Full text of DOI:10.1007/BF02901920

NOTES
ü
molecules and the change of the ratio of Ca to P[8 10]. In
Compositional change in hu-
man enamel irradiated with
MIR free electron laser
LIU Nianqing1, LI Yonggui1, ZHU Junbiao1,
ZHANG Liwen1, WANG Mingkai1, WU Gang1,
YANG Xueping1, Li Guangcheng1, Huang Yuying1,
DONG Yanmei2 & GAO Xuejun
order to study the effects of FEL irradiation on dental
composition, the distribution of elements in enamel irra-
diated and non-irradiated by FEL was investigated by
SRXRF and SEM.
1
Materials and methods
(ν) Samples. Experimental samples were prepared
in Stomatological College, Peking University. The ex-
tracted human teeth were cut into the slices with the
thickness of 0.5—1 mm along their longitudinal direction,
and were polished to produce a flat surface, named sample
1, 2 and 3 respectively. Three slices were irradiated by
FEL, then sample 1 was tested by SRXRF and samples 2
and 3 by SEM.
1. Institute of High Energy Physics, Chinese Academy of Sciences, Bei-
jing 100039, China;
2. Stomatological College, Peking University, Beijing 100081, China
Correspondence should be addressed to Liu Nianqing (e-mail: liunq@
ihep.ac.cn)
Meanwhile, the fragments of enamel of extracted
human teeth were ground by a mortar into powder and
pressed into the slices about 0.5 mm thick, as control
samples for the measurement of SRXRF.
Abstract
The purpose of this study was to investigate
compositional changes in human enamel irradiated with the
free electron laser (FEL). The exposure on dental enamel at
the wavelength of 9.64 Pm was observed with the Beijing free
electron laser. The distribution of elements in the irradiated
or non-irradiated enamel was measured by scanning electron
microscope (SEM) with energy-dispersive spectroscopy and
synchrotron radiation X-ray fluorescence (SRXRF) in Bei-
jing Synchrotron Radiation Facility (BSRF). The results
showed that the P/Ca ratio in the ablation region of enamel
at the maximum wavelength of infrared absorption was ob-
viously smaller than that at the non-maximum wavelength.
In the ablation region the ratios of P/Ca and Ca/Sr were
smaller than those in the non-ablation region. The distribu-
tions of P, Ca and Sr in the ablation region were heterogene-
ous due to the element change caused by FEL irradiation.
(ξ) MIR FEL. Beijing FEL was an S-band rf li-
nac-based FEL with its tunable wavelength region of
7—18 Pm whose double pulse time structure can be seen
in fig. 1[11]. Within each macro-pulse of 2—4 µs pulse
width and 3 Hz repetition rate there were more than 10000
micro-pulses with the width of 2—4 ps pulses and the
repetition rate of 2856 MHz. About 1 µJ energy per mi-
cropulse corresponds to about 5 mJ laser photon energy
per macropulse. The spot of FEL beam was about 100h
300 Pm2 depending on the slanting degree of the beam on
the sample surface.
Keywords: human enamel, Beijing free electron laser, synchrotron
radiation X-ray fluorescence, ablation, element distribution.
The conventional lasers in dentistry applications,
such as Er:YAG, CO2, Ho:YAG, possess thermal side ef-
fects within the vicinity of the irradiated area, due to their
inherent characteristics of monochrome and continuous-
ness of the long pulse width. The longer laser pulse causes
the rupture of hydroxyapatite structure due to the sudden
explosive gasification of the water, deep slits up 3 mm and
melting phenomena in enamel and dentin[1ņ6]. Comparing
with medium infrared FEL, MIR FEL has the characteris-
tics of wide wavelength tunability, ultrashort pulse (ps)
and high peak power. If the FEL is tuned to a characteris-
tic value at an absorption maximum of the major compo-
nent of the irradiated material, the material will be ionized.
High ablation efficiencies reduce the required energy per
pulse and the temperature increases for short pulse irra-
diation as well. The damage of soft or hard tissue within
vicinity of the irradiated area was limited[7]. Therefore,
FEL will offer a distinct combination of high ablation ef-
ficiency, low thermal side effects and the pain reduction as
a potential benefit of light source. Ogino reported that the
irradiated FEL induced desorption of ion from tooth den-
tine, the resonant multi-photon vibrational excitation of
Fig. 1. Time structure of double laser pulses of the S-band rf-linac-FEL
at BFEL.
Sample 1 and samples 2 and 3 were respectively
irradiated for 15 and 10 min (samples 2 and 3) with FEL
of 9.65, 9.46 and 10.6 Pm respectively.
(ο) SRXRF. The SRXRF experiment for the
measurement of sample 1 was performed at the BSRF[12]
.
The spot size of the white light was about 40 Pmh40 Pm.
Within the FEL irradiation region, sample 1 was scanned
at 11 and 17 points on the cross lines from its center
shown as X and Y in figs. 2 and 3 respectively. The control
sample was scanned at 3 points. An AXIL program per-
formed the processing of the X-ray spectra. The area of
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Chinese Science Bulletin Vol. 46 No. 23 December 2001

Fig. 3. Ratio of Sr/Ca changes in the ablation enamel to that in the
normal enamel along X and Y directions.
Fig. 2. Ratio of P/Ca changes in ablation enamel to that in the normal
enamel along the X and Y directions.
the peak in X-ray spectrum was proportional to the con-
tent of the corresponding element. When the net area of
the element peak in X-ray spectrum was 3 times more
than the background underlying peak, it was taken as the
limit of minimum detection (MDL).
(π) SEM. A SEM (KYKY2800 with operation
voltage of 15 KV and filament current of 75 PA) evalua-
tion of the enamel samples 2 and 3 was performed. The
beam spot about 1 Pm was scanned within the irradiation
and the non-irradiation areas of samples 2 and 3. The
scanning area of 1.5 mmh0.5 mm was chosen to scan
over the equivalent areas of sample 2. The P/Ca variation
of the irradiated samples 2 and 3 and that of non-
irradiated samples were obtained.
Fig. 4. FTIR absorption spectra of enamel and dentine.
10% water and 20% organic matrix (mainly collagen and
elastin). The wavelength of FEL irradiation was chosen
according to the IR absorption spectra. The maximum
absorption wavelength is located at 9.66 Pm, namely,
characteristic absorption wavelength of hydroxyapatite.
During the FEL irradiation on the samples, the visi-
ble light emission and the smell of the burning protein
2
Results and discussions
Fig. 4 shows the IR absorption spectra of the powder
sample of human enamel and dentine. The enamel is the
hard tissue in human body, which contains 95% hydro-
xyapatite (Ca10(PO4)6(OH)2), 4% water and other 1% or-
ganic matrix. Dentine consists of 70% hydroxyapatite,
Chinese Science Bulletin Vol. 46 No. 23 December 2001
2017

NOTES
were observed at the very beginning, then they decreased
gradually. Finally, under an optical microscope, an ellipse
crater, i.e. an ablation region was found. These results
suggested that there was a reaction of the organic matrix
and the mineral of the enamel during FEL exposure. No
more cracks were observed.
3
Conclusion
When the enamel was exposed to the characteristic
absorption wavelength of hydroxyapatite, P, Ca and Sr in
the enamel were reduced effectively; meanwhile, the light
and the ellipse ablation region could be observed. The
content change region of the elements was very small
(about 600 Pmh200 Pm). In the ablation region, the
change of the elements is denoted by P>Ca>Sr. The
measurement of SEM proved that the distributions of P
and Ca in the ablation region were heterogeneous. The
FEL might be a potential light source in the near future.
The SRXRF results of sample 1 showed that there
were higher peaks of P, Ca and Sr, as well as lower peaks
of Zn and Fe in the X-ray spectra of the enamel samples.
The ratios of P/Ca and Sr/Ca in the irradiated area of
sample 1 in the X and Y directions to these of non-irradi-
ated area were changed, as shown in fig. 2 and 3, respec-
tively. In addition, the lower energy X-ray, such as P
X-ray, was easily absorbed by the air and Be window of
Si(Li) detector. If the net area of P peak was less than
MDL, the value of P/Ca was zero (fig. 2). The distribution
of P, Ca and Sr in the ablation area was non-homogeneous
due to the element change caused by FEL irradiation. The
reduction of the element content in the center of the abla-
tion area was more than that in the margin. The reduction
of various elements was P>Ca>Sr. These results implied
that the light atom in enamel was easily removed and ion-
ized from crystal lattice during FEL irradiation at the
resonance absorption band. According to the changes of
the element content, the ablation region of the enamel
sample was about 600 Pmh200 Pm.
The results of SEM are in the following: the values
of P/Ca of the irradiated and non-irradiated enamels are
listed in table 1. The reduction of P in the enamel irradi-
ated with characteristic absorption wavelength of
hydroxyapatite 9.64 Pm was more than that of the non-
characteristic absorption wavelength of 10.6 Pm. The dis-
tribution of elements within the ablation area was consis-
tent with that by SRXRF. The cause of the P reduction in
the enamel irradiated with 10.6 Pm was perhaps the com-
burent of the organic matter in the enamel.
Acknowledgements This work was jointly supported by the National
High Technology Laser Technology Field Foundation (Grant No.
863-410) and the National Natural Science Foundation of China (Grant
No. 19975055).
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1.5 mm
10.6 Pm
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0.706
0.646
0.654
0.437
0.233
0.477
0.639
0.609
0.650
0.612
0.646
0.639
0.663
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Mean
0.528
0.156
0.642
0.019
0.682
0.504
0.012
(Received August 3, 2001)
2018
Chinese Science Bulletin Vol. 46 No. 23 December 2001