Well, you can almost see it in there behind the tinted shielding glass.
Well, you can almost see it in there behind the tinted shielding glass.
The Real Manual; it is worth taking a look at...
The manual can be found in a drawer below the counter just to
the left as you enter Room # ???? where the Furnas camera is
located.
Film Usage:
The film currently used is Kodak X-omat which may be stored by
L. Parfitt, though the M.R.G. keeps a box of there own in the
chemical fridge in the M.R.G. laboratory. The film needs to be
cut to fit the 12cm x 12cm film cassette for the Furnas camera.
( The other pinhold camera uses smaller squares of film). It
is highly recommended to use two pieces of film in the cassette,
this way the first acts as an attenuator to the intensity of the
X-rays so that while the first may be overexposed, the second
film provides a chance. Often both films will be usefull as the
more exposed may show very weak reflections while the less exposed
will avoid loss of detail caused by overexposure.
Operation:
The Furnas camera is primarily a fiber camera, though
samples of other types can also be used. A thick single
fiber may be positioned in front of the exit pinhole, but often,
especially in the case of thin fibers it is necessary wrap fiber
several times around a thin nothced cardboard doo-hickie to place
many mutually aligned fibers in front of the hole, as diagrammed
in Figure 1.
Figure 1. (a) Cardboard template and the same showing fibers
wound looser than they ought to be top be well aligned to get
many mutually aligned fibers into the beam. This sample holder
can then be taped to the exit pinhole of the collimator.
The data:
To obtain accurate measurements of d spacings or two-theta
values, it is neccessary to calibrate the patterns. Best calibrations
are obtained by running an internal standard. To avoid contaminatino
of samples, a two step method mayu be used where first an X-ray
pattern is recorded, then, without moving the film holder, the
sample is replaced by a standard and the film is further exposed.
In order to avoid overexposure of the film it is recommended
to use a highly crystalline standard so that less time is needed
to record the second pattern.
In the manual, it is shown how a focussing condition exists when
the length of the collimator tube from pinhole to pinhole mathces
the sample to film length. However, it is oftenmore convenient
to avoid changing tube lengths and often the difference in X-ray
focus will not be noticeably changed by making sure that this
focussing condition is maximized. When the collimator length
is changed, it is neccessary to use a Geiger counter to check
the alignment of the camera... often a slight tweaking of adjusting
bolts will cause an increase in X-ray intensity that will result
in a significant drop in exp[osure times required to record good
data. It has been found that the best local geiger counter
may be found in the lab of A.F. Yee ( they use it for their positron
annihilation experiment to monitor sources or something like that).
The geiger counter may be positioned inside the Furnas camera
elevated to accept X-rays that pass through the collimator. Often
it is neccessary to use a sheet or two of tin foil to attenuate
the X-rays to avoid saturation of the detector when alignment
is optimal. Best alignment is the goal as maximal X-ray flux
will significantly reduce film exposure times!!
A "penumbra" or ring shaped shadow can show on the film
as an artifact related to internal reflecetion of the X-ray beam
within the tube of the collimator. To prevent this penumbra effect
it is advisable to place a third aperture near the center of the
collimator tube although such an aperture was not supplied with
the machine. One was made, of lead, and inserted, but this aperture
could be removed by careless users and so it should be checked
for occaisionally by those that care of the quality of the data
obtained. This mid-tube aperture may be significantly larger
than those at the entrance and exit pinholes. It was found that
a decent aperture could be made from a 1 mm thick flat lead sheet,
cut to a perfect disk to fit in the bore blow the threads of the
joint between two of the long collimator pieces as shown in Figure
2. The actual aperture in this lead disk was cut by with a 2.5
mm drill bit turned slowly and carefully by hand (not in a drill).
Figure 2. Penumbra reducing aperture placement and simple fabrication technique.
Safety:
All users of the Furnas camera must first have taken the Radiation Safety Course provided by the University; new users should see Laraba Parfitt to register for the next scheduled session.
True safety comes from understanding how the generator works to generate X-rays and how the shutter system functions to release X-rays through the collimator and into the camera.
It is important that all users understand how to use a Geiger counter to monitor the allignment of the collimator as this task exemplifies the possible sources of exposure and how to avoid exposure.
Exposure times:
As hinted to several times above, the exposure time required to make a strong
pattern on the recording film is strongly dependent on the X-ray flux brought
to the sample. Sample crystallinity and thickness are also paramount. The optimal
thickness of a sample depends on the wavelength of the radiation used and the
atomic weigth/density of the sample. For Cu Ka radition
l=1.542Å and a polymer sample composed primarily of carbon (or
oxygen and nitrogen... as these have also relatively low atomic weights), the
optimal thickness may be determined to be on the order of ~3 millimeters.
However, it is often difficult to place that much sample in the beam without
losing alignment so often a compromise must be made and longer exposure times
can make up the difference.
Email Us: mrg_people@umich.edu
MAIN | RESEARCH | PEOPLE | EQUIPMENT | PUBLICATIONS