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DIP2000 user manual |
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DIP2000 stands for Diffraction Image Processor. Several models of the DIP2000 series exist. There are dual plate systems with 20 or 30 cm size plates and a single 30 cm plate system exists. The dual plate systems can have a single axis spindle or a three-axis goniostat with kappa geometry. The single plate system is called DIP1030, the dual 20 cm plate systems are called DIP2020 and the dual 30 cm plate systems are called DIP2030. The scanner is interfaced via a controller to a host computer. An overview of the internals of the dual plate system is shown here.
Overview of the DIP2000 image plate system
The image plate scanner : Principle of operation
Image plates consist of a support coated with a light sensitive Europium doped halide BaFBr:Eu2+. This material is a storage phosphor. As X-rays hit the image plate the Eu2+ is oxidized to Eu3+ and the released electrons are stored in a metastable state as so called "colour centres". This way latent images are stored on the image plate. Scanning of the plate with a red light laser causes release of the stored energy in the form of blue light. The laser light causes the electron in a colour centre to reduce Eu3+ to Eu2+ concurrent with atomic fluorescence. The intensity of the out coming blue light is proportional to the intensity of the X-rays that have hit the plate and can be measured with a photo multiplier.
After readout of the plate the plate material has not fully returned to its ground state. To be able to re-use the plate it is exposed by a bright halogen lamp, which completely erases the plate. Now it can be re-used for a following cycle of exposure, readout and erase.
Since the readout and erase of the image plate does take some time,
the DIP2000 system has two image plates. This way data collection can be
much more efficient then with a single plate system, since readout and
erase of the exposed plate can now be done during the exposure of the second
plate and there is no interrupt time due to readout of the plate. The only
dead time now left is the time needed to interchange the two plates.
Expose, readout & erase cycle image plates
Operation of the DIP2000 image plate
The DIP2020 and DIP2030 image plate scanners are scanners with two image plates. One plate can be exposed while the other plate can be read out and erased. Read out of one plate is done during exposure of the other plate by vibration free spiral read out. This is done by scanning the plate with a laser read out head, while rotating the plate: The plate rotation is done at constant angular speed, minimizing vibration and errors because of ramping up of the rotation speed. The read out head also moves by a rotary movement from the edge to the centre of the plate, thus covering the entire area of the plate, not leaving any blind areas. The resulting pixel size of the images is 80 x 80 for the 20 cm plate and 100 x 100 for the 30 cm plate. The photo stimulated luminescence generated by the laser read out is picked up by a double photo multiplier, which converts the light to an electronic signal. As soon as the first photo multiplier is saturated, the read out is switched over to the second photo multiplier. Thus the dynamic range is increased to more then 1:106.
The different steps of the process are depicted the following figures which have the following convention : The plate on the left side is on the exposure side, the plate on the right side is on the readout/erase side. The thing in the middle is the scintillation counter. The arm that moves over the right plate has both the readout laser as well as the erase lamp and so during the exposure of the other plate it sweeps over the plate twice, once for readout, with the laser on and once for erase with the laser lamp on. An erased plate is shown with grey shading.
The process starts with erasure of IP1. The plate is erased by an orange
coloured halogen light source, which is moved along the plate in the same
way as the read out head while the Image Plate is spinning.. Erasure of
plate 1 is done right in advance of the end of the exposure of plate 2,
to minimize the amount of spurious signals of cosmic rays on the plate
to be exposed and vice versa. After erasure the erase arm is moved
back to its original position and the two plates are interchanged, so the
freshly erased IP1 is at the exposure position.
Step 1: Start of data collection. IP1 is at the erase position and will
be erased.
Step 2: IP1 is erased. Now interchange the plates and move the read
arm back.
Step 3: IP's interchanged. IP1 erased and locked at exposure position. Start exposure of IP1.
Then IP1 is exposed for the total length of the exposure time. Shortly
before the end of the exposure of IP1, IP2 is erased to make it ready for
exposure.
Step 4: Exposing IP1. Start erasure of IP2.
Step 5: IP1 exposed, IP2 erased, now start
interchanging plates.
Step 6: IP1 exposed. IP2 erased, plates interchanged.
After the exposure of plate 1 and erasure of plate 2, the two plates
interchange position by rotating 180 around a central axis. This moves
plate 1 in the read out position at plate 2 in the exposure position. The
plate to be exposed is rotated to locate the position of a notch, after
which a solenoid lock fixes the plate in the same position for exposure.
This can be heard as a high pitch click. When this is done, the X-ray shutter
opens. While plate 2 is being exposed, plate 1 is spun around its axis
and then read out.
Step 7: Exposing IP2. Read out IP1.
After the readout the process waits until just before the end of exposure
of plate 2 and is then erased, again by rotating the plate and sweeping
over the erase light from centre to edge. After the end of the exposure
of plate 2 and the erase of plate 1, the two plates change position again
and the whole process is done over again and again, until the end of the
data collection.
Step 8: Exposing IP2. Move erase arm back.
Step 9: Exposing IP2. Start erasure of IP1.
Step 10: IP2 exposed, IP1 erased. Now interchange Image Plates.
Step 11: IP2 exposed, IP1 erased, plates interchanged.
Step 12: Exposing IP1. Reading IP2.
Step 13: Exposing IP1. Moving read arm back.
Step 14: Exposing IP1. Start erasure of IP2.
Step 15 :IP1 exposed, IP2 erased. ready for the next cycle. Go back to step 5.
The time frame of the process is as follows:
The plate rotation around the central axis allows the plates to be placed "off-centre" during exposure of the plate, thus increasing the resolution limit of the diffraction pattern. The plates can be offset to 20 degrees (or from -30 to +30 for older "Weissenberg-type 12 bit ADC based systems), around the central axis. This has no effect on the read out of the plates. The internal 2-theta movement is a unique feature of the double plate DIP2000 systems and does not require any extra stands or tables.
All commands of the host computer are transferred to the DIP2000 controller by an RS232 line. The sequence of all the commands can be shown on the monitor screen of the computer, which may be useful to see what is happening during data collection (see Appendix 1).
Imagefile transport is done via Ethernet and usually a second dedicated Ethernet card is installed to keep the other network traffic free from congestion by image file transport. The conversion from spiral to Cartesian coordinate system is done automatically and data are stored as Cartesian files on the disk of the computer system.
Two versions of the goniometer exist: a single axis phi-spindle and a three-axis kappa goniostat. Both of course carry the goniometer head, which keeps the crystal in the position of the intersection of the spindle axis (phi for single axis systems, omega, kappa and phi for three-axis systems) and the X-ray beam. The kappa goniometer is a combination of three parts, bearing the three rotation axes. All axes intersect in the centre of the diffractometer. The goniometer head is mounted on the phi axis, which is supported by the kappa block. The kappa block can be rotated about the kappa axis, which is carried by the omega block. The omega block in turn, can be rotated about the omega axis which is carried by the base plate of the diffractometer. The angle included by the omega axis and the kappa axis, , is about 55. The angle between the kappa axis and the phi axis is also about 55 and the goniometer can therefore access all directions in Chi within 100 of the zero position.
The single axis spindle is called the phi axis. With this type of goniometer, the crystal can rotate around one axis only. It has ample freedom to move the crystal along the spindle, so there are hardly any limitations in height of mounting of the crystal.
The distance between the centre of the goniometer (the crystal position) and the base of the kappa goniostat is 49 mm (ACA standard). Since some goniometer heads may already have a considerable height, the amount of freedom in height of mounting the crystal on the kappa goniostat is limited.
Internal scintillation counter
The DIP2000 system is equipped with an internal scintillation counter, which can be used to align the scanner with respect to the primary beam. The counter is mounted on the same arm as the two plates, so the arm can be rotated such that the counter coincides with the position of the primary beam.
The number of counts falling on the scintillation counter can be read out with a standard rate metre, which is situated close to or on top of the scanner. The range of the metre can be changed with a knob on the controller which varies form 1-100 kcps.
The count rate can also be displayed as a graph on the monitor of the
host computer (see chapter 4 for an explanation of the use of the software
to use the scintillation counter). Simultaneous use of the computer display
and rate metre will be of great help for an optimal alignment of the system.
The count rate can be checked while changing the height of the scanner
or while changing the setting angles of the monochromator or double focussing
mirrors. Since it is very easy to put the scintillation counter in place,
this can serve as a check prior to data collection to ensure the maximum
flux at the crystal position.