Physical process
Spallation
is a process in which fragments of material (
spall
) are ejected from a body due to impact or stress. In the context of
impact mechanics
it describes ejection of material from a target during impact by a
projectile
. In
planetary physics
, spallation describes
meteoritic
impacts on a planetary surface and the effects of
stellar winds
and
cosmic rays
on
planetary atmospheres
and
surfaces
. In the context of
mining
or
geology
, spallation can refer to pieces of rock breaking off a
rock face
due to the internal stresses in the rock; it commonly occurs on
mine shaft
walls. In the context of
anthropology
, spallation is a process used to make stone tools such as
arrowheads
by
knapping
. In
nuclear physics
, spallation is the process in which a heavy nucleus emits numerous
nucleons
as a result of being hit by a high-energy
particle
, thus greatly reducing its
atomic weight
. In
industrial processes
and
bioprocessing
the loss of tubing material due to the repeated flexing of the tubing within a
peristaltic pump
is termed spallation.
In solid mechanics
[
edit
]
Spallation can occur when a tensile stress wave propagates through a material and can be observed in flat plate impact tests. It is caused by an internal
cavitation
due to stresses, which are generated by the interaction of stress waves, exceeding the local
tensile strength
of materials. A fragment or multiple fragments will be created on the free end of the plate. This fragment known as "
spall
" acts as a secondary projectile with velocities that can be as high as one third of the stress wave speed on the material. This type of failure is typically an effect of high explosive squash head (
HESH
) charges.
Laser spallation
[
edit
]
Laser induced spallation is a recent experimental technique developed to understand the
adhesion
of
thin films
with
substrates
. A high energy pulsed
laser
(typically
Nd:YAG
) is used to create a
compressive stress
pulse in the
substrate
wherein it propagates and reflects as a tensile wave at the free boundary. This tensile pulse spalls/peels the thin film while propagating towards the substrate. Using theory of
wave propagation
in solids it is possible to extract the interface strength. The stress pulse created in this example is usually around 3 to 8
nanoseconds
in duration while its magnitude varies as a function of
laser
fluence. Due to the non-contact application of load, this technique is very well suited to spall ultra-
thin films
(1 micrometre in thickness or less). It is also possible to mode convert a longitudinal stress wave into a
shear stress
using a pulse shaping prism and achieve shear spallation.
Nuclear spallation
[
edit
]
Nuclear spallation from the impact of cosmic rays occurs naturally in
Earth's atmosphere
and on the surfaces of bodies in space such as
meteorites
and the
Moon
. Evidence of cosmic ray spallation (also known as "spoliation") is seen on outer surfaces of bodies and gives a means of measuring the length of time of exposure. The composition of cosmic rays themselves may also indicate that they have suffered spallation before reaching Earth, because the proportion of light elements such as lithium, boron, and beryllium in them exceeds average cosmic abundances; these elements in the cosmic rays were evidently formed from spallation of oxygen, nitrogen, carbon and perhaps silicon in the cosmic ray sources or during their lengthy travel here.
Cosmogenic
isotopes
of
aluminium
,
beryllium
,
chlorine
,
iodine
and
neon
, formed by spallation of terrestrial elements under cosmic ray bombardment, have been detected on Earth.
Nuclear spallation is one of the processes by which a
particle accelerator
may be used to produce a beam of
neutrons
. A particle beam consisting of protons at around 1 GeV is shot into a target consisting of
mercury
,
tantalum
,
lead
[1]
or another heavy metal. The target nuclei are excited and upon deexcitation, 20 to 30 neutrons are expelled per nucleus. Although this is a far more expensive way of producing neutron beams than by a
chain reaction
of
nuclear fission
in a
nuclear reactor
, it has the advantage that the beam can be pulsed with relative ease. Furthermore, the energetic cost of one spallation neutron is six times lower than that of a neutron gained via nuclear fission. In contrast to nuclear fission, the spallation neutrons cannot trigger further spallation or fission processes to produce further neutrons. Therefore, there is no chain reaction, which makes the process non-critical. Observations of cosmic ray spallation had already been made in the 1930s,
[2]
but the first observations from a particle accelerator occurred in 1947, and the term "spallation" was coined by
Nobelist
Glenn T. Seaborg
that same year.
[3]
Spallation is a proposed neutron source in
subcritical nuclear reactors
like the upcoming research reactor
MYRRHA
, which is planned to investigate the feasibility of
nuclear transmutation
of
high level waste
into less harmful substances. Besides having a neutron multiplication factor
just below
criticality
, subcritical reactors can also produce net usable energy as the average energy expenditure per neutron produced ranges around 30 MeV (1GeV beam producing a bit over 30 neutrons in the most productive targets) while fission produces on the order of 200 MeV per actinide atom that is split. Even at relatively low
energy efficiency
of the processes involved, net usable energy could be generated while being able to use actinides unsuitable for use in conventional reactors as "fuel".
Production of neutrons at a spallation neutron source
[
edit
]
Generally the production of neutrons at a spallation source begins with a high-powered proton
accelerator
. The accelerator may consist of a linac only (as in the
European Spallation Source
) or a combination of linac and synchrotron (e.g.
ISIS neutron source
) or a cyclotron (e.g.
SINQ (PSI)
) . As an example, the
ISIS neutron source
is based on some components of the former
Nimrod synchrotron
. Nimrod was uncompetitive for
particle physics
so it was replaced with a new synchrotron, initially using the original
injectors
, but which produces a highly intense pulsed beam of protons. Whereas Nimrod would produce around 2 μA at 7 GeV, ISIS produces 200 μA at 0.8 GeV. This is pulsed at the rate of 50 Hz, and this intense beam of protons is focused onto a target. Experiments have been done with
depleted uranium
targets but although these produce the most intense neutron beams, they also have the shortest lives. Generally, therefore,
tantalum
or
tungsten
targets have been used. Spallation processes in the target produce the neutrons, initially at
very high energies
?a good fraction of the proton energy. These neutrons are then slowed in
moderators
filled with
liquid hydrogen
or liquid
methane
to the energies that are needed for the scattering instruments. Whilst protons can be focused since they have charge, chargeless neutrons cannot be, so in this arrangement the instruments are arranged around the moderators.
Inertial confinement fusion
has the potential to produce orders of magnitude more neutrons than spallation.
[4]
This could be useful for
neutron radiography
, which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion, and study collective excitations of phonons more effectively than
X-rays
.
See also
[
edit
]
Spallation facilities
[
edit
]
References
[
edit
]
- ^
"Spallation Target | Paul Scherrer Institut (PSI)"
.
Psi.ch
. Retrieved
2015-12-12
.
- ^
Rossi, Bruno (1933). "Uber die Eigenschaften der durchdringenden Korpuskularstrahlung im Meeresniveau" [About properties of penetrating, corpuscular radiation at sea level].
Zeitschrift fur Physik
.
82
(3?4): 151?178.
Bibcode
:
1933ZPhy...82..151R
.
doi
:
10.1007/BF01341486
.
S2CID
121427439
.
- ^
Krasa, Antonin (May 2010).
"Neutron Sources for ADS"
(PDF)
.
Faculty of Nuclear Sciences and Physical Engineering
. Czech Technical University in Prague.
S2CID
28796927
. Archived from
the original
(PDF)
on 2019-03-03
. Retrieved
October 20,
2019
.
- ^
Taylor, Andrew; Dunne, M; Bennington, S; Ansell, S; Gardner, I; Norreys, P; Broome, T; Findlay, D; Nelmes, R (February 2007). "A Route to the Brightest Possible Neutron Source?".
Science
.
315
(5815): 1092?1095.
Bibcode
:
2007Sci...315.1092T
.
doi
:
10.1126/science.1127185
.
PMID
17322053
.
S2CID
42506679
.
External links
[
edit
]