Patent Number: 058728244
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As noted above, a californium source produces heavy ions in a not-so-predictable manner. Thus, a feature of the invention is the generation of heavy molecular ions in a predictable or controllable way. According to the invention, a uranium 238 (.sup.238 U) foil is bombarded with neutrons from a pulsed neutron generator. The neutrons contact the foil and because of the high (n, f) reaction cross-section (neutron to fission fragment cross-section) of .sup.238 U, heavy ions come off. The pulsed neutron generator has an adjustable repetition rate, or pulse rate, such that timing information that correlates the masses of heavy molecular ions can be extracted. With this timing information, the heavy molecular ion generation can be characterized and used more effectively in studying a substance. The invention as described herein takes advantage of the n,f reaction in the 238 isotope of uranium. In doing so, the reliance on the random break-up of californium is removed. In the system according to the invention, fission fragment generation is directly related to the neutron burst. That is, the time mark for the secondary desorption event, which corresponds to the desorption of ions from the sample, can be related to the timing of the neutron pulse that caused the primary emission event. A burst of neutrons from a pulsed neutron generator is allowed to impinge on a .sup.238 U foil, causing multiple fission events. Of these, a considerable number of fission fragments called primary ions will propagate toward and fall incident on the thin-film sample target, thereby inducing desorption of the sample target. These multiple fission events occur at a rate of several orders of magnitude (i.e., hundreds or thousands), and cause desorption of the heavy ions from the sample target. This results in multiple, time-correlated events instead of the single time-correlated event of a spontaneous fission using the californium source. In some cases, it will be possible to reduce acquisition times from hours to minutes or even seconds by using the .sup.238 U source. Also, it is possible to optimize TOF windows, as well as the number of pulse cycles, by varying pulse parameters appropriately. According to one embodiment of the invention, a mass spectrometer source contains a pulsed neutron generator with associated control electronics to provided an appropriate neutron flux in the direction of a fissionable material with a high-enough n,f reaction cross-section. The preferred fissionable material is .sup.238 U. Resultant fission fragments, hereafter referred to as primary ions, are then used to desorb heavy molecular ions from a target. The desorbed ions are hereinafter referred to as secondary ions. These secondary ions are then analyzed by a TOF mass discriminator. The timing sequence allowing for TOF mass discrimination, and any decay particle suppression, are associated with the timing sequence of the neutron pulses, resulting in a significantly improved degree of correlation between primary ion generation, secondary ion generation, and detection. A block diagram of a preferred embodiment of the invention is given in FIG. 3. Neutron pulses are sent to a .sup.238 U source 110-3 by a neutron generator 10-3. The number and rate of the neutron pulses 120-3 emitted from the neutron source 10-3 are controlled by pulse out signals 180-3 sent by the control electronics 20-3. The control electronics 20-3 are controlled by control signals 130-3 sent by a computer/analyzer 40-3. By way of example but not by way of limitation, in the preferred embodiment, the computer/analyzer 40-3 can be an IBM-compatible personal computer. As a result of the bombardment of the .sup.238 U source 110-3 by the neutron pulses 120-3, heavy mass ions (also called fission fragments) 140-3 are emitted from the .sup.238 U source 110-3 at a controllable and predictable rate. There is no need to use a complement particle as a time mark (as is done with californium as the radioactive source), since a time zero mark signal 150-3 is sent from the control electronics 20-3 to a Time-to-Digital Converter (TDC) 100-3 in coincidence with each burst of neutron pulses 120-3 emitted from the neutron generator 10-3. The fission fragments 140-3 emitted from the .sup.238 U source 110-3 due to the neutron pulses 120-3 hit the sample stage 60-3. These fission fragments 140-3 then pass through the sample stage 60-3, and impinge on the thin film sample 50-3 attached on one side of the sample stage 60-3. The sample stage 60-3 is positioned close to the .sup.238 U source 110-3, such that a substantial majority of the fission fragments 140-3 emitted from the .sup.238 U source 110-3 will impinge on the sample 50-3. With the closeness in location between the .sup.238 U source and the sample stage 60-3, the solid angle of the dispersal of the fission fragments 140-3 being ionized from the .sup.238 U source will not present a problem with respect to a certain percentage of the fission fragments 140-3 missing the sample stage 60-3. As a result of their masses and high velocities, some of the fission fragments emitted from the my .sup.238 U source are able to deposit large amounts of energy as they impinge on the thin film sample 50-3, allowing for the desorption of high molecular weight species from the sample 50-3. Molecules 160-3 are desorbed off the thin film sample 50-3 and accelerate toward a grid 90-3 held at a fixed potential, typically a ground potential. The grid 90-3 may be made of any of several standard types of grid material, such as a screen material. The desorbed molecules 160-3 then enter a drift region 70-3 with velocities inversely proportional to the square root of their masses. The amount of time needed to travel through the drift region 70-3 determines the mass of each particle desorbed from the thin film sample 50-3. When the desorbed molecules 160-3 exit from the sample 50-3, each of the desorbed molecules 160-3 have a relatively small amount of energy associated with them, typically around 100 electron volts (eV) of kinetic energy. These low energy particles will typically be desorbed off the sample 50-3 in various directions. However, as these desorbed molecules 160-3 get pulled into the drift region 70-3 by the attraction to the high potential at the grid 90-3, the desorbed molecules 160-3 will be pulled in line, so that the desorbed molecules 160-3 will travel through the drift region 70-3 in essentially parallel paths with respect to each other. The high potential at the grid 90-3 causes the desorbed molecules 160-3 to be accelerated towards the grid 90-3, where the desorbed molecules 160-3 then pass through the grid 90-3, and enter a field-free region, also known as the drift region 70-3. In the drift region 70-3, there are no forces acting upon the desorbed molecules 160-3, and so the heavier ones of the desorbed molecules 160-3 lag behind the lighter ones of the desorbed molecules 160-3, due to the fact that each of the desorbed molecules 160-3 enters the drift region 70-3 with the same kinetic energy, and since kinetic energy=1/2*mass*velocity.sup.2, the heavier mass ions will have slower velocities than the lighter mass ions as these ions pass through the drift region 70-3. These particles are then detected by a detector 80-3 at the end of the drift region 70-3, and the instant in time when each particle arrives at the detector 80-3 is recorded as a multi-stop signal 170-3. By way of example but not by way of limitation, in the preferred embodiment, the detector 80-3 can be a Dual Microchannel Plate, #C-701, manufactured by R. M. Jordan Company. Other types of detectors 80-3 may be used in the invention by one of ordinary skill in the art in keeping within the scope of the invention. The information concerning the particles passes through the Multi-stop Time-to-Digital Converter (TDC) 100-3, and arrives at the computer/analyzer 40-3. Also, by way of example but not by way of limitation, in the preferred embodiment, the TDC 100-3 can be a TOF2 manufactured by Schmidt Industries, a division of SI Diamond Technology. Note that other similar devices may be substituted for the TDC 100-3 as used in the preferred embodiment and still keep within the scope of the invention. The time it takes the particles to travel through the drift region 70-3 and impinge on detector 80-3 is compared against a time zero mark as determined by the time zero mark signal 150-3 received from the control electronics 20-3, and the time difference determines the mass of each of the desorbed particles. This time difference corresponds to the instant in time of a particular neutron pulse being emitted from the neutron generator 10-3 subtracted from the instant in time of an ion being detected at the detector 80-3, wherein the ion detected at the detector 80-3 was desorbed off the sample 50-3 due to the particular neutron pulse. The time determination and comparison can be performed in the computer/analyzer 40-3, or any type of processor as otherwise convenient. As mentioned above, each time zero mark as determined by the time zero mark signals 150-3 are correlated to a corresponding one of the bursts of neutron pulses 120-3. The above-mentioned structure performs the TOF mass discrimination. As described earlier, the californium source provides a significant primary ion yield, but is not actively controllable. The present invention uses a pulsed neutron generator to provide an adequate neutron flux to a suitable fissionable material. .sup.238 U is one such suitable fissionable material, since it has a relatively high n,f reaction cross section, which is approximately 1.2 barns for 14 MeV neutrons. Based on this, one can obtain reasonably high fission fragment yields. These fission fragment yields are strongly dependent on the neutron flux applied to the .sup.238 U by a neutron generator, which can be highly controlled by using current neutron tube technology. One such neutron tube that can be used for implementing the present invention is a pulsed neutron tube developed by Martin Marietta Specialty Components, Inc., which can deliver neutron fluxes in a pulsed mode. In one embodiment, the neutron pulses would each cause about 10000 fission fragments due to a burst of approximately 5-100 nanoseconds. The burst repetition rate would be on the order of 2000 bursts/second. The burst repetition rate can be controlled by appropriate control signals 130-3 sent by the computer/analyzer 40-3 to the control electronics 20-3. Based on the control signals 130-3 received, the control electronics 20-3 sends pulse out signals 160-3 to the neutron generator 10-3 at instants in time corresponding to the desired neutron pulse repetition rate. Using an approach according to the invention, what previously took hours to perform mass spectrometry could be done in a manner of minutes or even seconds. The ion source would then be coupled to a TOF mass discriminator, allowing mass analyzing capability up to 100,000 amu, which is well beyond the range of current conventional mass spectrometers. The time zero mark for the TOF analysis can be derived from the electronics used to drive the neutron generator. Another advantage of a system according to the invention is that all of the fission fragments generated would be synchronized. As a result, the background noise due to fission fragments that produce ions while the ions from the previous fission fragments are being analyzed can be eliminated, since these fission fragments associated with the background noise do not have a time mark associated with them. While preferred embodiments of the invention have been described, modifications of the described embodiments may become apparent to those of ordinary skill in the art, following the teachings of the invention, without departing from the scope of the invention as set forth in the appended claims.