Download The electron accelerator of the ISOF-CNR Institute

The electron accelerator of the ISOF-CNR
Institute: its characteristics and use
P. Fuochi, U. Corda, and M. Lavalle
ISOF-CNR Institute, Via P. Gobetti 101,
I-40129 Bologna, Italy
LINAC Laboratory: ISOF-CNR, Via Biancafarina 2485,
40060 Fossatone di Medicina (BO)
Italy
The electron accelerator
The Institute of Photochemistry and High Energy Radiation (now merged into the Institute for Organic Synthesis
and Photoreactivity) was established in 1970 in Bologna. One of the main tasks of the Institute, which became the
main reference centre for radiation chemistry in Italy, was to perform basic and applied research in the field of the
effects and application of ionizing radiation. For this purpose the Institute was equipped with 60Co γ-sources and a 12
MeV linear electron accelerator.
The 12 MeV linear accelerator (maximum energy with no load), built by Vickers, came into operation in
1973. It is an L-band (i.e. working at a radiofrequency of 1.3 GHz) travelling wave accelerator operating in the π/2
mode. The electron beam is pulsed and the pulse width can be varied from 10 ns to 5 µs with a repetition rate from
single shot up to 1000 pps. Thus the accelerator can be used in single shot or in continuous mode and a pulse counter
allows the delivery of any preset number of pulses. Pulse to pulse reproducibility is within ± 2% per day. The
maximum current (peak current) obtainable is between 5 and 7 A with 10 ns pulses and 1 A for long pulses (µs). The
most probable energy of the electrons is 11 MeV for pulses between 10 and 100 ns dropping to 8 MeV at 2 µs. The
beam energy can be varied by changing the pulse width and/or the average beam current.
The typical configuration for routine irradiation is:
pulse repetition rate: 50 Hz
pulse length: 2 µs
electron energy: 8 MeV
average beam current: 100 µA
peak pulse current: 1 A
Since there is no scanning system, in order to obtain good uniformity of dose distribution (≤ ± 10%) on the
sample under irradiation, aluminium scatter plates are used to spread the beam for samples having dimensions ≤ 110
cm2, while for samples up to 40 x 40 cm, a computer driven x-y moving system is used to move these samples in front
of the accelerator window.
The 12 MeV Vickers linear accelerator.
View of the accelerating waveguide.
The accelerator’s and the klystron’s rooms are
shielded with a copper Faraday cage to avoid
electronic noise in the irradiation room.
Scheme of the LINAC
1
Beam current, arbitrary unit
Energy spectra for the
10 ns, 1 µs and 2 µs beam pulses.
10 ns
0.8
2 µs
1 µs
0.6
0.4
0.2
0
6
7
8
9
10
11
12
Beam energy (MeV)
Depth-dose curves measured at 90 cm from the LINAC, scatterplate= 0.9 mm Al
1.6
Depth dose distributions
measured using the wedge
technique.
1.4
1.2
0.2 µs pulses
Ep = 11.5 MeV
Dose, a.u.
1
0.8
2 µs pulses
Ep = 8 MeV
0.6
0.4
0.2
0
0
2
4
6
8
10
12
14
mm in Al
16
18
20
22
24
26
28
13
Dosimetry and irradiation process control
A graphite charge collector is used to monitor the beam current and the electron fluence (e−/cm2). The
collected charge is led to a charge detection system placed outside the irradiation room and displayed on a digital
counter. Calibration against the modified Fricke chemical dosimeter allowed direct conversion of the collected charge
(e−/cm2) to absorbed dose (Gy). The modified Fricke chemical dosimeter is also the reference dosimeter for the
calibration of radiochromic films which are used as routine dosimeter. Alanine pellets are used as transfer dosimeter in
intercomparison dose measurements with other laboratories. An energy monitor allows to measure the energy of the
beam. Routine measurements of the beam energy and the fluence, and hence of the dose, are done regularly before and
after each irradiation sequence.
20
18
16
Dose (Gy/pulse)
14
12
10
8
6
4
2
0
0
The graphite charge collector.
0.5
1
1.5
2
2.5
10 -
3
3.5
4
2
Fluence (10 e /cm pulse)
Calibration curve for the graphite charge
collector vs. absorbed dose in “super Fricke”
solution.
4.5
e-
To the digital
current
integrators
100
mm
Aluminium cage dimensions:
Ceramic
Wall thickness: (14cm x 14cm x 10cm)
pillars
Connected
to ground
10 mm
2.
1.
0.95
long pulses (0.5-5 µs)
short pulses (0.05-0.5 µs)
0.90
0.85
0.80
1. front view of the energy
device
Energy ratio
0.75
0.70
2. cross-sectional view of the
energy device
0.65
0.60
0.55
3. calibration curves for the
energy device
0.50
0.45
0.40
0.35
3.
6.5
7.0
7.5
8.0
8.5
9.0
9.5
Ep (MeV)
10.0
10.5
11.0
11.5
12.0
Applied radiation research using the 12 MeV Linac
The electron accelerator is used in a number of researches and tests connected to industrial
applications which are listed below.
1. Treatment of food products and pharmaceuticals. Some studies have been conducted in
this field to evaluate the effectiveness of electron irradiation in decontamination of
pharmaceuticals, spices and poultry meat from pathogenic micro-organisms.
2. Curing of composites. Thick composite materials made of carbon fibres in organic
matrices (epoxy resins) have been cured with high-energy electron beams. The polimerization
of the methylmethacrylate in presence of rubber is under study.
3.
1.
4.
2.
COMPOSITE MATERIALS.
1. and 2.: glass fibres and epoxy resins layers,
aluminium honeycomb structure inside
3. and 4.: carbon fibres and epoxy resins layers,
aluminium honeycomb structure inside
5.
5. epoxy resin and carbon fibres composite
Applied radiation research using the 12 MeV Linac
3. Sterilization of human bone tissues for implants. Femoral heads are sterilized to be used
as biocompatible material in prosthesis implants, skull fragments are sterilized before their
implants in the same patient.
1.
2.
HUMAN BONE STERILIZATION.
1. disposable container for surgical specimen
2. the hermetic envelope, whit the container, is in front of the LINAC window,
ready for the irradiation
Applied radiation research using the 12 MeV Linac
4. Modification of semiconductor devices. Starting from 1982, research in the field of
electron irradiation of power semiconductor devices has been conducted at FRAE Institute
(now ISOF). In collaboration with Italian power semiconductor manufacturing industries an
extensive program of irradiation tests have been undertaken. Fast switching diodes and
thyristors, Gate Turn Off thyristors (GTO), power MOS transistors and Insulated Gate Bipolar
Transistors (IGBT) were irradiated with electron beams at different doses in order to achieve
modelling of the electrical parameters and specific adjustment of carrier lifetime. The results
led the above mentioned firms to introduce this technology for series production in place of the
conventional high-temperature heavy metal diffusion for lifetime control. Equipment and
methods for series production have been developed, tested and since 1986 put into operation
MODIFICATION OF SEMICONDUCTOR DEVICES.
1. silicon wafers
2. the system for the power semiconductor irradiation
3. power semiconductor devices in the process load unit
1.
2.
3.
MODIFICATION OF
SEMICONDUCTOR DEVICES.
1. silicon slice for thyristor
2. finished device: thyristor
3. and 4. final device set up:
3. stud case 4. press pack case
1.
2.
3.
4.
Applied radiation research using the 12 MeV Linac
The linac is also used to perform irradiation tests on MOS devices with ultra-thin oxides.
Present studies, conducted in collaboration with the Dept. of Physics and Dept. of Electronics
and Informatics of the University of Padova, on total dose effects (devices were irradiated up to
150 Mrad) include measurements of radiation induced leakage current (RILC) and soft
breakdown in gate oxides MOS capacitors. These studies have demonstrated that the RILC in
these devices is due to neutral traps generated in the oxide during irradiation and that RILC
conduction mechanism can be explained by a sort of electron tunnelling through gate oxide
assisted by radiation induced neutral traps. These results, obtained using the linac as electron
source, are similar to those obtained with γ-, x-rays and high energy ions with LET<10
MeVcm2mg-1. The use of a linac in place of gamma cells (usually used for total dose
experiments) results to be more convenient because of the reduced time to reach high doses
necessary to study the RILC phenomenon.
X-Ray generation
A water-cooled tantalum converter is used to produce X-rays. The target thickness
(1.5 mm) has been selected in order to optimise the bremsstrahlung production using
the 2µs pulses.
90
1
X-Ray distribution
Polar distribution of the
forward X-ray emission
from the converter.
120
00
e-
180
210
1
240
1
270
0.9
dose, u.a.
dose, a.u.
150
0.8
0.7
0.6
0.5
X-Ray penetration
Depth-dose profile of the
bremsstrahlung beam.
0.4
0.3
0
10
20
g/cm
2
30
LICHTENBERG TREES IN PMMA CILINDERS.
The “Lichtenberg tree” effect is the electrical discharge pattern resulting from an
instantaneous breakdown of an electrical field. The field results from the rest of
high-speed electrons injected into the material. When the field exceed the insulating
strength of the material a local discharge occurs whit consequent damage on the
material itself.