charged-particle detector are installed at ±135 to measure X-rays
and backscattered protons, respectively. A Mylar film of 100 lm
thick is located in front of the X-ray detector to stop backscattered
protons. During the measurement the capillary glass wall emits
fluorescence due to proton bombardment, causing the background
noise on the silicon charged-particle detector system. To attenuate
the fluorescence, a carbon foil of 100 lg/cm2 is also put in front of
the detector. The signals from these two detectors are processed by
amplifiers and recorded by PC-controlled multichannel analyzers.
To check the performance of the micro-PIXE system, a copper
thin plate was mounted on an aluminum target holder and
scanned with a 2-MeV proton micro beam extracted from a
£10-lm capillary. The variation of X-ray yield y is expressed as
a function of the beam center position x as follows:
yðxÞ ¼ cr2fcos1ðx=rÞðx=rÞsin½cos1ðx=rÞg ð1Þ
Here r is the beam radius and c is a constant. By fitting the measured
Cu Ka X-ray intensity profile with this formula, we evaluated
the effective beam spot size on the target. For the demonstration of
two-dimensional element mapping, copper fine mesh (£8-lm
wire, 1000 mesh/inch) was used in the present study. A 2-MeV
proton micro beam from the £10-lm capillary scanned the mesh
fixed on a sheet of graphite. The two-dimensional distribution of
copper was reconstructed from Cu Ka X-ray intensities normalized
by backscattered proton counts.
The energy spectra of protons focused by the capillary were
measured by a silicon charged particle detector mounted on the
target holder. A 50-lm thick tantalum plate with a pinhole of
100 lm diameter and a 0.8-lm thick aluminum foil were located
right in front of the detector to increase the spatial resolution
and prevent localized proton dose on the detector, respectively.
To examine the energy spectra of protons scattered in the capillary
separately, the detector assembly was placed 15 cm downstream
from the capillary outlet, resulting in an angular resolution of
0.67 mrad.
3.