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Many modern electronic components
exhibit increased sensitivity to low energy
proton radiation. To qualify new devices for space applications one needs
testing
in a full range of interesting energies. To answer such demands the low
energy facility
was developed in
the NEB area. It provides high proton fluxes and makes it possible
to
conduct radiation tests with energies
down to only few MeV. Generally, the design
of both low and high energy facilities is very similar.
The Philips cyclotron (also working as an injector) is used to
produce low energy
beams of protons, deuterons and different types of heavy ions. It is a variable energy
accelerator - up to 72 MeV for protons and up to 120 MeV· Z2/A for
heavy ions.
A medical irradiation
facility Opthalmological Proton Therapy Installation SIN -
OPTIS,
experimental installations in two other areas (one with a
neutron production target),
and isotope production station are supplied with particles from
this cyclotron.
It has a simple and immutable beam optics designed in accordance
with safety
operation conditions. Switching of the proton beam between different user
areas
can be done within very short time.
A schematic layout of the
component radiation test facility in the NEB area is
shown in Figure 1. Experimental instruments and devices are mounted on the
portable
stage (100 cm long, 80 cm wide) that can be easily moved and aligned
on the beam line. The
arrangement consists of energy degrader, XY-table with
a sample holder and a beam dump behind which a
laser positioning tool is mounted.
Plastic scintillators or ionization chambers used for
flux/dose monitoring are
located behind the beam exit window. Irradiation tests are performed in air.
Figure 1. PIF-NEB Experimental site (not in scale).
Energy degrader
At the moment aluminium plates of different
thickness are used to lower the initial
proton energy. Seven plates can be mounted perpendicularly to the beam
direction
using remotely controlled pneumatic pistons.
The energy
can be selected in a semi-
continuous manner between 6 and 63 MeV
by placing a proper material thickness
on the beam axis.
XY-table and sample
frame
Irradiated devices are mounted on
the standard holder frame that is also used in
PIF/PSI,
SEU/Brookhaven
and
HIF/Louvain
la Neuve. The frame is fixed on movable arms
of the
XY table that is mounted perpendicularly to the beam line.
The position can be changed
in a vertical and horizontal direction using servo-motors. The
distance between the holder
and the beam exit can be varied too. It provides easy access to the
front side of the test
frame either during set-up or system verification. The table with arms and the
sample frame
is shown in Photo 1.
Photo 1. PIF-NEB Side view with ionization
chamber, sample holder with XY-arm and laser.
Flux measurement and
calibration
Typical detection system consists
of the air filled ionization chamber - see Photo 2 - mounted
at the end of the beam pipe. The chamber is divided into four quadrants
allowing to monitor
the proton flux distribution over the beam line. At each energy, the conversion factor is
determined during calibration to
the plastic counter of known area - see Photo 3. The counter
is located exactly
in a place of the test sample.
Photo 2. PIF-NEB Ionization chamber.
Photo 3. PIF-NEB plastic detector on the
sample holder. Energy degrader and ionization chamber
are seen upstream.
Run control and data
acquisition
Irradiation procedure, supervised
by the computer, is highly automatic. For each run the most
important parameters (sample id, position,
energy, flux and accumulated dose) are updated
periodically on the computer screen and are also stored into a
file. Alarm signals are provided
in case
of any malfunctioning of the devices. The energy of the protons is remotely set
by
selecting proper thickness of the Al-degrader in front of the sample. The user panel
displayed on
a computer screen for
monitoring and control of the run is shown in Figure 2.
The position of the sample
on the beam is set either remotely with another computer operating
the XY table or manually.
Figure 2. PIF-NEB Run panel for control of the
test.
Laser tool and beam
dump
Test samples are centred on the
beam with a help of the laser system. The laser is mounted
separately, about 50 cm downstream from the irradiation platform.
The laser beam, aligned
to the proton axis, illuminates the rear side of the sample.
The protons passing behind the test sample are deposited in the
beam dump that is located
at the end of the platform. It consists of the PVC plate in order to reduce
activation of the area.
Energy spectra and
beam profiles
Examples of the energy spectra for
protons with energies of 64 MeV and 23 MeV are shown in
Figure 3. The measurements were done using plastics scintillator NE102 of
35 mm thickness
to stop the protons. (No corrections due to a finite resolution and
scattering effects are made.)
Beam profiles are usually set to flat during the setup using small
Copper plate located in the beam-
line about 2 m upstream. One assures the
uniform field of up to 5
cm diameter at each energy.
For smaller beam diameters the Copper plate must be removed. Without
the plate can achieve beam diameters as small as 0.5 cm.
Figure 3. PIF-NEB Examples of energy spectra.
Main features
-
Energy
range: 6 to 63 MeV
-
Proton
flux: <5· 108
p/cm2/sec
-
Beam
spot: circle, up to 9 cm diameter
-
Beam
uniformity: > 90% over 5 cm diameter
-
Flux/Dosimetry:
about 5% absolute accuracy
-
Irradiation
take places in air
-
Sample
frame Brookhaven and HIF compatible is fixed on the
XY table
are supervised by the computer.
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