Download Department of Energy - Center for Materials Research

The Keck slow positron beam, and other animals
David B Cassidy
Keck Antimatter Research Lab
Washington State University
September 2001
DISCLAIMER
The following is not meant to be a comprehensive overview of positron beams, and contains
elements that may be best described as (my) opinion. Therefore, any inaccuracies errors or
nonsensical claims are solely my responsibility and do not necessarily represent the other
researchers in the group
Introduction
The Keck antimatter laboratory is a new institution at WSU, operating under the auspices
of the Center for Materials Research, and closely connected to the department of Physics.
It is the latest addition to Professor Kelvin Lynn’s group, about which more may be
learned by clicking here. [Hyperlink to Kelvin stuff]. The primary purpose of this
laboratory is to conduct fundamental research in a variety of areas using positrons, the
antiparticles of electrons. Specifically, the laboratory will focus on experiments that
benefit from the availability of a high intensity positron beam.
The W.M. Keck foundation has provided a generous grant to WSU in order to allow the
construction of an intense accelerator based positron beam. This exciting project is
already underway, and progress on the installation of the deuteron accelerator is
described here. [Hyperlink to Alan stuff]
The following describes the construction and operation of a positron beam in the Keck
laboratory, specifically, the work occurring in parallel with the accelerator
implementation. However, I have also taken the opportunity (or, perhaps, the liberty) to
enucleate some of the basic elements associated with such contrivances, for the benefit of
those who may not be familiar with them. I hope my interpretations do not detract from
the details of our work, nor depreciate from my recounting of such efforts. They are
given only in the expectation that the audience is varied, or at least bizarre, and perhaps
even demented.
A fully functional beamline is being constructed, using the traditional positron source, the
radioactive isotope 22Na (“sodium twenty-two”). This philosophy behind the construction
of this beam is manifold. Initially it is intended for a very specific experiment, the
detailed study of the rare three-photon mode of electron-positron annihilation. However,
it is envisaged that the beam will also be suitable for general-purpose materials studies
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and, eventually, may be connected to the intense positron source created by the
accelerator.
The pictures in this section of what I shall henceforth refer to as the Keck positron beam
may evoke memories (of various forms) to anyone who has ever worked at Brookhaven
National Laboratory (BNL). Much of the equipment used in this beam (whose value is
estimated at $500,000) was generously donated by BNL. In fact, the elementary
components of the beam were once part of one of the very first positron beams,
constructed by Professor Lynn and associates quite some time ago. This does not mean,
however, that the Keck beam is antiquated!
The Keck beam is basically a magnetically guided positron beam with an energy range of
~ 0-75 kV, very similar to the one in use in the physics department at WSU. The most
significant difference between the two will be the use in the Keck beam of a solid rare
gas (Krypton) moderator, as opposed to the more durable, but less efficient, tungsten foil
moderator as used in the physics department. The equipment required for the Krypton
moderator (estimated value, $50,000) was also obtained at minimal cost via a
collaboration with Professor Karl Canter of Brandeis University.
Along with the vacuum hardware, a 30 mCi 22Na source was also obtained from BNL.
NB: the milli-Curie (mCi) is an antiquated unit of measurement describing the activity of
radioactive material. 1 mCi is equal to 3.7 x 1010 Bq, and one Bq (Becquerel) is simply
one disintegration per second. Thus, the 30 mCi source will have an activity of ~ 1.1 x
109 decays per second. 90% of these will result in the emission of a positron (with a 22Na
source). A typical efficiency value for a Krypton moderator (defined as the ratio of fast
+ particles emitted by the source to slow positrons in the beam) is 0.1%, and we
therefore expect to be able to produce a beam with an intensity of around 1 x 106 e+s-1.
The basic elements of a positron beam
In order to make a useful positron beam there are 5 essential ingredients one must have.
These are,
1)
2)
3)
4)
5)
A controlled positron source
A vacuum system
A transport mechanism
A detection system
Money
Naturally, the reality of any given circumstance is far more complicated, but I believe
that these five elements may be used to characterise positron beams, albeit in a
generalised manner. For example, the source of positrons may be split up into 3 realistic
sub-sections. The use of “off the shelf” 22Na is by far the most common, but there are
also beams around that use the bremsstrahlung shower from an electron linac to create
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positrons via pair production. Some use the production of intense sources in situ, either
by a cyclotron or a nuclear reactor (although usually this is not strictly in situ) and, the
methodology being instigated in the Keck lab, the use of a compact accelerator. This last
technique has provided only prototype beams so far, but a functional, data generating
positron beam produced in this way is expected in the Keck laboratory within the year.
All beams utilise some sort of vacuum system. However, some experiments are
performed ex vacuo, but these typically have no transport mechanism, and consequently
don’t qualify as beam experiments.
The transport mechanisms universally used in positron beams really fall into two
categories; they are either electrostatic or magnetic, and I shall say more about these later.
The detection can be done in a lot of ways and is determined by the type of experiments
being performed. This could be detecting the positrons themselves (e.g. in a scattering
cross-section experiment) or detecting electrons (e.g. in positron induced Auger
experiments). Alternatively, one may wish to measure either annihilation gamma ray
energy spectra or, equivalently, the angles between such gamma rays (e.g. in bulk defect
studies) or measuring the lifetime of positrons, (which tells you something of the local
electron density in the vicinity of the dying positrons).
The final criterion is money, and there’s not much I can say about that that isn’t obvious.
You can set up a beam and run it, but it is quite surprising how expensive such an
enterprise can turn out to be. Funding issues are very frequently of definitive importance
in positron physics, although I dare say this is not unique to the field!
So, the Keck beam naturally contains all of the above elements. I’ll go through them, and
perhaps mention some of the alternatives as we go along.
Positron source
As stated earlier, the positrons in the Keck beam will come from the decay of the
radioactive isotope 22Na. As I also mentioned before, this is by far the most common way
to do research with positrons, or antimatter in general for that matter. However, there is
now only one commercial enterprise from which this isotope may be obtained, and this is
by no means a situation that can only get better. Thus, this most common of
methodologies may soon be simply an historical curiosity. This is another reason why the
endeavours of the Keck laboratory are both interesting and potentially important to the
field of positron physics in general.
When 22Na decays the positron branching ratio is 90%, which means that electron capture
is limited compared to positron emission. That is, 90% of decays result in a positron
being emitted. Another point to note is that along with the positron there is a prompt
emission of a 1.28 MeV gamma ray, which is obviously correlated to the positron
emission. This can either be good or bad, depending on the type of experiment you want
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to do. For some timing experiments, where you want to know how long a given positron
has been around for, this gamma ray can be seen as a timing “start” signal. When that
same positron eventually annihilates, a 511 keV gamma ray can be the “stop” signal, and
the positron lifetime may be so measured. Alternatively, if you are uninterested in timing
signals, or use a different type of start signal (e.g. secondary electron emission, or a
signal from a thin film scintillator), this additional gamma radiation can simply result in
more background radiation, which is a bad thing. (NB: the decay of 13N is, in this regard,
superior because it does so without the emission of any other radiation (neutrinos don’t
count, at least in our detectors) and has a 100% branching ratio for positron emission.
(13N: all positrons, all the time!)
Regardless of which isotope emits positrons, the beta spectrum is really quite useless in
terms of beam formation. It is therefore necessary to take the broad energy spread of
these beta particles (whose end point energies typically extend up to the MeV range) and
compress it into a much smaller range. An energy spread of a few eV is required to make
a useful beam. This process is known as moderation, and the materials used to render this
effect are called moderators.
Karl Canter and colleagues (with credit definitely due to a number of others) as a
postdoctoral fellow at University College London (my alma mater, as it happens)
succeeded in producing what may be described as the first real positron beam, with a
moderator efficiency of almost 3 x 10-5. (Their beam had about 1.5 e+s-1!). This
arrangement used MgO powder coated on a venetian blind arrangement of gold foils.
Later advances, noticeably by Mills and co-workers were then made possible, and these
led to a rapid advancement in the understanding of positron-surface interactions and
related phenomena. This then led to further increases in moderator efficiencies, bringing
us to the modern day levels of around 1% in the best case scenario (obtained from solid
Neon moderators).
Our moderator is to be made from solid Krypton, grown on the second stage of a cryostat
that cools to 20K. A schematic of this arrangement is shown in figure (1a). The conical
arrangement is known to increase the efficiency by a factor of 2 or more (compared to a
flat geometry). The conical section is electrically, but not thermally, isolated from the rest
of the system by a sapphire disc. This allows a bias to be applied to the moderator to
promote extraction of the slow beam.
The krypton gas is introduced through a dedicated gas line that can be independently
baked and pumped out in order to maintain the integrity of the high purity Krypton gas.
The gas enters the system through a small aperture in the radiation shield and freezes on
the entire source mount (figure 1). Positrons are emitted from the sodium source with a
typical beta spectrum with an end point energy of ~ 0.5 MeV, and enter the frozen Kr on
the source window and the inside of the cone. Here they lose energy by a variety of
methods, chiefly electron-hole pair excitation and phonon scattering.
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Cone
Na source
source holder
Ceramic spacers
Sapphire disc
Screws to displacer
Figure (1a), A cross sectional representation of the source holder and substrate for the solid Krypton
moderator. Positrons are emitted from the small Ti window on the source (at the apex of the cone) and
collide with the walls, which contain a layer of frozen gas. The Positrons are then slowed down and emitted
with a very small fraction of their original energy and are guided out in a beam by an electric field (and
also an axial magnetic field).
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Figure (1b). The fast positrons from the source (filled arrowheads) hit the frozen Krypton gas on the walls
of the cone as well as the surface of the source (not shown here) and lose energy. Some of them are then
slowed down to form the beam (clear arrowheads). They are constrained to move along the axis of the cone
by an external magnetic field.
After the positrons slow down to near thermal energies in the Krypton they have a chance
of leaving the moderator. The basis of operation of rare gas moderators is fundamentally
different from the more common Tungsten (and other metals, such as Ni) moderators.
Those work by virtue of their negative work functions, so that after positrons lose energy
and diffuse, some may reach close enough to the surface to be “kicked out” by the
negative work function. Rare gas solids do not have a negative positron work function,
but the band structure is such that at low energies the rate of energy loss is very small,
and so although they are slowed to eV energies rapidly, they retain enough energy to
overcome the potential barrier at the surface and be emitted as “hot” positrons. This leads
to a high efficiency, but also to a relatively high energy spread (a few eV). Nonetheless,
since slow positrons are so scarce it is generally beneficial to optimise beam intensities,
even if it does cost in phase space. (Or, to put it another way, beggars can’t be choosers).
The vacuum system
What is a vacuum, house-cleaning appliances notwithstanding, and why does nature
abhor it so? (This question actually has some serious philosophical implications. I may as
well take this opportunity to point out the fact that I consider myself to be a pragmatist
(philosophically speaking). That is to say, my description of a vacuum is simply a
bounded region of space that has a lot less stuff in it than usual. Just how much less is as
important as you need it to be.
The way we describe such things is, as always, with some numbers. We use the units of
Torr (after the Italian scientist Torricelli). The normal pressure of the everyday world, our
atmosphere is, unsurprisingly, known as 1 atmosphere. This is about 760 Torr (more or
less; it depends on local weather conditions). The vacuum level required in this work is
about 10-10 Torr. This is actually a little lower than is usual in positron beams, but the
sensitivity of the Krypton moderator to ambient vacuum conditions means that I have to
keep things a little bit cleaner (i.e. I need to maintain a lower vacuum level).
In order to clarify exactly what these numbers mean we can calculate how many atoms or
molecules in a square centimetre these numbers correspond to. The ideal gas equation
comes in useful here, but I should point out that, like most of the laws of physics, it is
only an approximation. A real gas will only exhibit the properties of an ideal gas as the
pressure tends to zero, but since we are talking about very low pressures, this is ok. The
equation of state then just relates the pressure (P), volume (V), and temperature (T) thus:
PV  RT
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and R is the Molar gas constant (8.315 JK-1Mol-1). Using this we can draw a graph. I’ll
call it figure (2).
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Particle density (molecules/cm )
Particle density versus pressure at room temperature
10
19
10
17
10
15
10
13
10
11
10
9
10
7
10
5
10
-12
-10
-8
-6
-4
-2
0
1x10 1x10 1x10 1x10 1x10 1x10 1x10
2
Pressure (torr)
Figure (2) shows the molecular density (at room temperature) for various pressures in Torr.
At 1 x 10-10 Torr we see something like a million molecules in a square centimetre. This
is an extremely small number of molecules! (For example, in normal air there are around
1 x 1019 molecules per square centimetre. This is a difference of 13 order of magnitude.
The age the entire universe is (we think) only 12 orders of magnitude longer than one
day!
So, how are these low levels of vacuum achieved, and what are the factors that determine
them?
Pumping is achieved initially with by a dry scroll pump, which is a special sort of rotary
pump that doesn’t need any oil. These pumps are known as roughing pumps since they
“rough” the system out from atmosphere to a relatively high pressure of ~10-3 Torr. At
this pressure one may safely operate a turbo-molecular pump (usually just called turbo
pumps), and this will then bring the system down to ~ 10-8 Torr.
Oil free pumps are used in this system to eliminate the possibility of back-streaming oil
into the system, which would have dire consequences on our ability to grow a moderator.
Turbo pumps compress gas by momentum transfer from a set of extremely fast spinning
blades (around 50,000 rpm). The molecules are kicked out of the back of the pump, and a
backing (roughing) pump then removes the compressed gas, which is at a higher pressure
in the backing line.
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Turbo pumps are limited however, and at around 1 x 10-8 Torr or so pumping is
performed using ion pumps and cryo pumps, and the turbo/roughing pumps are isolated
from the rest of the system. These pumps typically bring the system to ~10-9 Torr or less,
depending on the vacuum history.
An ion pump is what is known as an entrapment pump. That is, while a turbo pump
removes gas molecules from the system, an entrapment pump merely contains the gas
within itself. In an ion pump a magnetically confined DC electron discharge of is used to
ionise gas molecules. These ions are then accelerated towards a Cathode where they
collide and release secondary electrons, which may then cause further ionisation. Ions are
more readily pumped since they are more reactive with surfaces and may also be
electrostatically accelerated with sufficient force to be embedded into the walls of the
pump. Different gases are pumped in different ways, and with different efficiencies, in an
ion pump but for the most part they are ionised and then trapped on a surface in the
pump.
Cryogenic pumps (cryo pumps) are also entrapment pumps. They work by cooling a large
surface area so that any gas molecules that come into contact with the cold surface freeze
out and become trapped. They eventually escape by thermal desorption, but since they
are cooled to quite low temperatures this takes a long time, and hence the system catches
more gas molecules than it emits, which is a pretty good definition of an entrapment
pump. Any gas can be pumped in this way is the surface is cold enough, although in
practice closed cycle helium pumps are usually used, and these do not pump hydrogen
very efficiently. Cryo pumps are extremely fast and reliable. I love cryo pumps.
When the system is initially pumped out with the turbo pump the dominant gas is usually
water, followed by hydrogen. The water comes largely from the desorption of water that
has physisorbed to the internal surfaces of the vacuum chamber after it has been exposed
to air (air is very wet stuff). By heating the entire chamber the desorption rate can be
increased so that the instantaneous pressure rises, and the water is pumped out of the
system (usually with a turbo pump). Thus, after this procedure the amount of water
remaining in the system is greatly reduced, and the dominant gas is then hydrogen, which
diffuses out of the steel walls themselves. Hydrogen is difficult (but not impossible) to
pump with ion or cryo pumps, and so we also have a Titanium sublimation pump on the
system, which can pump hydrogen. This works by resistively heating a titanium rod that
than coats the inside of a sleeve with a thin film of titanium. Since this is highly reactive
any gas that comes into contact with this film has a good chance of sticking, and thus
pumping occurs. (This sort of pump is known as a getter pump). Using all of these pumps
in combination we can reach pressures of ~10-10 Torr or, if great care is taken, perhaps
10-11.
The transport Mechanism
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The transport mechanism used in this system is magnetic. The slow positrons that leave
the moderator are given a little help by setting the whole moderator at some positive
voltage (around 100 volts in this case). This then accelerates the positively charged
positrons and sends them on their way down the beamline. The thing is, they do not all
come of in a perfectly straight line and so small angular deviations would quickly send
the beam into the walls of the vacuum system where they would be lost forever. To
prevent this a magnetic field is generated by the coils seen in figure (3) and the photos
(figure 4). This field runs along the axis of the beam line and the charged particles do not
like to cross the field lines, so they simply follow them.
In some systems electrostatic lenses are used to constrain the positron trajectories. These
somewhat more complicated to operate than magnetic systems and are usually only used
in applications where magnetic confinement is not appropriate. For example, for high
brightness applications, where re-moderation is usually required, or if double-differential
cross sections are to be measured, an electrostatic system would be the best choice.
However, for many applications one just wants to bombard a target with positrons and
look at the annihilation radiation, and in this case a magnetically guided system would be
more appropriate. These happen to be much easier to set up and operate and, coincidentally, are the most common type in use.
There is one problem with simply having an axial magnetic field to guide the particles,
however, and that is that not all of the positrons stop in the source. A significant fraction
are emitted as fast positrons and these may also travel along the beamline, although they
are not as strongly affected by the magnetic field since they spend less time in it. In order
to facilitate the removal of these particles, as well as gamma rays, from the beam a set of
deflection plates, called n ExB velocity filter is used.
These utilise two crossed fields, electric and magnetic (which is where the name comes
from, since it is common to use E to refer to an electric field, and B to refer to a magnetic
field). The exact details of how these work may be found here [hyperlink to Trochoidal
monochromater]. Basically, the slow particles (that is, the beam) are deflected by a
certain amount, and this allows them to pass through a small aperture that is offset from
the original beam axis by that same amount (in this system it is 1 inch). The fast particles
and gamma rays are not deflected by the same amount, so they never make it to the small
hole (except for a few which make it by chance) and are therefore blocked. In this way
the majority of what comes through the aperture is the required “clean” beam.
Now, the axial confinement caused by the magnetic field, and the axial deflection caused
by the ExB filter is sufficient to deliver a beam to the target region, but it is usually the
case that one also requires a more dynamic control over the actual beam energy. That is,
the 100 eV or so that is used to extract the beam from the moderator region is not
sufficient to implant positrons into the bulk of a sample, for example. It would be very
impractical indeed to have a set up whereby the extraction potential could be varied over
the range that many experiments require, which is anything up to 75 kV in this system.
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For this reason, the entire source region of the beam is designed to float at high voltage
(0-75 kV), with the target side connected through an accelerating tube (really just a big
potential divider). This allows the beam energy to be varied while the target chamber and
associated data acquisition systems remain safely at ground potential.
Leak valve for Kr gas inlet
Accelerating section
Residual gas analyser
Target
chamber
Retractable channeltron
and Fartaday cup
manipulator
Steering
coils
Magnet
Cryostat
Gate valve
Magnet stand
Electrical isolators
Cryo pump
Combination
Ion pump &
Ti sublimation
Ion pump
Figure (3). Everything to the right of the accelerating section (mounted on the electrical isolators) floats at
high voltage. The turbo-molecular pumps and associated backing lines are not shown, but can be attached
at various ports via an all-metal seal valve. Also not shown for clarity are the additional magnets and a
large isolation cage that surrounds the high voltage section.
Some systems, however, must necessarily be connected to the source end of the beam,
and thus float at high voltage. These include vacuum pumps, power supplies and the
cryostat required for the moderator. It is desirable to minimize the amount of equipment
that floats since arcing to ground can cause damage, and for this reason the cryostat
compressor is kept outside of the high voltage region and is connected to the expander
with especially designed isolating fittings. Power is supplied to the isolated section via an
isolating transformer (whose power limitations provide another reason to minimize the
floating load).
Detecting the positrons
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One of the aspects of positrons that makes them so useful is that fact that they annihilate
with electrons, producing gamma rays that are relatively easy to detect. The detection of
these 511 keV photons is by far the most common way to make measurements with
positrons, and usually this is achieved by using either a sodium iodide (NaI) detector or a
high purity germanium (HPGe) detector. In the 3-photon we are planning we may use
either of these types of detector. The choice used depends, as always, on the type of
experiment you want to do. HPGe detectors are quite expensive and have to be operated
at liquid nitrogen temperatures. However, they have excellent energy resolution (around
0.1%) so obviously if you need to be able to distinguish between photons whose energies
are similar, this type of detector would be the best choice. NaI detectors, on the other
hand, are relatively cheap and work at room temperature, and one can buy much larger
than are not available for HPGe. The drawback is that NaI detectors have much less
energy resolution (about 10% or so) and are not very useful for many studies (e.g.
Doppler broadening studies).
These two types of detectors both work in fundamentally different ways. NaI is what is
known as a scintillator, so that photons incident on the crystal excite molecular transitions
that result in the emission of light. This light is then amplified by a photomultiplier and
converted into an electrical pulse. In the very first scintillation detectors (around 1900 or
so) you had to turn the lights off and look for small flashes of light in a microscope to
count particles. Now we use electronic devices, which means that we need to convert the
flash of light associated with a particle into an electronic pulse, for which we use
photomultipliers. The light pulse is directed to the photocathode, which causes electron
emission via the photoelectric effect. Then this small current is focussed onto a dynode
chain, which is really just an electron multiplier, so that when a voltage is applied a
electron at the beginning of the chain will set in motion a cascade, and a measurable
pulse will emerge from the back of the dynodes. This pulse is collected at the anode and
forms the detector output.
HPGe detectors, on the other hand are solid-state detectors. The crystalline structure of
the semiconductor Germanium means that, as ionising radiation passes through, electron
hole pairs are created. However, the amount of energy required to make an electron hole
pair is quite small, so that a photon will typically create many electron hole pairs. Since
this translates directly into a pulse that is measures one can see that the energy resolution
will be correspondingly high. Unfortunately the small band gap energy (~0.77 eV) also
means that one must cool the crystal to avoid being swamped by thermal noise.
Germanium also has the advantage of being a high Z material, and therefore ahs a fairly
large stopping power, adding to the efficiency of HPGe detectors.
There are, of course, other types of detector around, but NaI and HPGe are routinely used
in positron physics for many applications.
.
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