PROCEEDINGS OF THE 7 INTERNATIONAL ... - Fizika

G. Adlys / Medical Physics in the Baltic States 7 (2009) 86 - 90
100 keV in the beam would be 90 %, the neutron flux
on the collimator output - approximately 1x10 9 cm -2 s -1 .
The dose produced by an accompanying gamma
radiation could reach 50 % - level of the total dose [2].
6. The neutron sources for therapy
In clinical practice, basically powerful cyclotrons or
nuclear reactors are used as neutron sources. Wide
utilization of such facilities is limited by high
construction and maintenance expenses and nuclear
safety reasons.
The initial clinical studies in boron neutron capture
therapy were carried out with fission reactor produced
neutron beam of thermal neutrons [9].
The nuclear reactor is the most powerful stationary
source of neutrons. At 1 MW heat power in the reactor
core, approximately 10 17 neutrons s -1 are produced. It is
quite natural that the beams of the reactor neutrons have
been used for the NCT and FNT purposes.,There are
still some active operating therapeutical beams
produced in nuclear reactors of various kinds and
powers in the world.
Another approach to using fission reactors has been
based upon the use of a fission converter which converts
a reactor’s thermal neutron to fission neutron beams.
This approach could be especially useful for the
adaptation of the existing medium power multipurpose
reactors for neutron capture therapy.
Modification of existing reactors, new reactors designed
specially for BNCT and accelerator based sources
would all have to be considered to meet the increased
need for neutron therapy.
The MTR reactor designed for medical research has a
power of 10 MW. Some other reactors developed as
sources of neutrons for NCT have a power ranging from
5 to 20 MW.
A BNTC treatment facility is in use at the VTT
Chemical Technology’s FiR-1 research reactor facility
(250 kW Triga reactor) in Espoo, Finland [10]. The
neutron beam from the reactor is moderated into the
epithermal (0.5-10 keV) energy range in the
aluminiumfluoride-aluminium layers.
Despite the nuclear reactor in its stationary regime of
operation has a constant power level and the beam
composition thereby providing standard expositions, a
powerful reactor is a very complex and expensive
facility whose maintenance demands quite strong
requirements for nuclear safety. With quite a rare
exception, the cancer therapy centers and clinics are
usually remote from the physics centers having nuclear
reactors.
The development of accelerator based neutron sources
(ABNS) has been of interest for almost three decades
[11]. The main goal of all the investigations was and is
to create ABNS that will produce epithermal beams
with a sufficient flux of neutrons and energy spectrum
as those produced by nuclear reactors.and to secure
more clinically friendly environment for the treatment
of patients than the environment at a nuclear reactor
facility is.
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Nuclear reactions on light nuclei are supposed to be
used for the production of neutrons. In some clinics
neutron generators based on T(d,n) 4 He fast neutron
sources are used [12]. However wide use of neutron
generators is limited due to a strong regulation on highactivity
tritium target application.
Experimental results specifying neutron yield data from
thick target on accelerator-based neutron source show
that the most prospective reactions considerd for the fast
neutron therapy are: 7 Li(p,n) 7 Be, 7 Li(d,n)2 4 He and
9 Be(d,n) 10 B.
Protons or deuterons with energies up to 2-2.5 MeV are
needed for the obtaining of neutrons.
Accelerator driven system (ADS) is one of the most
promising concepts under development [13].
Initial goal of this concept is nuclear waste
transmutation and energy amplification, but the progress
in this research field will be useful also for medical
purposes, especially looking for new neutron sources.
Energy amplifiers are subcritical systems, driven by a
proton accelerator for energy extraction from
components of spent nuclear fuel. The high energy
protons from the accelerator hit the spallation target and
generate some 20-40 neutrons per proton by spallation
phenomena. The generated spallation neutrons drive the
reactor to produce power under sub-critical conditions.
The power of such a hybrid system is regulated via the
proton current intensity of the accelerator. Studies by
Nobel Laureate Carlo Rubbia and his co-workers have
shown that using investigatd experimental hybrid
systems it would be possible to achieve reactor power of
1500 MW, if an accelerator which is able to accelerate
protons to 1 GeV (1000 MeV) and generate a proton
current intensity of 10-20 mA would be in operation
[14]. Even higher proton current intensities (100 mA
and more) are pointed out by the other designer groups,
[15].
ADS consist of three main components: the accelerator,
the spallation target and the subcritical reactor.
Usually spallation target as a neutron source and a
promissing tool for medical applications represents a
heavy material of high atomic number (Z) or high (N/Z)
ratio, like a lead or lead bismuth eutectic (LBE).
In medicine, radioactive elements are increasingly used
for diagnosis, therapy and pain relieving [16]. These
elements can be produced through neutron capture on
stable elements in an accelerator driven activator, as an
alternative to reactor production exploiting the process
called Adiabatic Resonance Crossing (ARC). The ARC
concept was first proposed by Carlo Rubbia for the
transmutation by neutron capture of long-lived nuclear
waste, and for radioisotope production for nuclear
medicine applications [17] The ARC effect is based on
the fact that elements to be activated have large capture
resonances at energies of a few eV. Another factor is
that the high energy neutrons in a medium with high
atomic number, low absorption cross-section and high
elastic scattering cross-section are slowed down through
a sequence of a large number of elastic collisions and
with low energy loss per collision. In this way most fast
neutrons reach the epithermal region. The epithermal