Abstract:
A method and apparatus for measuring the beam current of a particle beam in an accelerator by charging the capacitor across an operational amplifier and controlling the scaling of the amplifier output with a programmable gain amplifier (PGA). The out put of the (PGA) is sampled and storing with an analog-to-digital converter to acquire and store at least two digital voltage values. The two digital voltage values are using to obtain a value proportional to beam current. A field programmable gate array is used to implement digital logic to sample and hold output from the analog-to-digital converter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to devices used with a particle beam for measuring beam current with a Faraday cup in general, and in particular to systems which determine the beam currents of two or more different particle beams using the same Faraday cup.  
         [0002]     A beam of accelerated particles is useful in many scientific and industrial applications, from ion implantation for controlling material properties, to investigating fundamental principles of physics, to accurately dating ancient materials or following chemical reactions or fluid flows by monitoring isotope ratios. A mass spectrometer is one widely used application of particle beams. A mass spectrometer can be used to detect the ratio between the isotopes of a particular element. When one of the isotopes is radioactive, it typically has a low abundance in the environment because it must be constantly supplied from a source, such as the atmosphere or a parent radioactive material, or it disappears over time. Thus it is possible to date a material from the time the material was removed from a source of a radioactive isotope e.g., carbon-14 in the atmosphere. Various chemical and biological processes can cause a separation of isotopes, so that measurement of non-radioactive isotope ratios can be used to determine the diet of ancient animals from their remains, as well as the temperature or other conditions under which they lived.  
         [0003]     One of the most widely used isotope determinations is for  14 C. Carbon-14 is constantly generated in the upper atmosphere through the interaction of neutrons produced by cosmic rays with Nitrogen 14.  14 C forms carbon dioxide which taken up by plant life and so is incorporated into all living things. The amount of radioactive  14 C in the atmosphere has remained relatively stable over a period of thousands of years. However, once living matter dies, it no longer exchanges carbon with the atmosphere and so the amount of  14 C gradually decreases in accord with the half-life of  14 C of about 5,730 years. By determining the ratio of  14 C to non-radioactive  12 C and  13 C in an ancient sample, and comparing with the same ratio in a modem carbon sample from living material, it is possible to determine how many years have transpired since the source of the carbon in the ancient sample died.  
         [0004]     Since the late 1970s tandem electrostatic accelerators have been used as extremely sensitive mass spectrometers able to distinguish the atomic isotopic ratios of 17 orders of magnitude or more. For example, in a modem sample of carbon the ratio is 1.35×10 −12  between carbon-12 and carbon-14. Radioactive isotopes with long half-lives are difficult to measure by detection of radioactive decay if the sample size is small and the half-life of the radioactive isotope is large. For radioactive carbon-14, with a half-life of 5,730 years, a sample size of about one gram is generally considered necessary for conventional radioactive carbon dating. A one-gram sample of modem carbon contains approximately 10 −12  grams  14 C or approximately 5×10 10  atoms of  14 C and produces only 14 disintegrations per minute. By using an accelerator mass spectrometer (AMS), as much as 10 percent of the atoms of  14 C present in a sample can be directly detected. The result is that the concentration of  14 C can be measured with a precision of better than one percent in a modem sample, using a sample size of less than one milligram in only a few minutes. The ability to uniquely detect the presence of atomic isotopes finds many uses beyond carbon dating, for example using atomic isotopes as chemical labels.  
         [0005]     Mass spectrometry uses the principal that a charged particle is deflected more or less by a magnetic or static electric field depending on the velocity and mass of the particle. By the proper combination of magnetic and/or electrostatic analyzers it is possible to separate particles by mass and charge and thus to detect the mass and energy of individual particles. The unique detection of a particular atomic isotope, however, requires that all molecular isobars be eliminated. For example, in the case of  14 C molecular isobars of  13 CH and  12 CH 2  are perhaps one million times more prevalent than the  14 C to be measured. To detect  14 C, negatively charged particles of mass  14  are accelerated in the tandem accelerator through a potential of about one-half million volts to several million volts. The negatively charged particles of mass  14  are passed through a stripping column of rarefied gas in the high voltage positively charged electrode. The stripping column causes the particles to lose electrons and in the process breaks up any molecular isobars into their constituent parts. The positively charged ions are accelerated away from the positively charged high voltage electrode to ground and the particles of mass  14  are separated and counted.  
         [0006]     The isotopes of carbon occurring in nature are approximately 99 percent  12 C, 1 percent  13 C, and 10 31 13  percent  14 C. To obtain accurate determinations the amount of  14 C present it is necessary to compare the amount of  14 C detected to the amount of  13 C and  12 C present in the sample because it is the ratio, not the absolute amount of  14 C which is of interest. The amount of each carbon isotope measured while substantial is only about 10 percent of that which was originally contained in the sample. To obtain accurate results it is necessary to precisely measure the relative abundance of all three isotopes to account for all the loss mechanisms, which can effect the different isotopes of Carbon differently. In order to minimize the amount of power consumed by the high voltage electrode and minimize generation of radiation, it is desirable to minimize the beam currents which are accelerated. This is typically accomplished by accelerating the  12 C, and  13 C only for short periods of time between longer periods of time during which the  14 C beam is injected and analyzed.  
         [0007]     To minimize the overall size of the beam transport system on the injection side of the accelerator, a single Faraday cup which receives both the  12 C and  13 C beams can be utilized. It has also been known to use two parallel analog circuits to measure the beam current supplied to the common Faraday cup. Compensation for the differences in beam current between the  13 C and the  12 C is done by reducing the relative length of the  12 C beam pulse with respect to the  13 C beam pulse. Prior art analog integration circuits used several operational amplifiers which added to offset and gain errors, as well as limited gain adjustments. What is needed is a more accurate and flexible circuit for measuring beam currents over a wide range of beam currents.  
       SUMMARY OF THE INVENTION  
       [0008]     The beam current measuring method and device of this invention forms a precision pulse current integrator (PPCI) which includes a Faraday cup connected in current supplying relation to an integration capacitor connected between the inverting input and the output of an operational amplifier. The output of the operational amplifier is zeroed by a switch across the capacitor. A trigger pulse opens the switch across the capacitor and the integration capacitor begins to charge in response to a first beam current supplied to the integration capacitor from the Faraday cup. As the capacitor charges, the operational amplifier outputs a ramp voltage which is sent to a programmable gain amplifier (PGA). The output of the programmable gain amplifier is supplied to an analog-to-digital converter (ADC). The ADC is triggered at a time slightly after the beam current begins to accumulate on the integration capacitor, and the value determined by the ADC is stored in a first data register. After a first selected time the ADC is again triggered by a second trigger, and the value of the ADC corresponding to the second trigger is stored in a second data register. Logic contained in a programmable gate array (PGA) which forms a part of the precision pulse current integrator (PPCI) produces a measurement, which is a voltage corresponding to the definite integral of the beam current over the selected time, by subtracting the value of the first data register from the value of the second data register. The logic supplies the first beam current integration measurement to a first-in first-out (FIFO) memory. Following integration of the first signal, the integration capacitor is again zeroed by closing the switch across the capacitor. The first beam current integration measurement is divided by the time of integration i.e., the first selected time, to obtain beam current.  
         [0009]     A programable current source forms part of the current beam measuring device and is used to build a calibration table for the instrument. The calibration table contains the “effective” capacitance for each current measurement range as well as correction factors for removing small gain and offset errors in the PGA and the rest of the electronics.  
         [0010]     The beam current measuring device has three ways in which the voltage range can be scaled to best take advantage of the most accurate portion of voltage range of the analog-to-digital converter, e.g. the upper half of the range. Two of these, the size of the integration capacitor, and the programmed gain of the PGA, are adjustable during setup. The third way, the voltage range can be scaled by adjusting the length of time during which beam current is integrated. This third way can be adjusted rapidly so the sequential beam pulses can be scaled with respect to each other. Adjusting the length of beam current integration time allows a second beam of substantially lower current to be captured by the Faraday cup and applied to the same integration capacitor and subjected to the same gain in the programmable gain amplifier while making full use of the most accurate portion of voltage range of the analog-to-digital converter. The time selected for integration of the second beam current is selected approximately proportionately greater or less than the first selected time, as the beam current is proportionately greater or less. As described with respect to the first beam current integration measurement, a second beam integration measurement is supplied to a second FIFO memory.  
         [0011]     It is a feature of the present invention to provide a beam current monitoring circuit with increased beam current range adjustment.  
         [0012]     It is another feature of the present invention to provide a method of more accurately measuring beam current associated with an accelerated particle beam.  
         [0013]     It is a further feature of the present invention to provide a beam current monitoring circuit which is better adaptable to an integrated implementation as opposed to a discrete implementation.  
         [0014]     It is yet another feature of the present invention to provide a beam current monitoring circuit which minimizes analog components.  
         [0015]     It is yet a further feature of the present invention to provide a beam current monitoring circuit in which two beam currents can be compared using with the same analog circuit components.  
         [0016]     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic view of a prior art beam current measuring system based on analog components.  
         [0018]      FIG. 2  is a schematic view of the beam current measuring system of this invention.  
         [0019]      FIG. 3  is a schematic view of the control and measured waveforms of the beam measuring system of  FIG. 2 .  
         [0020]      FIG. 4  is a schematic plan view of a tandem electrostatic accelerator used for  14 C dating.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Referring more particularly to  FIGS. 1-4  wherein like numbers refer to similar parts, a prior art beam current measuring device  20  is shown schematically in  FIG. 1 . The prior art device  20  employs an analog circuit design to store voltages and to integrate beam current. A Faraday cup  21  in an accelerator  22 , as shown in  FIG. 4 , is arranged to receive a charged particle beam  26  by control of a bending magnet  24 . The charge on the accelerated ions forming the particle beam  26  produces a current which flows from the Faraday cup to the inverting input of the operational amplifier I 1  which forms part of a current integrator circuit  27  as shown in  FIG. 1 . An array of capacitors C 1 -C 5  is connected in parallel between the operational amplifier inverting input and the operational amplifier output, and the capacitors are arranged to be controlled by switches K 1 -K 4  to select a combination of capacitors for a particular integrator range. A switch SW 1  is also arranged in parallel with the capacitors, and the switch SW 1  is closed to zero the current integrator by shorting out the capacitors and causing whatever charge is injected into the circuit  27  to be drained to virtual ground formed by the inverting input of I 1 .  
         [0022]     At the beginning of an integration cycle, switch SW 1  is opened. If there is current present from a particle beam entering the Faraday cup  21 , then the integration capacitance e.g., C 5  begins to charge. As charge builds up on the integration capacitance, it causes the output voltage of the operational amplifier I 1  to increase forming a voltage ramp  29  shown in  FIG. 3 . A definite integral circuit  28  samples the ramp  29  at two different points and subtracts the two values from each other to produce a delta voltage. The combination of SW 2  and C 6  form a first sample and hold circuit  38 . A first control pulse causes SW 2  to close momentarily, recording the voltage at the point on the ramp where the pulse occurred. The ramp voltage continues to increase and the output of I 2  now provides the voltage difference between the present value of the ramp  29  and the value sampled by C 6 . A second control pulse is sent to a second sample hold circuit  40 , formed by the combination of SW 3  and C 7 . The value held by C 7  is the definite integral, i.e. the voltage change which took place during the period of time between the first and second control pulses. A gain circuit  30  follows the second sample and hold circuit  40  to increase the gain of the definite integral circuit output. The gain circuit  30  uses a third op amp I 3  which is used to provide additional x 1  or x 2  gain which is selected by switches K 6   a  and K 6   b . The combination of ranges of capacitors C 1 -C 5  and the gain circuit  30  together provide the beam current measuring device  20  with 10 operating ranges. The final voltage value from the output of the third operational amplifier I 3  is sent to an external ADC.  
         [0023]     A second particle beam of substantially different beam current can be collected by the same Faraday cup  21 , a second definite integral circuit  32 , and a second gain circuit  34 . The second definite integral circuit  32  can be used with a longer integration time so that the lesser slope (i.e. the lesser beam current of the second particle beam) produces a voltage ramp  33  which reaches a value of approximately the same magnitude as the first voltage ramp  29  as shown in  FIG. 3 . Thus two different particle beams can be measured with the same Faraday cup  21  and integrator circuit  27 . The current integrator circuit  27  uses a “clear” signal provided by another system (not shown) which controls the timing and duration of the “clear” signal. The definite integral circuit  28  uses “ch 1  begin definite integral” and “ch 1  end definite integral integrate” and circuit  32  uses “ch 2  begin definite integral” and “ch 2  end definite integral” shown in  FIG. 3 , which control the timing and duration of the “integrate” signals.  
         [0024]     The prior art beam current measuring device  20 , while functional, suffers from a number of limitations. It is very difficult to eliminate the offset and gain errors of the analog circuit because of the number of op amps involved. Both gain and offset errors are of importance because they contribute to a problem which is sometimes described as the “bathroom scale problem.” This occurs in inexpensive scales of the type used at home in the bathroom. The scale is reasonably calibrated over some range of weight values, but is not well calibrated above or below the good part of the range. In the same way the problem encountered when trying to design a pulse integrator using the prior art is that gain and offset error are difficult to correct over the entire range of measurement. Offset error is particularly a problem when voltages are at the lower end of the ranges of the analog circuits because then offset error contributes more to the measured value.  
         [0025]     The precision pulse current integrator (PPCI)  36 , shown in  FIG. 2 , solves the problem by eliminating most of the analog circuitry and digitizing the signal as soon as possible. The precision pulse current integrator (PPCI)  36  has a Faraday cup  21  and a current integrating circuit  27 , similar to those used in the prior art as shown in  FIG. 1 . However the remaining circuits of the device  36  are different. A programmable gain amplifier (PGA)  42  is connected to the output of op amp I 1  of  FIG. 2  and scales the output voltage by a programmed amount (e.g., a power of two: 1, 2, 4, 8) so as to provide an output voltage better matched to an analog-to-digital converter (ADC)  44 , which is in voltage receiving relation to the PGA  42 . Better matching of the input voltage by the PGA  42  to the ADC  44  results in better accuracy in the output of the ADC, i.e. the input voltage to the ADC is scaled to the range of the ADC for better accuracy. The ADC is controlled via a control signal  46 , shown in  FIG. 3 , from a trigger generator  48  shown in  FIG. 2 . At the rise of the control signal  46  the ADC converts the voltage received from the PGA into a digital word which is sent to a field programmable gate array (FPGA)  49  which stores the value in a data register identified as, present value monitor Low (pvmL) in  FIG. 2 .  
         [0026]     The timing of the control signals shown in  FIG. 3  are controlled by a master clock and controller represented by  FIG. 3  generally, and the clock-based timing signals shown therein. The clock for example employs a 4 MHz crystal controlled time base which provides 0.25 μs/step resolution, and thus integration times employed for the beam current measurements with respect to the accelerator  22  are all based on the same clock. After the passage of a selected period of time  54 , as shown in  FIG. 3 , a second control signal  50  shown in  FIG. 3  is sent from the trigger generator  48  to the ADC  44  as shown in  FIG. 2 . Again the rising voltage of the control signal  50  causes the ADC to convert the voltage received from the PGA  42  into a digital word which is sent to the FPGA  49  which stores the value in a data register identified as present value monitor High (pvnH) in  FIG. 2 . The FPGA  49  which forms a definite integral circuit which subtracts the value stored in the pvmL data register from the value stored in the pvmH data register, using hardware math, and stores the result in a third result data register identified as present value monitor Difference (pvmD). The value stored in the pvmD data register represents the definite integral of the current received by the Faraday cup  21  over the selected time period  54 . The digital contents of the pvmD data register is sent to a first-in first-out (FIFO) memory  56  by means of a data selection switch  58  which is normally set with a data selection pointer set to the pvmD data register as shown in  FIG. 2 . The FIFO memory  56  is loaded from the FPGA in response to a control pulse from the trigger generator  48 .  
         [0027]     As shown in  FIG. 2 , a second beam current can be measured with a second definite integral circuit  49  and a second FIFO memory  62  under the control of a second trigger generator  64 . The second trigger generator  64  supplies second voltage sample triggers  66 ,  68  separated by a second selected time  67  to an OR gate  69  which issues trigger signals to the ADC  44  from either the first trigger generator  48  or the second trigger generator  64 . As shown in  FIG. 2 , operating mode switches  70 ,  72  switch the second trigger generator, and the second FPGA  49  so as to operate with the Faraday cup  21 , the current integrating circuit  27 , and the ADC  44 . Alternatively, a second Faraday cup (not shown), a second PGA (not shown), and a second ADC (not shown) can be selected with the operating mode switches  70 ,  72  to form a separate independent beam current measuring device. Advantageously, the same front-end electronics, including the analog components, are used for each of two different beam currents alternatively received by the same Faraday cup  21 . The current integrator circuit  27  of  FIG. 2  uses a “clear” signal provided by the clock system as shown in  FIG. 3  which controls the timing and duration of the “clear” signal. The ADC uses an “integrate” signal provided by the clock system as shown in  FIG. 3  which controls the timing and duration between ADC trigger signals.  
         [0028]     The FPGA  49  has additional data registers which can be selected by the data switch  58  and read out into the FIFO memory  56  to provide diagnostics during manufacture or use. Possible selections shown in  FIG. 2  are: 
        off—nothing is loaded into the FIFO     pvm 2 —two words, making up the contents of the pvmL and pvmh data registers are loaded     ADC—each time the ADC completes a conversion, the contents of the ADC data register are loaded into the FIFO.     QA—the QA data register is loaded with a value; each time a FIFO trigger instruction is sent to the definite integral circuit  49  the contents of the QA data register are copied to the FIFO.     pvm—the setting normally used for beam current data collection.        
 
         [0034]     The “QA” settings allow software testing of the FIFO logic during manufacturing. The “ADC” and “pvm 2 ” settings may be used for other diagnostics.  
         [0035]     The gain and offset error contribution from the Programmable Gain Amplifier (PGA)  42  and the analog-to-digital converter (ADC)  44  are very low because the programmable gain amplifier and the analog-to-digital converter, contain resistors on which laser trimming is utilize to minimize the gain and offset errors. Further gain and offset errors are reduced or eliminated by calibration with a programmable current source  74 . The programmable current source  74  is arranged to supply a precise test current in place of the current from the Faraday cup input  21 . The programmable current source may have, for example, six current ranges between ±ten nanoAmp (10 nA) and ±one milliAmp(1 mA). The current source  74  is calibrated with a precise external ammeter. The same current source  74 , via an auxiliary output jack  76 , allows coupling of two or more precision pulse current integrators (PPCI) together to facilitate calibration from one common current source. Thus one current source can be used to calibrate all the precision pulse current integrators used on the accelerator  22  shown in  FIG. 4 . Because the desired output is a ratio between beam currents, errors in the calibration of the beam current caused by the programmable current source  74  cancel out.  
         [0036]     The current source  74  uses with a calibration table which is built and loaded at the factory by using a computer controllable picoammeter (for example a Keithley 486, available from Keithley Instruments, Inc., of Cleveland) to measure the current of the current source  74  while a calibration table is created. The table is stored in flash memory and is read as needed. The calibration table is used to determine the correct Range and Vernier inputs to apply to the current source  74  so the user selected current is provided.  
         [0037]     It should be understood that three or more different beam pulses could be monitored by the beam current monitoring device by adding additional computation and memory capabilities or by reusing existing circuitry.  
         [0038]     It should be understood that other logic implementations/technologies for the PPCI such as an application-specific integrated circuit ASIC, could be used in place of the field programmable gate array.  
         [0039]     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.