2007 International Nuclear Atlantic Conference - INAC 2007
Santos, SP, Brazil, September 30 to October 5, 2007
ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN
ISBN: 978-85-99141-02-1
STUDY OF THE CHARACTERISTICS OF NEUTRON MONITOR
AREA APPLIED TO THE EVALUATION OF DOSE RATES IN A 15MeV
RADIOTHERAPIC ACCELERATOR
Candido M. Ribeiro2, Karla C. S. Patrão1, Walsan W. Pereira1, Evaldo S.
Fonseca1, Ricardo A. Giannoni1, Delano V. S. Batista3
1
Instituto de Radioproteção e Dosimetria/Laboratório de Nêutrons (IRD/LN/LNMRI/CNEN – RJ)
Av. Salvador Allende, S/N – Rio de Janeiro, RJ
[email protected]
[email protected]
2
Instituto de Física ,Centro de Tecnologia, bloco A
Cidade Universitária-Ilha do Fundão
21949-900 Rio de Janeiro RJ
[email protected]
3
Setor de Física Médica - Instituto Nacional de Câncer (INCa/MS)
Praça da Cruz Vermelha – Centro, Rio de Janeiro, RJ
[email protected]
ABSTRACT
Currently, in Radiotherapy, the use of linear accelerators is becoming each time more common. From Radiation
Protection point of view, these instruments represent an advance in relation to the cobalt and caesium
irradiators, mainly due to absence of the radioactive material. On the other hand, accelerators with the energies
superior to 10 MeV produce contamination of the therapeutic beam with the presence of neutrons generated in
the interaction of high-energy photons with high atomic number materials from the own irradiator. The present
work carries through measurements in a linear accelerator of 15 MeV using three neutron area monitors for a
comparison of the response of these instruments, evaluating its adequacy to this measurement. Characteristics of
use and operation associates to parameters such as: monitor dead time, monitor gamma rejection, and
calibration results are also analyzed in this study.
1. INTRODUCTION
Currently [1,2,3], in Radiotherapy, the use of linear accelerators is becoming each time more
common. From Radiation Protection point of view, these instruments represent an advance in
relation to the cobalt and cesium irradiators, mainly due to absence of the radioactive
material[4].
On the other hand, accelerators with the energies superior to 10 MeV produce contamination
of the therapeutic beam with the presence of neutrons generated in the interaction of highenergy photons with high atomic number materials from the own irradiator[5]. Additionally,
photo neutrons can interact with materials around the treatment room, modifying the initial
spectrum and causing other types of interactions which privilege the production of gamma
capture. In this way, the measurement of this spectrum of photo neutrons can be necessary in
a radiometric survey.
For that is important to verify the response of the instrumentation used in radiation protection
for the commercially available neutron measurement, analyzing its performance ahead of this
spectrum in particular. The present work carries through measurements in a linear accelerator
of 15 MeV using three neutron area monitors for a comparison of the response of these
instruments, evaluating its adequacy to this measurement. This work has for purpose the
choice of equipment of more adequate measurement and with determination based on the
characteristics of the place of measure (radioterápico center) as well as in physical and
metrological conditions of the equipment.
2. METHODS AND MEASUREMENTS
To investigate the response of different area monitors in neutrons fields in the treatment room
of clinical linear accelerator CLINAC (15 MeV) and in the surroundings of this room,
measurements with the three instruments (Table 1) had been done[6,7]. Additionally a gamma
monitor area was used to evaluate the influence of this type of radiation in the response of
neutrons monitors.
Table 1. Area monitors
Model
Radiation
Quantity and Unit
MIR
gamma
Ambient dose
equivalent (µSv/h)
FH 40 G-L
neutron
Ambient dose
equivalent (µSv/h)
ASP-1
neutron
Ambient dose
equivalent (mrem/h)
Dineutron
neutron
Ambient dose
equivalent (mrem/h)
Previously this work had been carried through measurements in this treatment room in order
to verify the detectable presence of neutrons in interest points as command accelerator and
the door room. Based on previous experience, it was chosen for the measurements the
external (Fig. 1) and internal side of the door room accelerator and the command table.
INAC 2007, Santos, SP, Brazil.
Figure 1. Three area monitors next the door
outside room treatment.
3. RESULTS
The Table 2 shows numerical results for the area monitors used. The measurements had done
with 400cGy/min, 40X40, 1 min. The results present absolute expanded uncertainty (k=2,
level of confidence 95%) calculated according to ISO GUM [7].
INAC 2007, Santos, SP, Brazil.
Table 2 Numerical results for the area monitors
Places
door (floor)
door (Fig.1)
Command table
door (Fig.1)
door (inside room)
door (outside room)
door (inside room)
Gantry
Position
Gamma
area
monitor
Neutron area monitors
FH 40 G-L
ASP-1
Dineutron
MIR
(µSv/h)
(mrem/h)
(mrem/h)
(µSv/h)
o
5.89± 0.37
0.5
0
8.5
o
6.84 ± 0.34
-
-
-
o
-
0.5
0.01
-
0
o
6.54 ± 0.24
0.5
0.30 ± 0.19
7.0
90o
5.98 ± 0.13
0.72 ± 0.17
-
6.5
270o
6.16 ± 0.43
0.55 ± 0.05
0.90 ± 0.19
8.0
180o
6.63 ± 0.14
0.6 ± 0.09
-
7.5
90o
518 ± 10
30 ± 41
-
-
270
680 ± 25
38 ± 4
-
-
o
601 ± 16
31 ± 6
-
-
0
0
0
o
0
The values gotten for the measures carried through in the previous position and subsequent to
door of the room of the accelerator if find inside of typical values of literature [1,2,4]. The
reason between the average values, gotten with this monitor, before and after the door is of
approximately 120, corresponding the 2.08 deci-reducing layers.
The discrepancies found between the two first neutron monitor in table 2 point out the
necessity of a better evaluating the reply of the monitors of area for neutrons ahead of the
created conditions of measurement in the rooms of clinical accelerators.
The neutrons detector Dineutron does not possess calibration adjustments, so that, it is not
possible adjusts it to measure in the ambient dose equivalent standard.
4. CONCLUSIONS
The measurements realized with the monitor Eberline FHT 752 showing coherent during the
period of measurement, demonstrating the adequacy of use of this monitor. All the neutron
monitors have equivalent dead time, gamma rejection and calibration uncertainties are
equivalent for FH-40 G-L and ASP-1. The gamma dose rates at the points of measurements
were from the same order from neutron dose rates, so those gamma rates did not influence in
the neutron monitor readings. There were problems with the two others neutron monitors and
this must be investigated in future measurements. A new set of neutron monitors will be used
for new measurements besides those which were used in the present work. Besides that, new
INAC 2007, Santos, SP, Brazil.
measurement points must be studied where the gamma rate is much more higher than neutron
dose to investigate the influence in neutron monitor readings.
ACKNOWLEDGMENTS
We would like to acknowledge the help from INCa e IRD support.
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INAC 2007, Santos, SP, Brazil.