result ir is immediately clear [har ooly seeds less (han 1.0 cm longcan be irradiated uniforrnly aloog (he axis. This method oí determining beamprofiles seems to be comparable in accuracy with that reported by Okabe.3 ,but not better than the method reported before4, using a mlcrophotometer tocontinuously scan the optical density of the irradiated foil. ~hen the tablewith the PVC foil is rotated under the scanning beam, a uniform distributionis obtained. Consequently, this design was used for the experiments. Alongthe axis parallel to the scanning direction (figure 3) there is no problem.becau"se the beam was scanned 25 cm long and the samples placed in themiddle of the field. These results are similar to Grünewald's, S who studiedthe homogeneity of dos e distribution on rotating samples.

D~pth- dost' distributiotl. - The depth-dose distribution for a block of PVC,1.5 mm thick, containing 5 foils of thickness 0.3 mm each, inside the glasscontainer, is shown in figure 4 (see curve 1). It is possible to see a non-uniform distribution along the block; the increase in the relative dose after1.2 mm is due to the effect of backscattered electrons, produced in the baseof the glass container by the incident electrons. The relative dose is givenin terms of 111 , where 1 and Tare the percentages of transmittance

1 1(measured at 354 mil of the irradiated PVC with respect to a non- irradiatedfoil) for any foil and for the fírst foil, respectively. In this condition it isnot possible to irradiate the seeds uniformly (thickness, for Dah/ia, 0.7 mmand for Dioscoua, 0.6 mm). For this reason, it was necessary to design asystem with ab30rbers,to average the effect of the contribution of both theincident elecHons and secondary electrons produced by backscattering in thebase or in the walls r' 'he container, in such way that the seeds would beirradiated l'i1iformly. Figure 3 shows a group of systems, composed of PVC

DO!limrtry jor Seed!l FA 127

( O)

Scanníng direction

70c", --l oluminium table

( b)

1.50.3

PVCAl

UUAl 0.1PvC 1.5Al 0.3

PVC L5mm

4 Pyru 910$$container

Al 0.3 Al 0.3 Al 0.35PVC 1.5 PVC 1.5 PVC 1.5Al 0.3

Al aluminium foil.PVC polJYJnil chlOfid. foil.

Fig. 3. _ a) Seheme of (he arrangemen( used for 1.0 MeV electron irradiatioo, lhalshows the glass. container with the samples passing under the scanningelectron beam at a constant velocity. The rotating table moves atw = 3 rpm, so the samples are under (he beam 0.5 seconds.

b) Scheme of [he systems of PVC and aluminium foils used for the studyof [he dep[h- dose distribution. The thickoess of the foils is gi"en iomm.

FAI28

l.,

"1'"T¡T, I ..•..2 -+- ------1.2,-o-4 ..•.. r5 ..•-• -\- /

1.1 //

/1.0 ---4('

O.,

O.,

0.7

_-+-

Reye s el al

O., O., O., 1.2

Fig. 4. Depth-dose distribution foc PVC block s oí different systems used in theexpetiments. The distribution i5 gi~en in T/T1 ' where T and TI are thepercenls ol transmittance, at 345 m,u. with respect to a non" irradiatedfoil, foc 3ny foil and foc the fíest foil, respectively.

FA129

and aluminium foils, [har weee used until (he final design was ohtained.Figure 4 shows rhe deprh- dose discriburion obrained for ,he PVC block for ,hesystems, alter several experiments. It is clear [hat in system 6 a uniformdiscriburion is obrained 'o a deprh of 0.6 mm. The deprh for rhe seeds p 5 isdetermined by che relation

PPYCP5 = -- PPYCP5

(1)

where PPyC is ,he deprh or penecration obrained for PVC, P5 and PPYC are(he d,ensicies {oc (he seeds and PVC respectively. As (he Vab/ia, Dioscoreaand PVC densiries are 1.156, 1.173 and 1.380 g/ce respecrively, rhe deprhfoc Dahlia and Dioscorea are, respectively, as large as 1.193 and 1.176times rhe deprh for PVc.

Figure S and 6 show (he depth.dose distribution (in relative units)versus penetraríon {or seeds oC Dahlia and DioscOTea. Feoro (hese results,we have selected system 6 foc irradiatíon, because oí (he uniformity in (hedistribution. If BSAC is used as a secondary dosirneter, (he seeds must beirradialed wirh a base of polyelhylene foils (of similar densilY as PVC)until (he total thickness, seed plus foil, equals 1.5 mm, in order to have thesame thickness for the sample and the dosimeter. This arrangement allowsuniform distribution in the seeds.

Dos~ rat~. - Dose rate in the samples (DR)s can be measured by the relation

(DR )5 (2)

where (DR)d is the dose rate for a secondary dosimeter irradiated under thesame conditions as the samples. S and Sd are the mass stoppingm 5 mpowers for the samples and dosimeter, respectively. mSBSAC was calcu"

laled for 1.0 MeV eleccrons by lhe following relation6

where (-dE/dx)c. BSAC and PnSAC are the specific energy 105S and densi'"

FABO

1.3

1.0

o.,

o .•

0.7

Sp_1-.-I -t-3 -0-4-.-S~.~-

G"'" """""'01 -.---J!U!l2.

_..-11

r//

----

Reyes er al

0.341 0.71 1.0. 1.44. 1.80

Oopt~ 1•• 1

Fig. 5. Depth •.dase distribution foc Dahlia. The distribution is given in relativeunits, obtained from rhe values oí Y/Y for PVC corcected by (he fractior.

1oC [he den sities oC PVC and Dahlia.

F.~ 131

1.5

//

-

////

~ ~/>y

_.-ti.l>.-

//

,/./

S"te",.I ..•..2 -+-5 -0-4 -<1-5 ..••-.~-

l. O

1.2

O.,

0.8

0.7 '\anra,. \....ttMc.n •••o, - "- "- .-+DIo'cor.o --

0.55 0.70 1.05 1.40 1.75D,plh ( mm)

Fi,c.6. J)el'lh.dosC' distribution for Dio~co'f'a. Tht" di ••rribution is ¡l;i,.C'n in rclJ.-l¡vC unirs. obrained from the ,'alues of TIT

tfor PVC cOtrecred by rli(,

fr3crioll of thc densities of PVC and Dio:<¡co,pa.

F .'0132 Reye s et al

ty far BSAC. The value of mSBS.'oC is 1.588 \teV cm2/g, calculated from the

formal definition6 using che data oí Nelms7 and flenricksen8 and with [hefollowing values: N = 0.737 X 1021 atoms/cc; Z = 9.337; 1= 126.98 eV andPnS.'oC = 2.226 g/cc. The value of mS, is calculated assuming that the

specific energy 1055 foc biological material is equal to [he specific energyfoc water, so

= O/POj)(-dE/dx)c, water

(4 )

(5)

Using (-dE/dx) = 1.90 \leV cm'/g, calculated before. furc, water1.0 \feV electrons, [he dose rate foc Dahlia and [)ioscoT~a are as large as1.0346 and 1.0187 the dose rate fut BSAc. respectively. \hth these valuesir is possible to selecr [he adequate electron beam current [Q give (he rc-quired dose rate foc [he irradiating conditions descríbed befare.

,\CKNOWLEDGEMENTS

The authors hereby extend eheie thanks to Unioo Carbide ~1exicana,

S. A. for samples of PVC kindly supplied through their División de Plásticos;to S. A. Reyes L. who performed [he ESR measurements; ro \l. A. Limón \1.

and F. Velázquez S. foc their assistance in the experimental work; to\frs. \lary Ann \l. de Valladares and A. A. Valladares for proof reading themanuscript.

REFERENCES

1. E. Adem Ch. Tesis Profesional, Facultad de Ciencias, UNA\t (1968).2. 1. G. Draganié. et al. Riso Repon :'-lo. 22 (1961) p. 26 et seq.3. Shigeru Okabe. et al. Jap. J. Appl. Phys. 5 (1966) 68.4. J. Reyes L., Rev. \lex. Fís. 17 (1968) 1.5. T. Grünewald, Kerntechnik 8 (1966) 68.G. ).'iX'.T. Spinks and R.). \lioods, "An lntroduetion to Radiation Chemistry"

John Wiley, N. Y. (1964).

D03imet,y /0' Seed3 F A133

7. A. T. Nelms, U. S. A. National Bureau of Standards, Circular 577 (956).8. T. Henricksen and J. Baarli, Radiat. Res. 6 (957) 415.9. R. Mondel and J. Reyes L., Rev. Mex. FÍs. 18(969) 152.

RESUMEN

Se presenta una técnica para calcular la dosis absorbida en semillasde Dahlia y Dioscor~a irradiadas con electrones de 1.0 :\-kV de energía, asícomo el diseño de irradiación emplc:ado para obtener una distribución de do.sis uniforme en la dirección de penetración en las muestras.