Atomic bomb and accident dosimetry with ESR: Natural rocks and human tooth in-vivo spectrometer

0883-2889/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press plc

Appl. Radiat. Isot. Vol. 40, No. 10-12, pp. 1021-1027, 1989 Int. J. Radiat. Appl. Instrum. Part A Printed in Great Britain. All rights reserved

Atomic Bomb and Accident Dosimetry with ESR: Natural Rocks and Human Tooth In-vivo Spectrometer MOTOJI IKEYA and HIROSHI ISHII Department of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan ESR dosimetry of some construction materials at Hiroshima and Nagasaki was carried out to determine the A-bomb radiation dose. Some minerals exposed to low-levelnatural radiation over a given geological time period can be also used to determine the intense A-bomb radiation dose. Finally, an ESR cavity and a special NdBFe (Neomax) magnet system for in-vivo measurement of radiation dose of a human tooth without extraction is designed and manufactured.

Introduction Electron spin resonance (ESR) dosimetry of atomic bomb radiation has been made for tooth enamel and shell buttons at Nagasaki (Ikeya et al., 1984) following the paleodosimetric ESR dating of fossil shells and bones (Ikeya, 1988). Although the tentative A-bomb radiation dose in 1965 (T65D) was revised into the dose system in 1986, (DS86), based on the theoretical calculation and the experimental thermoluminescence (TL) measurement of roof-tiles and bricks (Ichikawa and Nagatomo, 1986), further investigation with different techniques is strongly hoped to refine the dose. This is especially true as the ESR dosimetry can determine the A-bomb or accident radiation dose to persons directly by using shell buttons and tooth enamels. Extensive studies on tooth enamel extracted during dental treatment have been made by several investigators (Hoshi et al., 1985; Tatsumi and Okajima, 1985; Pass and Aldrich, 1985). Some basic studies of the secondary electron equilibrium have also been made to separate the dental x-ray effect (Shimano et al., 1988). Recent work to determine the A-bomb dose with natural quartz (SiO:) grains exposed to natural radiation during a given geological time has been successful by using relatively unstable Ge centers on a geological scale but stable on a historical time span (Ikeya et al., 1966). In spite of the enormous dose of the accumulated natural radiation, accident or Abomb radiation at a high dose rate can be resolved. The depth profile of the radiation dose gives the approximate average energy of radiation. The present work describes the ESR dosimetry of A-bomb radiation using certain construction materials. The new objects examined with ESR will expand the scope of studies and help to refine the

A-bomb or accident radiation doses. Finally, a new ESR apparatus to determine the radiation dose of a tooth in situ without extraction is described, although this first test machine may not be practical in many respects especially with regard to protection against the hazard concerning the magnetic field and microwave leakage.

Theoretical Background One can determine either intense accident or atomic radiation dose, in spite of the enormous accumulated dose of natural radiation, since the dose rate is orders-of-magnitude different. It was shown that the concentration of radiation-induced material defects at a time t (y) after their formation at the annual dose rate o f / ) (Gy y- t) is described as n ( t ) = a/)z[l - e x p ( - t / z ) ]

(1)

where a is the defect formation efficiency (Gy-1) and the z is the defect lifetime. The saturation concentration of defects is a/)z. The saturation level is extremely low for defects with a short lifetime, while those with a relatively long lifetime show an appreciably large saturation level. If the environment of radiation for an old sample is changed from/5 t o / ) ' for a time t', the equilibrium of the defect concentration also changes. The concentration is obtained by solving the equation dn / d t = al~" - n /z

(2)

with the initial condition for t = 0 being, no = a/)z. This lasts for a short period of t' so as to give the accident dose: AD = / ) ' t ' . The signal intensity is enhanced by such radiation but decreases as a function of the time to the equi-

1021

1022

MOTOJI IKEYAand HIROSHIISHn

librium a/)T. The concentration of defects calculated using equation (2). substituting with a new initial condition of no = a(/}T + a ( D r + AD) at t = 0, after returning to the dose rate. Here, A D denotes atomic bomb accident dose. The solution of equation (2) n = a[/)r + A D exp(

-

t/z)].

can be /9'=/3 /)'t')= natural dose or is (3)

Hence. if some signals that have small lifetimes on a geological time scale but have sufficiently long half times on a historical time scale, they can be used for determination of the accident dose. Typical formation curves of defects with a short lifetime T] and a large life time z, by both natural and accident (Abomb) radiation are shown in Fig. 1. When the saturation level is sufficiently high for defects with a long lifetime, no further creation of defects is made, due to the interactions among defects in spite of the high dose rate o f / ) ' . Another background used in the previous work is the depth profile of the defect concentration due to the self-absorption of radiation. As the values o f / ) ' described above depend on the depth x as /3' = D6 e x p ( - / ~ x ) with the mass absorption coefficient #, one can determine the equivalent dose E D as E D = / ) z + A D O exp( - / ~ x ) . '

Experimental

(4)

Results

Construction materials: ('onerete a n d granite

Only roof-tiles and bricks that had been once-fired to anneal natural defects to zero have hitherto been used to determine the ,' ray dose with the thermoluminescence (TL) technique (Ichikawa et al., 1966). Following the previous work on bridge pillar granite at Motoyasu bridge in Hiroshima, granite river bank and concrete pavement are the objects of the present ESR dosimetry. Some shell pieces embedded in the concrete were also recovered. Figure 2(a) shows the ESR spectra at 0.005 m W (to observe the E' centers) and I m W (to observe the Ge centers) for quartz grains of Motoyasu bridge granite previously used for the study of the self-attenuation effect. Figure 2(b) shows the ESR spectrum measured at liquid nitrogen temperature. In addition to the quadrupole splitting of the A1 centers signal, the signal due to Ti centers is also observed. The microwave power dependence shown in Fig. 3(a) clearly indicates that E' centers have to be measured at a low microwave power, while the so-called Ge centers at g = 1.997 must be measured at an increased microwave power. A similar microwave power dependence was measured at 77K for both A1 and Ti centers (Fig. 3b). The saturation occurs at a much higher microwave power of 100 roW. Figure 4(a) shows the depth profile of both E' and Ge centers in terms of ESR signal intensity as a function of the depth from the surface. In the case of

Natural

radiation (dose r a t e : D)

Accident radiation t(O')l

(O)

i

02~T2

T2

~DT 1

// //

1i

;i

Fig. 1. Formation of defects b3 natural and accident radiation for defects with a long lifetime and a short litetime (r: and r 0. The saturation intensity is proportional to adz. The dashed curve after the accident dose rate /3' indicates saturation due to the interaction among defects.

Ge centers decrease of the intensity is apparem from the surface to the inside as reported previously (Ikeya et al., 1986). There is no appreciable change of ESR signal intensity of AI and Ti centers with depth [see Fig. 4(b)]. A similar experiment using construction materials such as concrete wall has also been made. The main difficulty is the relatively poor reproducibility of the measurements due to the different concentration in each grain: this happened on some beach sands (Ikeya, 1988). G a m m a - r a y irradiation of the quartz grains inside the rock shows the formation of Ge centers but the E' centers intensity is decreased, as shown in Fig. 5(a). Since the concentration of E' centers is saturated, further 7-ray irradiation reduced the concentration. It is known that oxygen vacancies are not created by 7-rays- The enhancement of the E' centers in quartz grains at a geological fault is due to internal deformation or dislocation (Ikeya et al.. 1982). One can determine the A - b o m b or accident dose from the enhancement of Ge centers [see Fig. 5(a)] and from the decrease of E" centers. The estimated accident doses in the Motoyasu bridge granite at the surface are 150 + 100 Gy, 280 ± 60 Gy for E' and Ge signals respectively: the estimation using the decrease of E' centers is less accurate. The ;,-ray dose of DS86 at a ground distance of about 100m is about 200Gy, roughly in agreement with the present work. H u m a n tooth dosimeter

A piece of tooth extracted upon dental treatment was used in ESR dosimetry of A - b o m b survivors (Ikeya et al., 1984). A trial of ESR in-vivo dosimeter using human tooth without extraction is now being developed in our laboratory. Figure 6 is an illustration of the ESR cavity system with a special permanent magnet with a field-sweep coil. The conventional electromagnet with a pole-piece gap of about 70 mm is too narrow for placing the head in the gap.

Atomic bomb and accident dosimetry with ESR

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(a) 0.001

mW

Goin x 2.5

0.01 mW

0.1 mW

.

1 mT

(b)

77"K

(

) 5mT

Fig. 2. ESR spectra of quartz grains in granite exposed to A-bomb radiation at Hiroshima: ground distance of 132 m from the hypocenter. Measurements were at (a) T = 300 K, and (b) T = 77 K. Magnets with a large gap of 25 ~ 30 cm can be manufactured, but the coil becomes too large to secure the field uniformity. It will become too expensive just for the purpose of dose measurements. Hence, we have designed a permanent magnet of the shape shown in Fig. 6(b), using the intense magnet material of NdBFe (Neomax) in collaboration with the Sumitomo Special Metal Co. Although we have manufactured the test magnet with NdBFe, the field uniformity along the z-direction is 1 mT with the 1st test magnet, for the region

of 1-2 mm at a hole in the TE~0: microwave cavity. This is too large to observe the signal in tooth enamel; the linewidth is 0.2 m T around g = 2.0025 for polycrystalline hydroxyapatite of the enamel, which is broadened. The separation from the overlapping organic signal at 2.0045 is difficult. An effort is being made to reduce the field gradient to secure __.0.1 m T at the tooth front of an approx. + 3 m m region. One of the other difficulties we have experienced in the in-vivo observation is that the tooth, especially the inner dentine part, contains a considerable

1024

MOTOJI IKEYA and HIROSH1 ISHII (a) t-;

fJ 300K

~Ixl

E'

0.001

0.01

0.1

1

10

100

Microwave power (roW)

(b) ~: ~3

• Al &Tt

o

amount of water, leading to microwave toss. In a separate study of tooth dosimetry with dentists, the technique of carefully removing the dentine part by professionals, played a major role in measuring low doses of a few tens of mGy (Shimano et at., 19891. Although the ESR sensitivity is increased by using a new spectrometer (JEOL-RE1X) and a computer, in order to separate the accumulated natural radiation dose and the dose of dental x-rays, we need several orders-of-magnitude higher sensitivity to use the present test magnet and TEl0: cavity system to detect accident radiation doses without extracting the tooth. The signal of organic radicals at g = 2.0045 must be separated carefully so as not to overestimate the radiation dose: this was often carried out during the early stage of the research by several people with

.,.aI A/&- .''~

(a)

&t

g

Af .d

W

0.001

I Illllll[ 001

] Iltilll] 0.1

I Illllll

[ 1

I I Iltlll] 10

I Jlllllt] 100

Microwave power ( m W I

f, E

Fig. 3. The microwave power dependence of each signal at different temperatures: (a) 300 K, (b) 77 K.

n03 UA

0

100

200

300

Dose ( G y )

(b)

/

(a) •

o

J5

c

q

o o

~

U

o o

>,

o

o E E rr ~3 uJ

E

...,...r•

I.s.l

•

0

Ge

10

r

L

20

30

i

I

40

60

• _o_o_e_o

i

I

60

70

•

0

J

i

J

1 O0

200

J

_

300

Dose ( G y )

80

Depth (cm)

(c)

(b) q

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•

•

AI

Ti

0 Ti

' lo

I 20

q 30

E 40

A

t

?

50

100

Artificial

I ~ = ' - ~ ' ~ 50 60 70

e•

Depth (cm]

Fig. 4. The depth profile of (a) E' and Ge centers, and (b) AI and Ti centers from the surface of the granite at Motoyasu bridge.

dose

(Gy)

Fig. 5. The change of signal intensities of (a) E' and (b) Ge centers. *** Denotes intensities of Ge and E' centers, which gives an indication of the radiation dose corresponding to the signal intensities of E" and Ge centers at the surface of the granite exposed to A-bomb radiation, and (c) A1 and Ti centers by artificial irradiation.

(o)

Yoke

i Wave guide

lN\\\\\\\.\\\\\\'--\\\i

~

~

\

/

Microwave unit

ESR

Neomax(NdBge)

spectrometer

Fig. 6(a) A schematic illustration of the ESR dosimeter reader for the in-rico h u m a n tooth measurement. The front molar of the lower jaw is attached to the end hole of TE,L, microwave cavity. (b) A photograph of the magnet NdBFe together with the microwave cavity.

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Atomic bomb and accident dosimetry with ESR insufficient experience. Efforts to increase the sensitivity by redesigning the microwave cavity and field modulation method are presently going on, in order to establish the in-vivo ESR dosimetry of human teeth for accident dosimetry and for dental analysis. One remark must be made about the microwave and magnetic field safety of the apparatus. As systematic studies are proceeding on the safety of the magnetic field, because of the widespread clinical use of NMR-CT or magnetic resonance imaging (MRI) at high fields of around IT, one might assume that the effect of a 330 mT magnetic field would be less than that used in MRI. However, since we made a small magnet with an open magnetic circuit, the magnetic field gradient is considerably large. Thus it is still too early to use the present apparatus as a human dosimeter from the safety point of view, although one of the authors (MI) has tested it for a short period. The microwave radiation safety is of greater concern. The U.S. regulation safety limit is 1 mW/cm 2, 100 times higher than that of the U.S.S.R. at 10/~W/cm2. We normally run the ESR spectrometer at a few mW, more than the U.S. regulation. In fact, surface skin burning of the fingers occurred during the test. Water-soaked cotton and fabric in a polyethylene envelope must be placed in the mouth to absorb the leaking microwave radiation, and special attention must be paid to protect the eyes with wet goggles. The microwave power must not be raised more than a few mW to avoid injury by leaking microwaves. This is another reason why the sensitivity of the present study is low. The power should be increased to 200mW to obtain a sufficiently high signal intensity with this cavity equipped with a hole, since the actual power leaking from the hole is much lower than that indicated at the attenuator.

Summary The ESR dosimetry of A-bomb radiation at Hiroshima and Nagasaki is continuing using building materials such as concrete cement and granite and

1027

gneiss river bank. The theoretical background of the accident dosimetry using samples that have been exposed to continuous natural radiation for long geological periods has been described. E' and Ge centers are the main defects in quartz grains. A preliminary test study of m-t,&c tooth dosimetry with ESR has also been described. Acknowledgements--We thank Dr Kobayashi and Mr Konishi of the Sumitomo Special Metal Co. for help in designing and measuring the uniformity of the field of oral dosimeter magnet, and Dr T. Miki, Ms A. Kai and Dr M. Hoshi for the use of their samples in preliminary experiments. This work is supported by the 14th Research Grant from the Nissan Science Foundation and also by the Grant-in-Aid for Developmental Research from the Ministry of Education. Scienceand Culture (No. 62890007).

References Hoshi M., Sawada S., Ikeya M. and Miki T. (1985) ESR dosimetry for A-bomb survivors. In ESR Daang and Dosimetry, pp. 407-417. lonics Co., Tokyo. Ichikawa Y., Higashimura T. and Sidei T. (1966) Thermoluminescence dosimetry of gamma rays from atomic bombs in Hiroshima and Nagasaki. Health Phys. 12, 395-405. Ichikawa Y. and Nagatomo T. (1986) Thermoluminescent Measurements. In U.S.-Japan Workshop for Reassessment of Atomic Bomb Radiation Dosimetry (Radiation Effects Found.). Ikeya M. (1988) Dating and dosimetry with electron spin resonance. Magn. Reson. Rer. 13, 91-134. Ikeya M., Miki T., Kai A. and Hoshi M. (1986) ESR dosimetry of A-bomb radiation using tooth enamel and granite rocks. Radiat. Prof, Dosim. 17, 181-184. Ikeya M., Miyajima J. and Okajima S. (1984) ESR dosimetry for atomic bomb survivors using shell buttons and tooth enamel. Jpn. J. Appl. Phys. 23, L697-L699. Ikeya M, Miki T. and Tanaka K, (1982) Electron spin resonance dating of infrafault materials. Science 215, 1392-1393. Pass B. and Aldrich J. E. (1985) Dental enamel as m-riro radiation dosimeter. Med. Phys. 12, 305-307. Shimano T., Iwasaki M.. Miyazawa C., Kai A., Miki T. and Ikeya M. (1989) Appl. Radiat. lsot. 40, 1035-1038. Tatsumi J. and Okajima S. (1985) ESR dosimetry of irradiated human teeth. Med. Phys. 12, 397-405.