X-ray or gamma ray systems or devices – Beam control – Shutter or chopper
Reexamination Certificate
2000-03-29
2002-07-09
Porta, David P. (Department: 2832)
X-ray or gamma ray systems or devices
Beam control
Shutter or chopper
C378S145000
Reexamination Certificate
active
06418194
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of particle and radiation beam choppers and more specifically, the present invention relates to the field of choppers for x-ray beams.
BACKGROUND OF THE INVENTION
Many experiments in physics, chemistry, biology, materials science, and other disciplines require irradiation of a sample by a short burst of x-ray radiation after which the response of the sample is studied in detail. X-ray irradiation is an important area of study for it allows for the investigation of high energy bonding within the material. A typical experiment may consist of excitation of a system by an x-ray pulse and then measuring the fluorescence emitted during the system's de-excitation phase. In a pump-probe experiment, a system may be prepared by laser excitation and then have a pulsed x-ray beam used to monitor the time evolution of the system. A typical energy for the x-ray radiation is of the order of a few keV. Inasmuch as the time evolution of the phenomena under study may be as short as several microseconds, x-ray pulses on the order of microseconds or less are required.
The primary source of high flux x-ray pulses is a synchrotron storage ring, although use of laser-produced plasma is becoming more frequent. The salient features of synchrotron storage rings are the high flux per unit solid angle, the beam collimation and the x-ray energy tunability. With the advent of third generation storage rings, the temporal structure of the stored charge has been exploited for time resolved studies.
Short duration x-ray bursts typically are produced when a continuous x-ray beam passes through a mechanical shutter that is open for a very short time. One x-ray chopper is disclosed in A. D. LeGrand, et al.
Nuclear Instruments and Methods in Physics Research
A
275
pp 442-446 (1989), so designated as prior art in FIG.
1
. Briefly, an x-ray beam
2
is incident perpendicular to a rotating wheel
3
. On the outer periphery of the wheel is a slot
4
having width d. The x-ray beam is transmitted beyond the plane of the wheel only when aligned with the slot
4
. This system delivers an x-ray burst whose duration is given by
t=d/fC Equation 1
where f designates the rotation frequency of the wheel, d designates the width of the slot and C designates the wheel circumference. The above expression underscores the necessity of having a large wheel circumference C at the same time as a-very high rate of rotation.
The centrifugal force acting on the wheel varies as f
2
C. Thus, strong mechanical stresses are encountered as one increases either the wheel circumference or the rotation frequency. To date, rotation periods less than 1 millisecond are rarely achieved. The typical wheel circumference is less than 0.5 meters.
Another chopper arrangement, depicted in
FIG. 2
, also as prior art, utilizes a cylinder of length L rotating around an axis defined by a shaft
15
. An x-ray beam
20
is incident in a direction parallel to the axis. The cylinder is rotated by a motor
25
. At a specified radius of the cylinder a rectilinear channel
35
has been cut parallel to the shaft. The cylinder rotates about its axis as shown, and clockwise as seen looking down the beam direction. Points A and B indicate the boundaries of the channel where the beam enters. Likewise, points C and D define the boundaries of the channel where the beam exits.
The duration of the transmitted x-ray beam is determined as follows. Consider a point E fixed in space at a radius r and intermediate between A and B and a corresponding point F fixed in space and intermediate between C and D. Points E and F define a line parallel to line AC. A photon entering at E will be transmitted through the beam chopper only if it reaches F before the cylinder's rotation has brought C to the point F. If s is the distance CF, then C reaches F in a time s/fW, where f is the rotation frequency of the cylinder and W is the circumference of the cylinder at the radius r. The photon's travel time is L/c where c is the speed of light. Thus a photon entering at E will exit at F if s≧LfW/c. Only the photons entering the channel between E and B will be able to pass through the rotating cylinder. The distance EB is in effect the effective width d
eff
of the channel, with
d
eff
=d−s=d−LfW/c. Equation 2
The length of the burst in this case is
t
eff
=d
eff
/fW Equation 3
The above instrument for producing a short radiation burst requires a long cylinder, a fast rotation rate and a large cylinder diameter. (In any event, L must be large enough to ensure absorption in the cylinder when the x-ray beam is not being transmitted through the channel and the circumference must be large enough to allow the transmitted x-ray beam to clear the motor
25
.) Such a long cylinder with a large diameter entails a large mass and a large moment of inertia. This entails a large stress on the bearings of the motor. This is especially relevant when dealing with a horizontal beam and consequently a cylinder rotating in the horizontal plane, inasmuch as considerable torque is placed on the motor bearings.
Another disadvantage of the rotating cylinder configuration is that unless the walls of the channel are oriented in the radial direction, as illustrated in
FIG. 2
, rather than parallel, the effective width of the channel increases the deeper the x-ray beam traverses the channel.
In addition to the mechanical problems with current beam choppers described above, there is the stability of the rotation speed of the wheel or cylinder to be considered. Synchrotrons provide bursts or packets of radiation at specific intervals. However, typical choppers often lack the motor speed control necessary to synchronize with the temporal structure of the storage ring so as to take advantage of the full time structure of the storage ring without resorting to expensive electronics.
Thus there is a need in the scientific community for a fast x-ray beam chopper that overcomes the large size of the above designs, is economical to operate, and that can utilize the natural time structure of synchrotron storage rings for time resolved measurements.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a fast x-ray-chopping device that overcomes the disadvantages in the prior art.
Another object of the present invention is to provide a fast x-ray-chopping device that can be synchronized with the temporal structure of a storage ring. A feature of the present invention is that the rotation of the beam chopper can be phased-locked to the synchrotron storage ring orbital frequency. An advantage of the present invention is that the chopper can take advantage of the variable loading pattern of the storage ring (e.g., by admitting only a predetermined packet of energy) to take full advantage of a single x-ray pulse or a pulse train therefrom.
Yet another object of the present invention is to provide a compact fast x-ray-chopping device. A feature of the present invention is that the moving item that chops the beam is a thin rotating disk. An advantage of the present invention is that the rotating assembly has a low mass and a low moment of inertia, thereby allowing for rapid fine-tuning of the rotational speed of the disk. A further advantage of the present invention is that the rotating assembly is integrally molded to the motor rotor and hence coupling to a motor is not necessary.
Yet another object of the present invention is to provide a fast x-ray-chopping device, the performance of which can be monitored accurately. A feature of the invention is that the chopper can be made to also admit and transmit an optical beam in addition to the x-ray beam. An advantage of the present invention is that the performance of the chopper can be ascertained by studying the transmitted optical beam.
A further object of the present invention is to provide an economical fast x-ray-chopping device. A feature of the present invention is that it comprises a commercially available m
McPherson Armon
Mills Dennis M.
Barber Therese
Caress Virginia B.
Dvorscak Mark P.
Porta David P.
Smith Bradley W.
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