Transcript Photoneutron Distributions around 18 MV X

Photoneutron Distributions around 18
MV X-ray Radiotherapy Accelerators
using Nuclear Track Detectors
Fazal-ur-Rehman,
H. Al-Ghamdi,
M. I. Al-Jarallah (Presenting Author)
N. Maalej
King Fahd University of Petroleum and Minerals
Outline
•
•
•
•
Introduction
Methods
Results
Conclusion
Introduction
• Neutrons contaminate the high energy
Linear Accelerator (LINAC) photon beam
in radiation therapy and contribution dose
to the patient
• We studied the distribution of thermal
and fast neutrons in the LINAC room
during irradiation with an 18 MeV photon
beam
Methods
• Neutrons are generated by high energy photon
interaction with the high Z materials of the
LINAC head
• Fast Neutron Measurement:
– Bare NTDs are used
– Tracks are generated by protons from the (n,p)
recoil in the detector material (C12H18O7 )
• Thermal Neutron Measurement:
– NTDs are covered with Li2B4O7
– Tracks are generated by -particles from 10B(n,)7Li
and 6Li(n,)3H nuclear reactions
Methods
Fig.1 Experimental arrangement showing the location of CR-39 NTDs around a
radiotherapy linear accelerator.
Methods
Isocente
r
-3
-2
-1
0
1
2
3
x
1m
1
1m
2
3
4
Maze
Entrance
5
y
Maze
Insid
e
Fig. 2 Experimental arrangement showing the location of CR-39 NTDs
around a radiotherapy linear accelerator.
Methods
• NTDs advantages over active
detectors:
– There is no pulse pileup problem
– No photon interference with neutron
measurement
– No electronics are required
– Less prone to noise and interference
Results
18
Fast neutron relative intensity X 10
4
16
small room
large room
14
12
10
8
6
4
2
0
-4
-3
-2
-1
X
0
1
2
3
4
Transversal distance from isocenter (m)
Fig. 4 Fast neutron relative intensity as a function of transversal distance from
isocenter of linear accelerators located in a small and large treatment rooms.
Results
18
small room
large room
Fast neutron relative intensity x 10
4
16
14
12
10
8
6
4
2
y
0
0
1
2
3
4
Longitudinal distance from isocenter (m)
Fig. 5 Fast neutron relative intensity as a function of longitudinal distance from
the isocenter of linear accelerators located in a small and large treatment rooms.
Results
8
5
SRSA
LRSA
Thermal neutron relative intensity x 10
7
6
5
4
3
2
1
0
-4
-3
-2
-1
x
0
1
2
3
4
Transversal distance from isocenter (m)
Fig. 6 Fast neutron relative intensity as a function of longitudinal distance from
the isocenter of linear accelerators located in a small and large treatment rooms.
Results
8
small room
large room
Thermal neutron relative intensity x 10
5
7
6
5
4
3
2
1
y
0
0
1
2
3
4
Longitudinal distance (m )
Fig. 7 Thermal neutron relative intensity as a function of longitudinal distance
from isocenter of linear accelerators located in a small and large treatment rooms.
Conclusion
• The fast neutron distribution along the
transverse and longitudinal directions is
symmetrical about the beam axis.
• The fast neutron relative intensities fall in
an exponential-like fashion with distance
from the isocenter of the accelerators.
• The thermal neutron distribution is uniform
at all the locations along transverse and
longitudinal directions.
• The larger size LINAC room had lower fast
and thermal neutron relative intensities at
all locations.
Acknowledgment
• Physics Department of King Fahd
University of Petroleum and Minerals
(KFUPM).
• The Department of Radiotherapy, King
Fahd Specialist Hospital.
• Dhahran Health Center, Dhahran, for
utilizing their accelerator facilities in this
study.