Nuclear Instruments and Methods in Physics Research B 225 (2004) 521–527

www.elsevier.com/locate/nimb

Radiation induced radical in barium sulphatefor ESR dosimetry: a preliminary study

M.A. Sharaf, Gamal M. Hassan *

National Institute for Standards (NIS), Laboratory of Ionizing Radiation Metrology, Tersa Street, El-Haram,

El-Giza, P.O. Box 136 Giza, El-Giza, Cairo 136, Egypt

Received 3 March 2004; received in revised form 11 May 2004

Abstract

Barium sulphate (BaSO4) was irradiated by c-rays and analyzed with electron spin resonance (ESR) to study

radiation induced radicals for materials as radiation dosimeter. The ESR spectrum for the radical species is charac-

terized by a hole-type center with g factor of 2.019, 2.0127 and 2.0103 and electron-type center with g factor of 2.0039,

2.0025 and 2.0001. The dosimetric signal with splitting factors of g ¼ 2:0039 is ascribed to SO�3 radicals and 5G

linewidth. The response to c-ray dose ranging from 5 to 103 Gy, energy dependence calculation and the thermal stability

have been studied. The number of free radicals per 100 eV (G-value) was obtained to be 0.25± 0.06 and 0.9± 0.18 for

BaSO4 and alanine, respectively. The lifetime of radicals and the activation energy were estimated from Arrhenius plots

to be approximately 325± 60 days, and 0.50± 0.09 eV respectively.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Electron spin resonance; Barium sulphate; Radiation dosimetry

1. Introduction

Barium sulphate mineral consists of divalent

cation Ba2þ and a divalent tetrahedral molecular

anion, SO2�4 . It has an orthorhombic crystal form

[1]. In the unit cell, the SO2�4 and Ba2þ ions lie on

the mirror planes and Ba2þ ions link the SO2�4 ions

in such a way that each Ba2þ ions coordinated by

twelve oxygen [2]. Studies by Melikove and Vu-

kovic [3] have shown the presence of barium and

* Corresponding author. Tel./fax: +20-27-416-936.

E-mail address: gamalhassan65@hotmail.com (G.M. Has-

san).

0168-583X/$ - see front matter � 2004 Elsevier B.V. All rights reser

doi:10.1016/j.nimb.2004.05.025

sulphate vacancies in the barium sulphate lattice.

It is also observed that the concentration of these

vacancies can be very high. Sulphoxy radicals like

SO�2 , SO

�3 and SO�

4 associate with barium vacan-

cies.It was reported earlier [4–6] that BaSO4 shows

efficient TL in the presence of certain suitable rare

earth impurities, particularly europium in the lat-

tice. ESR technique was used as a tool for imple-

menting the information achievable by the

thermoluminescence (TL) technique [7,8]. Also a

photostimulable X-ray storage effect had been re-

ported for BaSO4 doped with K [9]. The mainadvantage of this method is the linearity of dose

response over more than six orders of magnitude

ved.

522 M.A. Sharaf, G.M. Hassan / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 521–527

that of conventional X-ray films. In all these cases

the ESR dosimetric properties of BaSO4 were not

investigated.

ESR spectra of barite (powder and a singlecrystal) had been studied extensively. In addition

to SO�3 signals at g ¼ 2:0036, 2.0025 and 2.0001, a

hole-type center with g factors of 2.0191, 2.0127

and 2.0103 was observed [10] although no ESR

dosimetric properties has been done so far.

The main objectives of this research are to

study: (I) the ESR spectrum of BaSO4 powder, (II)

the dosimetric properties, (III) the thermal stabil-ity of the dosimetric peak and (IV) energy depen-

dence calculations.

2. Experimental technique

Reagent grade BaSO4 powder from Aldrich

chemical company was used. The samples weresealed in small polythene capsules, 100 mg each, to

be irradiate at room temperature with 60Co gam-

ma radiation to doses ranging from 5 Gy to 1 kGy.

The dose values have been calibrated using sec-

ondary standard system (NPL electrometer, 2560

and its thimble NE-2561 ionization chamber) with

a combined uncertainty 0.31%. The doses were

calibrated in term of absorbed dose to wateraccording to technical report series (TRS-398).

Traceability to the primary dosimetry standard

of BIPM, France, was provided.

ESR spectra were measured with an X-band

ESR spectrometer (Bruker, EMX) at room tem-

perature using a standard rectangular cavity (4102

ST) operating at 9.7 GHz with a 100 kHz modu-

lation frequency. The ESR parameters were cho-sen to provide the maximum signal-to-noise ratio

for non-distorted signals. The microwave power

and modulation amplitude were 2 mW and 1G,

respectively. The response time constant was 40 ms

with the field-sweeping rate of 100G/164 s. The

intensity of each sample was measured 10 times as

the peak-to-peak height and average values of

these measurements were plotted. The standarddeviation was about 0.5% from the mean value.

Standard samples of MgO doped with Mn2þ and

weak pitch were used to calibrate the ESR inten-

sity and the g-factor of the signal. The G-value was

obtained from the integrated absorption spectra of

weak pitch and compared with those of irradiated

materials of BaSO4 and alanine.

Isochronal annealing was performed by heatingirradiated samples at temperatures varying from

25 to 300 �C for 15 min at each interval. Isother-

mal annealing was carried out at 100, 150, 200 and

250 �C and at room temperature for 7 months.

3. Results and discussion

3.1. ESR spectra

Fig. 1 shows typical ESR spectra of BaSO4

irradiated by 60Co c-ray of 100 Gy with the unir-

radiated one. The spectra contain two type peaks:

A and B which ascribed to hole and electron cen-

ters respectively. Signal A, recorded in the unir-

radiated sample and characterizes by a narrowlinewidth triplet Fig. 1(a), typical of a hole-type

center with g factor of 2.019, 2.0127 and 2.0103.

Fig. 1(b–d) shows two different ESR signals A and

B, observed in the irradiated samples. Signal B is a

triplet signal electron-type center with g factor of

2.0039, 2.0028 and 2.0001. Comparing the ob-

tained g-values of signals A and B with those of

SO�4 and SO�

3 in the literature [5,10], the signal Acould be related to the SO�

4 radical and the signal

B to the SO�3 radical. The slight differences among

published and measured g-values may arise from

differences in distortion of the molecular structure

or in lattice environment of the radicals. The

electron-type center with the line width 5G at

g ¼ 2:0039 was used as dosimetric peak.

3.2. Power dependence

The intensity of some signals saturates at low

microwave power ðPÞ but that of others increasesin proportion to P 1=2 up to high microwave power.

Appropriate setting of power level is therefore

necessary for the ESR measurement. Fig. 2 shows

the microwave power dependence of signal Bintensity on a logarithmic scale. The signal inten-

sity increases as a function of power up to 2 mW

and decreases with the further increases in micro-

wave power.

3440 3460 3480 3500 3520 3540

3440 3460 3480 3500 3520 3540

3440 3460 3480 3500 3520 3540

3440 3460 3480 3500 3520 3540

g = 2.0191

g = 2.0127

g = 2.0103

Hole Center (A)

ES

R In

tens

ity (a

rb. u

nit)

Magnetic Field (G)

Unirradiated

(b)

g = 2.0001

g = 2.0025

g = 2.0039

Hole Center (A)

Electron Center (B)

ES

R In

tens

ity (a

rb. u

nit)

Magnetic Field (G)

10 Gy

(c)

ES

R In

tens

ity (a

rb. u

nit)

Magnetic Field (G)

20 Gy

Electron Center (B)

(d)

ES

R In

tens

ity (a

rb. u

nit)

Magnetic Field (G)

100 Gy

(a)

Fig. 1. ESR spectra of unirradiated and c-rays irradiated BaSO4 as observed at room temperature. The field modulation was 1G at 100

kHz and microwave power was 2 mW.

M.A. Sharaf, G.M. Hassan / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 521–527 523

3.3. Dose response

The signal B of ESR spectrum for BaSO4 is

observed only after c-ray irradiation and well

defined increase in amplitude with doses ranging

from 5 Gy to 1 kGy. The observed relationshipbetween absorbed dose and the peak-to-peak

height of the ESR first derivative line suggests

a linear function in the dose range of 5–103 Gy

as shown in Fig. 3. The dosimetric signal B has

a lower sensitivity than that of alanine

(BaSO4/alanine ¼ 0.6). However, we expect that

doping this material with light activators in

certain concentrations may improve its sensitiv-ity.

3.4. Spin concentration

The radical formation efficiency, G-value at an

intermediate dose range was determined by double

integrating the derivative spectra of the B signal at

g ¼ 2:0039. The area of standard weak-pitch wascompared with that of irradiated BaSO4. The

absolute spin concentration was estimated within

the error of 15% by using the following equation:

n ¼ 6:3� 1013G ðGy�1 g�1Þ; ð1Þ

n ¼ Asample � nweak�pitch

Aweak�pitch �Dose ðGyÞ � ðgÞ¼ 6:25� 1013G; ð2Þ

25 50 75 100 125 150 175 200 225 250 275 300 3250.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

ES

R In

ten

sity

(ar

b. u

nit

)

Temperature (oC)

BaSO4

Fig. 4. Isochronal annealing of the intensities of BaSO4. The

relative intensities are shown as a function of the annealing

temperature with a heating duration of 15 min at each step. The

peak to peak height of derivative line was taken as the intensity

of the signal.

1 10 100 1000102

103

104

105

106

ES

R In

ten

sity

(ar

b. u

nit

)

Dose (Gy)

Alanine BaSO

4

Fig. 3. The comparison dose–response of ESR signals for

BaSO4 and alanine as a function of the dose from the source of60Co.

1E-3 0.01 0.1 1 10 100 1000102

103

104

105

ES

R In

ten

sity

(ar

b.u

nit

)

Power (mW)

BaSO4

Fig. 2. Microwave power dependence of the dosimetric BaSO4

signal irradiated by c-rays.

524 M.A. Sharaf, G.M. Hassan / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 521–527

where Asample, Aweak�pitch, nweak�pitch and m are the

areas of integrated signals of sample and weak

pitch, number of spin in weak peach (1013 spins/

cm) and the mass of the sample respectively. The

G-value of the whole area are 0.9 ± 0.18 and

0.25± 0.06 for alanine and BaSO4, respectively.

3.5. Thermal stability

3.5.1. Isochronal and Isothermal annealing

The isochronal annealing of BaSO4 for tem-

peratures ranging from 25 to 300 �C was shown in

Fig. 4. The relative intensity ðn=n0Þ of BaSO4 de-

creases with the increases in temperature up to 150

�C and B signal losses �7% of its original value.

The relative intensity increases with further in-

crease in temperature up to 225 �C then decrease

again with the temperature. This may be due to the

effect of water content in the sample or due tooxidation at high temperature.

Fig. 5(a) describes the variation of the relative

intensities of signal B normalized by that before

annealing ðn0Þ as a function of isothermal

annealing time for BaSO4 samples heated at dif-

ferent temperatures in a closed ESR tube. They

were plotted logarithmically considering first-

order decay. In first-order kinetics, the decrease ofthe radical concentration n at constant tempera-

ture is written as

dn=dt ¼ �n=s ¼ �m0 expð�E=kBT Þn; ð3Þ

where s is the lifetime of the radical at the tem-

perature T , m0 the frequency factor, E the activa-

tion energy of the radical, t the annealing time and

kB the Boltzmann constant.

For second-order decay characteristics of

dn=dt ¼ �kn2, where k is the second-order decay

constant as in Fig. 5(b). The annealing results are

Table 1

ESR dosimetric properties (g-Factor, G-value, lifetime at room temp

Sample g-Factor G-value

Hole center Electron center

BaSO4 g ¼ 2:0190 g ¼ 2:0039 0.25± 0.06

g ¼ 2:0127 g ¼ 2:0025

g ¼ 2:0103 g ¼ 2:0001

2.0 2.2 2.4 2.6 2.8 3.0 3.2101

102

103

104

105

106

Lif

etim

e (m

in)

(1000/K)

BaSO4

Fig. 6. The lifetimes against the reciprocal temperature ð103=T Þobtained from isothermal annealing experiments for BaSO4.

Extrapolation of the straight line gives the lifetime at ambient

temperature.

Fig. 5. Isothermal annealing study of BaSO4 as a function of

time and fitting to (a) first-order decay ðn=n0Þ, (b) second-orderdecay ð1=nÞ.

M.A. Sharaf, G.M. Hassan / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 521–527 525

not straightforward. They neither indicate first-

order nor second-order kinetics. The decay kinetic

would be a mixture of first- and second-order

kinetics for which the following equation wasproposed [11,12]:

dn=dt ¼ �n=s � kn2; ð4Þ

where k is the decay constant for second-order

decay. The exact value for n ¼ n0 at t ¼ 0 is given

as

nðtÞ ¼ n0e�t=s

1þ ksn0ð1� e�t=sÞ : ð5Þ

3.5.2. Fading and Arrhenius plot

The fading in radical concentration ðnÞ for

BaSO4 at room temperature for a period of 7

months reveals a small change in the signal

intensity (�7% of its original value). This means

that the radiation-induced free radicals are stableat ambient temperature. Fig. 6 shows Arrhenius

plots of lifetime s calculated from second-order

kinetics of BaSO4 at different temperatures. The

lifetimes at room temperature are estimated from

Arrhenius plot by extrapolation of straight line

to be about 325± 60 days.

The activation energy ðEÞ has been calculated

for BaSO4 from the Arrhenius plot of I=s versus1000=T , the activation energy (E ¼ 0:1958� Slope)was calculated to be 0.5 ± 0.09 eV. All the ESR

data are summarized in Table 1.

3.6. Energy dependence

The mass energy absorption coefficient ðlenq Þ andmass stopping power coefficient ðScol:q Þ for BaSO4,

alanine and soft tissue were calculated using the

following equations [13–15]:

erature, and activation energy) for BaSO4

Lifetime ðsÞdays

Activation

energy (eV)

Effective atomic number

Zeff

325± 60 0.50± 0.09 47.03

526 M.A. Sharaf, G.M. Hassan / Nucl. Instr. and Meth. in Phys. Res. B 225 (2004) 521–527

len

q

� �compound

¼ WZ1

len

q

� �Z1

þ WZ2

len

q

� �Z2

þ

ð6Þand

Scol:q

� �compound

¼WZ1

Scol:q

� �Z1

þWZ2

Scol:q

� �Z2

þ ;

ð7Þwhere WZ , ðlenq ÞZ and ðScol:q ÞZ are the weight fraction

of the molecular weight, mass energy absorptioncoefficient and mass stopping power coefficient for

the element Z in the compound, respectively.

The ðlenq Þ values for BaSO4, normalized to the

corresponding values for water, are nearly energy

independent for high photon energy ranges from

0.6 to 5 MeV and energy dependence for photon

energies, less than 0.6 MeV and greater than

5 MeV as shown in Fig. 7(a). The electron energydependence calculated for BaSO4 divided by the

corresponding values for water is energy depen-

dent for all energy range except at �10 MeV as

10-1 100 101

10-1

100

101

102

10-2 10-1 100 101 102 103

10-1

100

101

(a)

(Uen

/P) M

ater

ial/

(Uen

/P) W

ater

Energy (MeV)

(BaSO4 / H

2O)

(Alanine / H2O)

(Soft tissue / H2O)

(b)

Co

llisi

on

(S

P) M

ater

ial/C

olli

sio

n (

SP

) Wat

er

Energy (MeV)

(BaSO4 / H

2O)

(Alanine / H2O)

(Soft tissue / H2O)

Fig. 7. Mass energy absorption coefficient of X-rays and mass

stopping power of electrons for BaSO4, alanine and soft tissue

relative to that of water.

represented in Fig. 7(b). Energy dependent for

BaSO4 may ascribed to its high value of the

effective atomic number ðZeff ¼ 47:03Þ. This

behavior could be improved by replacement ofbarium with light element such as lithium.

4. Conclusions

The preliminary ESR analysis of radiation-ind-

uced free radicals in BaSO4 represents an intere-

sting and relevant approach to dosimetry. Theresults obtained from studying BaSO4 compound

may be summarized as follows:

(1) The signal, electron type centre, at g ¼ 2:0039seems to be the most suitable one for ESR

dosimetry, which increases linearly with the

dose.

(2) The lifetime and activation energy are 325± 60days and 0.50± 0.09 eV, respectively.

(3) The fading of the dosimetric signal at room

temperature is very small. The stability of the

radical at room temperature indicates that

BaSO4 loses 7% of its initial value after a per-

iod of seven months; this value should be

taken in consideration for using it as dosimeter.

(4) Further investigations are needed to improvethe sensitivity of BaSO4 by doping with light

activators and study the experimental energy

dependent extensively.

The obtained ESR dosimetric properties of

BaSO4 show that, it could be used in some appli-

cations such as radiotherapy and transfer dosi-

metry.

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