A: The ND&M Handmonitor uses a high-gain, proximity focused image
intensifier
for amplifying scintillation light bursts which are not visible for the
human eye. These very faint light burst are amplified to such a high
degree
that they become visible on the back screen of the intensifier even
under
daylight conditions. The whole system is built into a small housing,
which
also contains power supply boards and a battery for convenient,
wireless,
one-handed operation.
A: The Handmonitor is usually used for aligning neutron beams in a
neutron
physics experiment. Another application, where our Handmonitor is used
for quite often, is finding leaks in neutron shieldings. It is very
convenient
to have a Handmonitor for this task, because in many cases you can
locate
the leak and its dimension at the same time by just looking at the
screen
of the Handmonitor, without performing annoying scanning procedures,
remembering
numbers, and so on. In most cases, you save a tremendous amount of time
if you use our Handmonitor compared to using conventional detectors.
A: Because your time is precious. The Handmonitor saves a lot of time -
you don't have to wait until your film is developed, you don't have to
build any fixtures or holders four your film cassette. Instead, you can
quickly scan over much larger areas of your experiment than you would
if
you are only eqipped with films - and you won't be the first one who
finds
unexpected leaks in shieldings, unexpected reflexes from beams or
background
neutrons from your neighbour's experiment. And - do not forget - you
see
your beam in real-time! That means that you can quickly discover
mechanical
instabilities in your setup, or thermal effects, which you would never
see on film!
A: That depends on the wavelength, of course - if you work with thermal
beams, you can expect to see about 10 to 20% of the neutrons that enter
your Handmonitor. At longer wavelenghts - say, about 4 to 5 A, the
efficiency
increases to about 40 to 50%. At even higher wavelengths, the
efficiency
drops again to lower values. The reason for this is that the
scintillator
that is used in Handmonitor is not very tranparent for its own light
(you
may ask, why don't we use another scintillator then? The reason is,
that
our scintillator has a very low gamma sensitivity. Other scintillators
do not even come close to this low gamma sensitivity.). So, any neutron
that is captured close to the entry window of the Handmonitor gives
only
a weak light spot. The brightest light spots have their origin in
neutrons
that are captured close to the exit plane of the scintillator. Very
cold
neutrons are immediately absorbed in the scintillator and such give
only
a low amount of light. If you mainly work with very cold or ultracold
neutrons,
we can equip your Handmonitor with a special, very thin scintillator
that
gives rise to very bright scintillation light spots for longer
wavelengths.
A: Beware - keep in mind that any structure that you put in a neutron
beam
scatters neutrons and/or produces secondary gamma radiation! So, before
you try to operate your neutron Handmonitor in any beam, ask your
radiation
protection staff before to give you advice! Tell them to make
measurements
for you of the radiation level you will be exposed to! For very intense
beams, consider using some form of mechanical expansion of your arm -
the
Handmonitor is equipped with a standard 1/4'' camera thread for this!
So,
use some form of a camera tripod, microphone stand or the like to keep
distance from the beam and the Handmonitor! Switch the Handmonitor to
the
lowest gain (gain setting 1) before putting it in the beam! If the gain
is not high enough, try gain setting 2 or 3. If the screen of the
Handmonitor
is very bright even at the lowest gain setting, remove the Handmonitor!
Do not use the Handmonitor in this beam then! Caution: The Handmonitor
can be activated in a strong beam in a very short time to quite a high
degree! So, after using the Handmonitor, put it behind a gamma
shielding
and let decay for about half an hour. Before you remove the Handmonitor
and take it away to a non-radiation- controlled area, measure its
activity
or tell your radiation protection staff to do so. Do not violate the
allowed
the allowed activity levels when carrying the Handmonitor.
A: We can equip your Handmonitor with other scintillators, of course.
In
fact, we have already made a special X-ray Handmonitor for visualizing
6 keV X-rays. For that, we also changed the usual aluminum entrance
window.
We used a carbon entrance window instead, to get as low absorption as
possible.
A: The AFG, together with a ND&M Readout Unit, turns your
Handmonitor
into a full functioning, computer operated, two-dimensional neutron
detector
with outstanding resolution. The RCP (Real-time Centroiding Processor)
mainly does the same thing, but for a completely different range of
applications.
The AFG is either good for getting computer readable pictures of your
neutron
field at HIGH fluxes and LOW resolution (0.1...0.2mm) or for getting
images
of your neutron fields at LOW fluxes with ULTRA-HIGH resolution
(0.04mm).
A: If you want to use the AFG, you have to mount your Handmonitor on a
ND&M Readout Unit (the Handmonitor is powered by the Readout Unit
then).
Inside the Readout Unit sits a CCD camera which looks at the screen of
your Handmonitor. The CCD camera delivers 25 pictures per second at a
resolution
of 512 times 512 pixels to the AFG. The AFG digitizes each picture and
removes weak light bursts in each picture. Each new picture is added to
the formerly digitized picture then. At the end of the exposure time,
you
get a matrix of 512 times 512 brightness values with a resolution of 16
bits. This matrix is then read in by the PC. Our AFG software analyzes
this matrix then and calculates the center of gravity of each found
light
burst, which corresponds to the coordinate of an impinging neutron.
After
this analysis, which usually takes less than a fraction of a second on
a Pentium PC, you can save the coordinates of the neutrons in an output
file (with a choice of several file formats).
A: The RCP does mainly the same thing as the AFG, but much faster. The
AFG has one disadvantage: The light bursts (=neutrons) in the summed up
pictures are not allowed to overlap. The calculation of the centers of
gravity would become impossible, otherwise. This fact limits the
maximum
count rate of the AFG to about 100 neutrons per second on the whole
detector
area. Most neutron experiments that really need the ultra-high
resolution
of the AFG are experiments where you have to deal with low fluxes,
anyway
(perfect crystal interferometry, UCN experiments, fundamental physics
experiments,
to give a few examples). These types of experiments are ideal
applications
for the AFG. If you are lucky, you can use higher fluxes. Then the RCP
is the right thing for you. The RCP also digitizes pictures (50 "half"
pictures per second), but these pictures are not summed up for later
analysis.
Instead, each picture is immediately analyzed for neutron events and
the
coordinates of each found neutron are also immediately made available
to
the PC. For this, a really impressive computation speed is needed
(about
900 million calculations per second). The RCP has special hardware on
board
for this task. But, even this high computational speed does not allow
to
get the same resolution as on the AFG. This is the reason why the RCP
has
a somewhat lower resolution of about 0.1mm in vertical direction and
0.065mm
in horizontal direction.