Well, you can almost see it in there behind the tinted shielding glass.
The Furnas camera is currently set up for recording 2-dimensional X-ray scattering patterns on film. The point of this camera is to obtain 2-dimensional data on film, 2-D data is only of interest if the sample is oriented.
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.
