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I built an X-ray backscatter imaging
system from parts found on eBay. This system works by scanning a very thin beam
of X-rays across the target, and measuring the amount of backscatter for a
given beam position. The beam is scanned mechanically by a
rotating chopper
(collimator) wheel, and by tilting the rotating wheel on an orthogonal axis.
The output image is generated on an oscilloscope by matching the horizontal
scan speed to the rotating wheel, and using a potentiometer to measure the
vertical axis position. The scope's brightness (z axis) is controlled by the
amount of backscatter signal received by a large-area detector. Thus, the image
is constructed bit by bit. I used a long-exposure shutter on my camera to see
the image formed by the moving oscilloscope trace.
turn to your left hold your hands above
your head hold still
for 10 seconds
you
just as I suspected
trying to sneak an
allen key through
hey everyone I've
been working on this
project for a while
this is a x-ray
backscatter imaging
system very popular
in airports these
days and this works on
the principle that
if you shoot x-rays
at something some of
the x-rays will
penetrate through
the object some will
be absorbed and some
will be back
scattered or
reflected off and if you
catch the back
scattered reflection you
can make an imaging
device like this
just like is used in
airports one of the
problems with
imaging things with x-rays
is that we don't
have the equivalent of
a lens for x-rays so
for a standard
light based camera
we can just use a
piece of glass to
make a lens and that
will focus the light
rays down on to the
film or the sensor
but with x-rays it
doesn't work that
way we don't have a
material that can
bend x-rays like we do
for light rays so an
alternative way to
build an imaging
device is to scan a
very thin beam of
x-rays across the
object that we want
to image and then
use a very large
detector to catch the
backscatter which is
how this system is
built this wheel
here has four very
small slots cut in
it the slots are
about forty thousand
seven inch width
and height and the
x-rays are generated
here in a tube and
the x-rays go through
that very thin
channel which causes them
to come out like a
beam like this so the
x-rays come out of
this disc in a beam
here and as the disc
spins the x-ray
beam moves in an arc
like this across
the field of view of
the imaging device
to catch the back
scatter I have a very
large detector here
this is built with a
phosphor screen
inside used in x-ray
film cassettes and
then I have a
photomultiplier tube
here that detects
the light from the
phosphor screen so
what happens is the
x-rays come out of
the of the tube hit
the object back
scatter hit the
screen here which turns
into a very dim
light source and the
light source is
conduct collected by
this photomultiplier
tube which is
really just a very
sensitive photo
detector device from
here the electrical
signal is channeled
into a very simple
op-amp circuit and
then into my
oscilloscope and the
oscilloscope shows
the intensity of the
signal coming back
from this detector
as the intensity on
the screen of the
oscilloscope the beam
is scanned across
the oscilloscope
screen at a rate
that matches the
rotational speed of
my chopper disk
which is about 330
RPM for this example
here and I also
measure the Y position
of the x-ray emitter
device with a
simple potentiometer
and that controls
the vertical
movement in order to make
an image I just use
a very long shutter
on my camera just
aimed at the
oscilloscope screen
and for now all I've
been doing is moving
the x-ray head by
hands just to scan
the image very
quickly obviously a
better way to do
this would be to
have a worm gear or
something drive the
the x-ray head up
and down and a very
smooth controlled
rate but I just want
to get this working
quickly so I decided
to just push it by
hand most of the
x-rays are contained
within this device
by this LED shield
here and only a very
very thin beam
makes it through the
shield and then an
even thinner beam
makes it through the
Chopper wheel so the
amount of x-ray
actually coming out
of this device is
very very small you
can see how the
detector works if I
wave a plastic rod
in front of the
device with the wheel
not spinning and the
detector running
you can see the
output on the
oscilloscope change
as the plastic rod
moves through the
beam the beam has a
size of about maybe
a quarter inch about
a foot and a half
away from the x-ray
source i spaced the
slots in the Chopper
wheel 90 degrees
apart because I
eventually want to
do a much wider field
of view currently
this tube is limited
to 25 degrees total
angle and so I'm not
really using the
devices full capability
in fact most of the
time it's not even
emitting x-rays so
it would be a much
more efficient
device to have eight
slots cut in it if I
didn't need the
field of view but I
have some other
tubes that I want to
put in there that
have a wider field
so I wanted to leave
that option open
currently the entire
scan across the
field of view takes
about 12 to 15
milliseconds the position
of the disc is
sensed with a simple opto
of sensor and this
just triggers a
delayed scan on the
scope so that the
left and right
margin of the image lines
up with the left and
right edge of the
screen this system
is very comparable to
airport
scanners in terms of
beam size radiation
dose scan speed time
and all of that and
I got a lot of my
information from a
patent which I'll
link to in the
description and that
had quite a lot of
useful info just
regarding like pixel
size and count and
line count and all
that kind of stuff
as usual I ended up
getting most of
these parts on eBay the
photomultiplier the
x-ray tube the power
supplies for the
x-ray all the machining
for this project was
done here and I
spent a good portion
of this week doing
the machining which
was quite extensive
doing precision
rotary motions is not
easy and I ended up
positioning this
this rotating wheel
kind of like the
wheel on a car where
it has a taper
bearing and a thrust
bearing I guess
most cars would have
to taper bearings
squeezed together in
this case I didn't
really need quite
that much radial load
support so I just
did one taper bearing
and a thrust bearing
I might do another
video about this in
the future so let me
know if you have any
questions and I'll
try to address all
of them in the next
video there's kind
of a lot to talk
about here and
hopefully I'll have a
better performing
system after a few
additions okay see
you next time bye
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