Proceedings of IBIC2012, Tsukuba, JapanTUPB79USE OF GAFCHROMIC FILMS TO MEASURE THE TRANSVERSEINTENSITY DISTRIBUTION OF A LARGE-AREA ION BEAM
Y. Yuri, T. Ishizaka, T. Yuyama, I. Ishibori, S. Okumura, and A. Kitamura (Ogawa),
Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, JapanT. Yamaki and S. Sawada, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Japan
A technique of forming a large-area uniform ion beamby multipole magnets is developed at the TIARA
cyclotron facility of Japan Atomic Energy Agency. The
quality of the uniform ion beam is described by the crosssectional area and uniformity. A technique has, therefore,
been developed to measure the two-dimensionaltransverse intensity distribution of the ion beam using
Gafchromic radiochromic films, which are widely used
for dose distribution evaluation in radiation therapy. The
coloring response of Gafchromic films irradiated with ion
beams is investigated as a change in the optical density of
the film. It has been found that the optical density
increases linearly with the ion-beam fluence and that, thus,
the relative transverse intensity distribution of ion beams
can be measured using the film at practical fluence ranges
for materials and biological research. Furthermore, it isconfirmed, by evaluating the microscopic pore areadensity distribution in a track-etched polymer film, thatthe uniform intensity distribution of the multipole-focused
beam is realized microscopically, too.

INTRODUCTION
A research and development (R&D) study is in progresson formation and irradiation of a large-area uniform beamusing multipole magnets at the ion accelerator facility,
TIARA, of Japan Atomic Energy Agency (JAEA) [1]. It isnecessary to evaluate the quality of the uniform beam
both precisely and handily. As a possible technique, we
have, thus, employed Gafchromic radiochromic films(Ashland Inc.) [2] for the evaluation of the cross-sectional
area and uniformity of the large-area uniform ion beams.
The film, whose color turns blue due to radiation
exposure, was originally produced for dose evaluation in
X-ray or gamma-ray radiotherapy, but can be also applied
to the intensity distribution measurement for different
kinds of beams. The use of the film is suitable for thepresent purpose since the film has various characteristics
such as high spatial resolution, large area, relatively lowdose range, and easy handling.
Therefore, ion-beam irradiation experiments for thefilm calibration were performed at the TIARA
azimuthally-varying-field (AVF) cyclotron [3] toinvestigate the coloring response. For precise and handy
evaluation, general-purpose scanners were employed toanalyze a change in the optical density of irradiated films,
instead of using a dedicated two-dimensional
spectrophotometer.
Furthermore, the beam intensity distribution determinedby the Gafchromic film was compared with themicroscopic area-density distribution of track-etchedpores in a polyethylene terephthalate (PET) film [4].

CALIBRATION OF GAFCHROMIC
FILMS
Procedure
Two types of Gafchromic films have been chosen forthe present study [2]: One is HD-810, whose active layer(6.5 μm thick) is behind a 0.75-μm-thick surface layer
and coated on a 97-μm-thick polyester substrate. The
other is EBT2, whose active layer (30 μm thick) is put
between 80-μm and 175-μm polyester layers. According
to the manufacturer [2], the available photon dose rangesof the films are 10~400 Gy and 0.01~10 Gy, respectively.
The following procedure was taken for film calibration:
First of all, the films were irradiated uniformly with 10MeV 1H and 520-MeV 40Ar beams from the AVF
cyclotron in TIARA. The beam current was measured bya Faraday cup near the target. The irradiation time wascontrolled by an electrostatic beam deflector from 10-3 to
101 s, depending on the fluence, intensity of the beam andthe film type. Then, the irradiated film was read bygeneral-purpose scanners to digitize into a TIFF image
with 16-bit RGB color intensity values. Two different
kinds of flat-bed scanners were employed for film
reading: Canon LiDE50 (reflection type) and Epson ES10000G (reflection/transmission type). The irradiatedfilms were scanned in more than one day after irradiation
to prevent the color variation right after irradiation.
Finally, the optical density dX was determined for each X
of
RGB
color
valuesbytheequation:d X = log10 ( 216 − 1 X ) .

Results
Figure 1 shows the fluence response of HD-810 films
irradiated with 10-MeV H beams and scanned by LiDE50.
The optical densities of all three color intensities increaselinearly with the particle fluence at a low fluence and thenare saturated at a high fluence. The optical density
obtained from the red color component is the largest in
the linear-response region. On the other hand, the blue
component is the least sensitive. This reflects the fact thatthe absorption of the irradiated film is the highest in the
wavelength between 650 and 700 nm [2]. Thus, thefluence of 10-MeV H beams can be measured from 1 ×
109 to 2 × 1011 cm-2 with a moderate S/N ratio bychoosing an appropriate color component of HD-810.
When the films were scanned by ES-10000G with atransmission mode, the optical densities were slightly
ISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

TUPB79

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 1: Optical density of Gafchromic film HD-810
as a function of the 10-MeV H beam fluence. LiDE50
was used for film scanning. The background value (i. e.,
the optical density of a non-irradiated film) of about 0.
04 has been subtracted from each optical density.

Figure 3: Relative transverse intensity distribution ofan octupole-focused uniform beam measured with a
Gafchromic film. The HD-810 film was irradiated with
10-MeV H beam of a few nA for 90 s. The left andright panels show the two-dimensional (2D) distribution
and the vertical distribution along the beam central
axis, respectively.
environmental temperature and humidity. Therefore, theresponse curve obtained here does not always guarantee
the absolute calibration of the films. However, we haveconfirmed, by repeated experiments, that there always
exists the linear regime of the fluence response as long as
the optical density is below unity, as shown in Fig. 1.
Employing this feature of the Gafchromic-film response,
the relative transverse intensity distribution of the beam
can be measured handily without caring about various
external conditions.

Figure 2: Optical density of Gafchromic film EBT2 as afunction of the 10-MeV H beam fluence. LiDE50 was used
for film scanning. The background levels of 0.15 and0.20 have been subtracted from the optical densities of red
and green components, respectively. For the b lu ecomponent, the background level has not been subtracted
since the optical densities at finite fluences are smaller thanthat of a non-irradiated film.
smaller than in Fig. 1 although a similar linear response of
the optical density was observed in almost the same
fluence region.
As shown in Fig. 2, the fluence response of EBT2 to
the H irradiation behaves very differently from Fig. 1. The
optical density from the blue color is always high and lesssensitive to beam irradiation due to the yellow
fundamental color of the film. Only the optical densityfrom the green component is approximately proportional
to the fluence at a low fluence. As expected, the available
fluence range (from 108 to 3 × 109 cm-2) of EBT2 is about
two orders of magnitude lower as compared to HD-810.
Similarly, it has been found that, for 520-MeV Ar, theresponse of HD810 is linear in the fluence on the ordersof 107 ~ 109 cm-2.
In this calibration experiment, film’s external
conditions were not strictly controlled, such as the fluence
rate, elapsed time from irradiation to film reading, andISBN 978-3-95450-119-9
2

MEASUREMENT OF THE TRANSVERSEINTENSITY DITRIBUTION
We apply the above measurement technique to theevaluation of the uniform beam formation using multipole
magnets at the TIARA cyclotron [1].
An ion beam extracted from the cyclotron, which often
has an asymmetric intensity distribution, is firstly
smoothed to a sufficiently Gaussian-like transverse
distribution by multiple Coulomb scattering of a thin foil.
Then, the tail of the Gaussian-like beam is folded into theinside by octupole magnets so that the transverse
distribution is made uniform on the target. In a series ofthese processes of the uniform beam formation, various
relative distributions have been measured using
Gafchromic films. As an example, the relative intensity
distribution of the uniform beam measured is shown in
Fig. 3. The rms uniformity of the distribution is 5% in thecentral region of 9 cm × 9 cm.

COMPARISON WITH THE TRACKETCHED PORE DISTRIBUTION
Another different method of the distribution
measurement has been performed in order to verify thereliability of the above technique [4]. We here employ a
track-etching technique, which involves irradiation of apolymer film with heavy-ion beams and chemical etchingof resultant tracks in the film. Microscopic pores, each ofwhich corresponds to an incident ion to the film, can beTransverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan

(b)

High

0.5
Intensity [arb. unit](a)

TUPB79(c)
Optical density from HD-810
Track pore density from PET

0.4
0.3
0.2
0.1
0.0
0

10 20 30 40 50 60

Vertical relative position [mm]Low 1cm
Figure 4: Relative transverse intensity distributions of the Ar uniform beam obtained from HD-810 and PET films.
(a) The 2D distribution from the HD-810 film. The spatial resolution of the scanned film is 50.8 dpi. (b) The 2D
distribution obtained from the track-pore density of the PET film. The area density of the pore was counted at 64 squaregrid points, each of which has an area of 200 μm2 . (c) The 1D distributions along the vertical direction, extracted
from (a) and (b).

Procedure
Both HD-810 and PET films were irradiated with theoctupole-focused 520-MeV Ar beam under the same
condition: The cross-sectional size and fluence of thebeam were adjusted to 5-cm square and 1 × 108 cm-2,
respectively. For the HD-810 film, the beam uniformity
was determined from the relative intensity distribution
through the optical density. On the other hand, the PET
film was etched with an alkaline solution to form trackpores on the film. 64 square-grid points with amicroscopic area (~200 μm2 each) on the film wereobserved using a scanning electron microscope. Then, therelative standard deviation (RSD) of the pore area-density
distribution was evaluated as a measure of the beam
uniformity.

Results
The beam intensity distribution is compared between
the two films in Fig. 4. Figure 4(a) is the 2D relative
intensity distribution given by the HD-810 optical density.
Figure 4(b) shows the 2D relative intensity distribution
obtained from the microscopic track-pore density of the
PET film, which is composed of 64 data points. The two
distributions do not coincide exactly, but a similartendency can be seen as shown in Fig. 4(c) where the 1D
distributions extracted from Figs. 4(a) and 4(b) are plotted.
We have actually confirmed that the RSD (12%) of the 64
data points in Fig. 4(b) is equal to the uniformity of the
distribution in Fig. 4(a).
In the case where the uniform distribution was
deformed purposely using a steering magnet [1], the beam
uniformity determined from the optical density
distribution was deteriorated to 22%, which, again, agreed
with the RSD of the track-pore density distribution. Thus,
the present evaluation of the microscopic distribution
using 64 data points is adequate in a statistical sense.
We can conclude that the technique of measuring thebeam intensity distribution through the optical density ofGafchromic films reflects an actual beam intensity
distribution and that the uniform intensity distribution
formed by multipole magnets is realized microscopically.

SUMMARY
We explored the adaptability of Gafchromic
radiochromic films HD-810 and EBT2 for measuring thetransverse beam intensity distribution of energetic ion
beams. For this purpose, the beam irradiation experiment
was performed at the TIARA AVF cyclotron. The fluence
response of the films’ optical density was investigatedusing flat-bed scanners. It has been found that the opticaldensity increases linearly with the ion-beam fluence at a
low fluence. The available fluence ranges of the films areoverlapped with those often required for materials andbiological research using ion beams.
The Gafchromic-film technique of measuring therelative transverse intensity distribution is useful in the
beam evaluation although it is difficult to determine theabsolute fluence distribution. Using the technique, wehave actually confirmed that an ion beam with a largearea uniform transverse distribution is formed bymultipole magnets, and evaluated the cross-sectional area
and uniformity. The relative intensity density distribution
determined by the Gafchromic film was verified with thehelp of the track-etching technique. It has been
demonstrated that the uniform intensity distribution
formed by multipole magnets is realized also in a
microscopic viewpoint.

REFERENCES[1] Y. Yuri et al., Nucl. Instrum. Methods Phys. Res. Sect.
A 642 (2011) 10.
[2] http://gafchromic.com/[3] K.Arakawa et al., Proc. 13th International Conference
on Cyclotrons and their Applications, Vancouver,
Canada, 1992, p. 119.
[4] A. Kitamura (Ogawa) et al., to be presented at 8thInternational Symposium on Swift Heavy Ions in
Matter, SHIM2012, Kyoto, Japan, 2012.
ISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○formed on the film. The pore distribution should
correspond to the transverse beam intensity distribution.