Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 69-71, 2005

AcceptedExternal Scanning Proton Microprobe –A New Method for In-Air Elemental Analysis –T. Sakai,* M. Oikawa, and T. Sato
Advanced Radiation Technology Center, Japan Atomic Energy Research Institute (JAERI), 1233 Watanuki,Takasaki, Gunma 370-1292, Japan
Received: November 15, 2004; In Final Form: April 21, 2005
A novel external scanning proton microprobe system has been developed at JAERI Takasaki. The system enables
multi-elemental mapping of samples in-air environment with spatial resolution of 1 µm. The elemental analysis isbased on Particle Induced X-ray Emission (PIXE) and Particle Induced Gamma-ray Emission (PIGE) techniques.
Sodium and heavier elements can be analyzed by the former method while light elements such as fluorine andboron can be detected by the latter. This system has already been targeted for a wide variety of applications infields such as biomedical research, dental study, environmental science, and geology.

1. Introduction
The scanning nuclear microprobe is regarded as a powerful
technique for two dimensional elemental analysis because it canbe employed for almost all ion beam analytical methods, suchas Particle Induced X-ray Emission (PIXE), Particle Induced
Gamma ray Emission (PIGE), Rutherford Backscattering
Spectrometry (RBS) and so on.1 The PIXE technique exhibitsa superior sensitivity compared with analogue electron microanalysis. Minimum detection limits for PIXE are at the level ofppm for most elements. However, conventional PIXE setups
have no sensitivity for elements lighter than neon due to low
energy X-ray attenuation in the energy dispersive X-ray detector
window. On the other hand, the PIGE technique is widely used
for detection of light elements such as lithium, beryllium, boron,and fluorine. The RBS method is sometimes used for carbon,nitrogen, and oxygen analysis in biological specimens. These
analytical techniques compensate for and complement eachother.
In the TIARA facility, JAERI Takasaki, a novel externalscanning proton microprobe system with spatial resolution of 1µm was developed.2 The external beam, which enables in-air
proton irradiation of the samples, has many advantages: itrequires no special sample preparations against a vacuum environment, makes for easier handling and observation of thesamples, and reduces damage caused by heating and charging.
The system is already being applied to elemental analysis inmany research fields.3 This paper outlines an experimental
setup of the in-air micro-PIXE and -PIGE system. Several
new applications and related techniques are also introduced.

Mylar and/or 5 µm-thick polycarbonate foils were mainly used
for the window. The foils also play the role of sample backing
to minimize the distance between the window and the samples.
A window is attached on each sample holder, which is anannular disk made by acrylic resin; the tapered hole has a 1.2
mm diameter on the atmospheric side. Samples are set onholders; up to six holders can be mounted on the revolving
stage. The final resolution of the extracted proton beam can be
kept at 1 µm.2, 5
X-ray detectors for PIXE. Two conventional X-ray detectors and a unique backward X-ray detector were operated
simultaneously for PIXE analysis. Figure 1 shows a schematic
diagram of the configuration of these three X-ray detectors,and Table 1 indicates the properties of the detectors.
The hp-Ge X-ray detector has moderate energy resolution
and poor detection efficiency below 2 keV because of its Ge Labsorption edge and backscattering proton absorber (60 µmthick polypropylene). The Si(Li) X-ray detector was set at asymmetrical position with the hp-Ge X-ray detector withrespect to the beam axis. The energy resolution of the Si(Li)detector is excellent, and this higher energy resolution results
in a better signal-to-background ratio and smaller peak overlapping, especially in the low-energy regions. The detector
window was 8 µm-thick Be and attached with an annular typeabsorber (100 µm-thick Mylar), which has a center hole 3 mm
in diameter, so that the detection efficiency below 2 keV isSample
hp-Ge X-ray detector(Ortec IGLET-X)
Backward X-ray detector(Aptec PS305-D7.5C remodeled)

2. Instruments
External scanning microprobe. A proton beam accelerated by a TIARA 3 MV single-ended machine was extracted toa light-ion focusing system.4 The typical beam spot size is 1µm with a beam current of a few hundred pA. The maximum
scan area is 1 mm2 for 3 MeV proton beams.
A beam exit window, which separates samples from thehigh vacuum beam line, is essentially required for the externalscanning microprobe. A thin organic film is an ideal material
for this purpose, because it is composed of lightweightelements and has good physical properties. Four µm-thick*Corresponding author. E-mail: tsakai@taka.jaeri.go.jp. FAX: +81273-346-9690.

Proton microbeam

Si(Li) X-ray detector(PGT LS30135)

Beam exitwindowA

Current meter

Beam dump(Glassy carbon)

Figure 1. Schematic diagram of the three X-ray detectors and thesample cooling system arrangement with a sample.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

70SakaiJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

TABLE 1: Characteristic Properties of Three X-ray Detectors
for PIXE Analysis
Si(Li) X-ray
detector

Backward X-ray
detector

Manufacturer,
model no.

Ortec IGLET-X

PGT LS30135

Aptec PS305-D7.5C
remodeled

Active area

100 mm2

30 mm2260 mm2

Solid angle~0.2 sr~0.13 sr~π sr

Energy resolution
at 5.9 keV X-ray

160 eV135 eV260 eVAbsorber
Detectable
Elements

60µm-thick annular 100µm-thick 500µm-thick glassy
polypropylene
Mylarcarbon> Si=> Na=

3428keV( 10 B)Countshp-Ge X-raydetector

10440keV( 23Na)10 2> Mn=

fairly good. On the other hand, the backward detector can beused to improve the detection efficiency X-rays above 6 keV.
The detector (Aptec PS305-D7.5C remodeled) has a 260 mm2
active area with 260 eV energy resolution. The endcap of thedetector was converted to an L-shape to set the detector crystal
just behind the sample. Although the configuration provides avery large solid angle, a proton beam dumper must be attachedin front of the detector window. This 500 µm-thick high purity
glassy carbon (Tokai Carbon co. ltd.) attached as the beam
dump also plays the role of low-energy X-ray absorber. The
backward detector has five to ten times higher efficiency thanother detectors for X-rays above 6 keV.6
Nuclear reactions and gamma-ray detectors for PIGE.
A nuclear reaction 19F(p, αγ )16O was used for fluorine analysis.
This reaction emits gamma-rays of 6.13 MeV, 6.92 MeV, and7.18 MeV.7 The energy is much higher than the backgroundgamma-ray energy in the experimental room; therefore thistechnique has a good signal-to-background ratio. The gammarays of the reaction are detected by a 4" NaI scintillation
detector (Bicron, 4x4H4/3.5A), which is located just behindthe sample instead of the backward X-ray detector.5
Boron has two stable isotopes, 10 B and 11 B. The mostintense gamma-ray from the proton-bombardment of boron has
an energy of 2.12 MeV resulting from the first excitation levelof 11B.8 However, the sensitivity is not good when the protonenergy employed is below 3 MeV. Another possible reaction
is 10 B(p, α γ ) 7 Be, which emits 428 keV gamma-rays. The
cross-section has a wide resonance at the proton energy of 1.54
MeV9; thus, the latter reaction was adopted for boron analysis.
The peak of 428 keV is Doppler broadened and subject tointerference with the 440 keV gamma-rays of 23Na. This isimportant for biological applications, because many biological
specimens contain sodium. Since higher energy resolution isrequired to distinguish these elements, we have remodeled acoaxial hp-Ge gamma-ray detector (Ortec 1601-1231- S-2)
which has 100 cm3 crystal volume, similar to the backward Xray detector.10 The energy resolution of the detector is 1.7 keVat 1.33 MeV gamma-rays from a standard 60Co source. Figure
2 shows an example of the gamma-ray spectrum from 10 B
containing a biological sample. The sample was prepared forthe study of BNCT (Boron Neutron Capture Therapy) at
Tsukuba University. Two peaks are barely distinguished at428 keV (10B) and 440 keV (23Na).
Data acquisition system. In prior experiments, a PC-based
multi-parameter data acquisition (DAQ) system was used forthe micro-PIXE and -PIGE measurements.5, 11 The DAQ software has been updated and the PC upgraded to be the latest
one capable of simultaneously processing the signals fromthese multiple detectors. The improved DAQ system canmanage up to four independent detectors with a total count rateof 30 kHz. Collected data are saved to the hard drive as binary
files without disturbing the data acquisition. The DAQ-PC isconnected to the local area network (LAN) via an Ethernet
link, so that the data file can be accessed from other PCs. The1010100200300

400500

Energy ( keV)

Figure 2. A sample gamma-ray spectrum from 10 B containing abiological sample in PIGE analysis.

file can also be copied to the FTP server automatically for apreset time period, so that the data can be retrieved easily overthe Internet.11, 12
3. Applications
Biomedical applications. One of the major applications inTIARA micro-PIXE is the analysis of biological specimens.
The observation of pharmaceutical compounds diffusing within
the cells is a key technique to reveal the agent uptake mechanism at a cellular level. The measurement of elemental distributions within the tissues is also important for physiology andpathology. Many biomedical research programs have adopted
TIARA micro-PIXE6, 13, 14 because of the superior capability ofits multi-elemental analysis with good spatial resolution.
Most raw biological samples are very vulnerable to ion beam
bombardment because they are subject to heat damage. Sample
cooling is considered to be an effective technique to reduce thisdamage. A CryoJet® (Oxford instruments) sample cooling system
has recently been installed. In this system, a flow of evaporated
gas from liquid nitrogen is blown onto the sample to keep thefrozen state of the raw biological samples. The flow temperature can be set down to 90 K within the stability of ±0.1 K. The
combination of this technique with the external scanning microprobe enables the elemental mapping of frozen raw biological
specimens.6
Dental study. Fluorine uptake and distribution in teeth arenoteworthy subjects in the dental research field, because thefluoride ion exhibits cariostatic properties on the teeth’shydroxyapatite.15 However, the small atomic number of thefluorine and its strong chemical reactiveness prevent nondestructive quantitative chemical analysis except by ion beam
analysis. The combination of micro-PIXE and -PIGE techniques is an excellent tool for measuring the microstructures offluoride distribution in teeth because the teeth’s shape can bemeasured by PIXE while fluorine maps can be taken by PIGE,
simultaneously. This combined technique has discovered
different uptake process from several fluorine-releasing dentalmaterials.16
The total charge of the proton beam is indispensable forquantitative analysis. However, in the case of thick and nonconductive targets like teeth, direct measurement of the beam
current is not possible. An indirect periodic beam chopping
method has been developed and used for beam current moni-J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

External Scanning Proton Microprobe
17

toring for quantitative analysis.
Environmental science. A large number of studies on aerosol
analysis using ion beam techniques have been reported, but only
a few studies on single particle analysis have been reported.18
The analysis of a single raindrop or single aerosol particle isexpected to give new and interesting information about air
pollution, and various airborne particles, such as single raindrops,
individual fog droplets, snow crystals, and Kosa (yellow sand)dusts have been analyzed. 18−20 These results have revealedchemical and physical properties of individual particles and havegiven detailed information on multiple scavenging processes in
the atmosphere.
Intense efforts have been made to clarify the migration
behaviors of toxic heavy metals in soil, such as cadmium, leadand arsenic, for the conservation of the living environment.
Clay and apatite are promising candidates as sorbents forheavy metals to decontaminate soils and water. 21 Uptake
processes of heavy metals in those sorbents are being studied
using the micro-PIXE technique.22 The external beam has
another advantage for analysis of thick non-conducting targets
such mineral specimens: No conductive material coating on
the sample surface is needed. Ambient gas provides a path forthe charge leak away from the insulating samples, so that theproblem of charge build-up on samples is eliminated.8
4. Summary
The TIARA external scanning proton microprobe system andrelated techniques were developed. This novel system has been
applied to various research programs and has proved to be auseful tool. Several excellent results have been discovered.
Recently, two unique apparatuses have been installed in thesystem. The multiple X-ray detection system succeeded in
drastically improving X-ray detection efficiency. This system
is used in usual operation of micro-PIXE analysis. Anotherimprovement is a sample cooling system that blows cryogenic
gas onto the sample. The sample cooling technique is very
attractive method for biological applications, because it enables
elemental mapping of a frozen raw biological specimen without
serious irradiation damage.
Acknowledgement. The authors would like to thank Drs. M.
Fukuda, K. Arakawa, Y. Ohara (JAERI), and all the staff of theJAERI electrostatic accelerator group. They also acknowledge
Mr. S. Nomiya (Raytech Corporation, Utsunomiya, Japan), andDrs. H. Naramoto and M. Katagiri (JAERI) for remodeling thedetectors.

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