Table of Contents
AccelNET provides a complete AMS data collection system capable of managing data collection for any species of interest. All collected information is logged to disk. The system may be reconfigured by operator command to change ion species.
AMS data logging is comprehensive. Data for every multicup (stable isotope off-axis Faraday cup) are logged in disk files and time stamped with the jumping cycle number in which the measurement was taken. Event data for each detected rare isotope particle is logged and is time stamped with the jumping cycle number and event number within the jumping cycle.
A "setup" file is written containing other information such as the terminal voltage, rare isotope, charge state, etc. for each measurement.
All of the logging files are written in ASCII to make them easy to view, manipulate and input to other programs.
As each cathode measurement is completed, a summary of the measurement is written to a disk file and displayed on the screen.
When the runlist is completed (all cathodes are measured), a summary file is written containing the average currents, rare isotope counts, average ratios, standard deviations etc. for all cathodes in the runlist. Other summary files may be created containing tab separated fields suitable for use by Quattro and Excel.
After all measurements for a list of cathodes have been completed, the datasets gathered may be reanalyzed off line if necessary. Parameters such as the rare isotope gates may be changed and the datasets reprocessed to obtain new summaries.
The summaries are then analyzed to perform normalization, background subtraction and so on.
NEC has written software using the PV-Wave (a data analysis package) language which performs both post measurement functions. NEC calls this software IsoNET.
IsoNET is divided into two programs. One program is used for reanalysis (off line data reduction) of the datasets. It traverses a list of datasets specified by the operator, reevaluates them and writes new summary files.
The second program performs normalization, background subtraction and calculates conventional AMS results and uncertainties.
The sequential beam injection system consists of an ion source, an electrostatic spherical analyzer, a bending magnet with an insulated chamber, three off axis Faraday cups and an X/Y steerer.
The post acceleration portion of the system consists of a bending magnet, three off axis Faraday cups, an electrostatic cylindrical analyzer and a gas ionization chamber. The figure titled "AMS system components" is a simplified line drawing of the accelerator layout.
Two of the cups at each end of the machine are used for carbon AMS measurements. The two cups plus a third set of cups can also be used for other species of AMS.
The ion source may be either a multicathode solid SNICS source or a multicathode gas SNICS source. In both cases a negative ion beam is accelerated from the source and passed through an electrostatic spherical analyzer (ESA). The ESA removes energy tails in the beam, which are produced by the cesium sputtering process. In systems containing two ion sources the ESA is made rotatable to select which ion source is to be used.
The beam then passes through a bending magnet with an insulated chamber. The bending magnet is set to bend 14C to the correct angle for injection into the accelerator. 12C and 13C are bent past the angle needed for accelerator injection into a set of offset Faraday cups which are monitored by AccelNET AMS. A power supply connected to the insulated magnet chamber is controlled by a signal supplied by the sequential injection control electronics. Voltages are sequentially applied to the magnet chamber to inject 12C and 13C.
A set of X/Y steerers follows the bending magnet. The steerers are also controlled by the jumping system. A different set of voltages is applied for each injected species to correct for slight possible differences in beam position and direction which may be produced by the different gap voltages applied to the magnet chamber.
The beam jumping control electronics may also be used for a simultaneous injection system. In this case bending magnet chamber bias and the steerers are not required but an electronic chopper is used to reduce the average intensity of the 12C ion beam to about 1% of its DC value. The data collection process is otherwise unchanged.
Beam jumping waveform and gating signals are generated by a set of CAMAC modules designed and manufactured by NEC. Below is a description of each of the modules which make up the system. Please refer to the figures titled "AMS data collection System block diagram" and "AMS carbon data acquistion waveforms".
The "Sequence Controller" CAMAC module generates the timing information. A single jumping cycle may be as long as 4 seconds but is more typically about 100ms. A 4MHz clock and 24 bit programming registers provide a resolution of 0.25us/step.
The Sequence Controller may be programmed to free run, meaning that the next jumping cycle starts immediately after the previous one ends, or may be line synced, meaning that the next jumping cycle is started at the next zero crossing of the AC power line. The choice of free run or line sync and the polarity of the zero crossing may be set by the user.
Jumping cycles are counted by the computer. At the end of each jumping cycle a LAM (Look At Me) signal is generated by the Sequence Controller, which then pauses until the LAM is acknowledged by the computer. LAM processing may be disabled by the user for diagnostic purposes.
A ribbon cable connects the Sequence Controller to the other CAMAC modules making up the jumping system. This cable carries the timing data to the other modules.
One jumping cycle is made up of several "states". The Sequence Controller outputs a state number which is used by the rest of modules.
The Sequence Controller contains 16 timing comparator registers. There is one comparator register for each state.
At the beginning of each jumping cycle a counter register is initialized to zero. This register is incremented by the 4MHz clock.
The comparator registers are programmed to increment the state number at the appropriate delta T for each state. The value of the comparator register for the current state is compared to the counter register. When the value of counter register is greater than the value of the comparator register the state number is incremented.
The maximum number of states used in a jumping cycle is programmable. The jumping cycle ends when the value of comparator register for the maximum state number to use is exceeded.
Typical carbon AMS is done using 7 states in the following way. See "AMS system components" for more information.
This is the rest state. When the sequencer is not performing a jumping cycle. i.e. it is waiting for the next AC line trigger or for the LAM to be acknowledged it is in this state.
This state may be used to provide a predelay, for example, if line sync is in use one can delay the start of the actual jumping cycle for some amount of time after the trigger.
Power supply voltages slew to the correct value for injection of the first abundant isotope.
High energy side measurement of the first abundant isotope.
Power supply voltages slew to the correct value for injection of the second abundant isotope.
High energy side measurement of the second abundant isotope.
Power supply voltages slew to the correct value for injection of the rare isotope (14C). Usually for carbon AMS this means 0 volts on the magnet chamber, other species such as aluminium may require different settings.
Rare Isotope data collection and low energy side measurement of both abundant isotopes.
Not used.
A Sequenced D/A converter CAMAC module provides the waveforms needed to drive the jumping power supplies.
Usually three Sequenced D/A Converters are used in a system. One converter drives the injection magnet chamber bias power supply. The other two are used to drive a set of power supplies connected to steerers installed in the beamline after the injector bending magnet.
The Sequenced D/A Converter contains four analog value registers. Another set of registers associates a value register with the state number provided by the Sequence Controller.
If the value registers are assigned in this way:
Abundant isotope #1
Abundant isotope #2
Rare isotope. Usually this value is zero for the magnet chamber power supply but may be something else for the steerers.
Not used.
Then the state numbers from the Sequence Controller would be assigned as follows:
VCreg2
VCreg0
VCreg0
VCreg1
VCreg1
VCreg2
VCreg2
Not used.
The Gate Generator CAMAC module provides various signals used to control the AMS data collection system. The module provides 8 separate outputs. Each output may be individually programmed to be on or off during any state. The outputs are differential to allow them to drive long cables. Usually the outputs are connected to the "Quad Receiver" described below at the destination end of the signal.
Typically two channels are used to clock the low energy transient recorders, two channels clock the high energy transient recorders, and one channel is the rare isotope gate.
See the sections which follow for more information.
The Quad Receiver CAMAC module is a simple module containing four channels of differential receivers and two TTL level outputs for each receiver. The differential receivers are connected to the gate generator outputs. The outputs may be individually programmed by DIP switches inside of the CAMAC module to provide a high true or low true output.
AccelNET AMS provides a complete facility to control AMS data collection. A supervisory program controls the changing of the cathode in the ion source and starting and stopping of data collection. The status of the accelerator is monitored, and the program will pause data collection in the event of a problem such as a beamline valve closing due to a vacuum fault. Data collection startup is inhibited in cases where the operator may have failed to open a Faraday cup or a beamline valve or other accelerator problems may interfere with data collection.
The supervisory program may be configured to call other programs at various points in its execution. For example, one might implement a program to log accelerator parameters and run this program each time a cathode is measured.
The supervisory program operates from a list of cathodes (a runlist) generated by the user. This list specifies measurement parameters including the number of times to measure the cathode, the warmup time, the data collection time and the data collection mode.
The cathodes may be divided into groups within the list and each group measured consecutively or independently. Data may be collected for a fixed amount of time or a fixed number of rare isotope events.
When collecting for a fixed number of events, a time limit is also used to prevent excessive collection time on cathodes which have low count rate or problems such as low current. This mode collects until one of the limits is reached.
The cathode runlist may be traversed in two ways.
One way is to measure each cathode once and then go to the next cathode in the list. When this method is used the cathode list is traversed as many times as necessary until all cathodes in the list have been measured the number of times specified for each cathode.
The second way to traverse the cathode list is to measure each cathode repeatedly until the cathode has been measured the number of times specified in the runlist. When working in this mode another program in AccelNET can be used to analyze the measurements as they occur and make decisions about whether to go on to the next cathode or measure the same one again.
This is typically used in applications where it is desired to make several short measurements of a cathode and after some minimum number of measurements make a decision based on the scatter of measurements whether to measure again or go to the next cathode.
AccelNET provides a program module which makes this decision based on the results of the most recent N number of runs. It is possible to write program modules using other selection criteria.
IsoNET also provides a "best run" selection feature for off line analysis.
Abundant isotope collection is performed by a set of current amplifiers and CAMAC transient recorders. The current amplifiers are controlled by the computer and may be adjusted over a wide range.
The output of each current amplifier is connected to a transient recorder. A transient recorder is a type of list mode ADC. During each jumping cycle the transient recorders are individually clocked by signals from the gate generator at the appropriate moment to capture the value of the pulsed current waveform.
Usually each transient recorder is clocked once per jumping cycle. The isotope is sent to the offset Faraday cup by the beam jumping system, and a snapshot of the current value is taken and placed in the internal memory of the transient recorder.
Occasionally (approximately every 10 seconds) data collection is paused, and the list of measurements is uploaded to the computer where the information is placed in disk files for later processing.
Each time a block of data is uploaded numbers such as the average current and isotope ratios are updated in AccelNET.
NEC offers an option to this system which allows the transient recorders to be clocked more than once per jumping cycle. Using this option one can capture several datapoints within each Faraday cup current pulse and perhaps better quantify the measurement.
Rare Isotope data collection is handled by a gas filled, delta-E detector, a set of NIM amplifiers and logic modules, a CAMAC peak holding ADC, a CAMAC multichannel counter and a CAMAC list processor.
A list processor is a CAMAC module which performs CAMAC I/O operations independently of the computer. It contains a large RAM so the data from the I/O operations it performed can be stored.
The detector contains up to six plates for measurement of Etotal, dE1, dE2, dE3, dE4, and dE5.
The analog outputs from the NIM amplifiers, a trigger signal generated by the NIM electronics and gated by a signal from the jumping electronics are fed into the peak holding ADC.
A program is loaded into the list processor by AccelNET and the list processor is enabled. Each time the ADC receives the trigger signal from the NIM electronics it performs a conversion. At the end of the conversion the ADC asserts a LAM signal.
The list processor waits for the LAM from the peak holding ADC to trigger it. Each time the list processor is triggered it runs the program which has been loaded into it.
The program performs a number of CAMAC I/O operations to copy the data from the ADC to the list processor and clear the LAM. It then goes to sleep until the next trigger.
Periodically data collection is paused, and the data collected in the list processor's internal memory are uploaded to the computer.
The rare isotope data collection system uses two channels of the CAMAC multichannel counter.
The ADC trigger signal from the NIM electronics is split and is sent to the ADC and to one channel of the CAMAC counter module. Each time the ADC is triggered the counter is incremented. This provides a count of the number of ADC triggers supplied by the NIM electronics. When the data are uploaded from the list processor this register is read and a parameter containing the total number of ADC triggers is updated.
When the uploaded data are processed the number of events contained in the data block is counted and a parameter containing the total number of events processed by the list processor is updated.
If the ADC is already busy performing a conversion when another trigger is received, a particle event will be missed. By comparing the two numbers one can get an idea of the number of missed events (detector system pileup). Normally the number of missed events is very small, usually less than 0.1% of the total number of events.
Another channel of the counter module is used to count the number of jumping cycles which have occurred. It is connected to a signal from the jumping electronics which causes the counter to increment at the end of each jumping cycle. Each time the list processor is triggered the cycle number is read from the counter and placed in the block of event data. This allows individual events to be stamped with the cycle number in which they occured.
All aspects of rare isotope data collection are controlled by configuration files. Parameters such as the number of channels of data read from the ADC can be changed. This makes it possible to use the system with other types of particle detectors and perhaps use the system for other types of event counting.
For example, a solid state detector requiring only one ADC channel could be used. The program loaded into the list processor would be changed to only retrieve data from a single channel. This has the side effect of allowing more events to be stored in the list processor's memory and decreasing the data collection dead time because a smaller number of CAMAC I/O operations need to be performed.
It is also possible to locate CAMAC hardware needed for other types of experiments in the same CAMAC crate and load different programs into the list processor for the various configurations.
HISTmngr is a display tool which allows viewing of the histogram, contours, and gates which have been defined for the species of interest. Any of the defined histograms or contours may be viewed and the event gate settings may be changed from this program.
When rare isotope data are collected and uploaded into the computer the individual event data blocks are processed according to rule sets contained in a configuration file.
The format of the rare isotope event file and sets of one and two dimensional histogramming arrays (contours) are specified here. An event gate may be defined for each spectrum.
As each event data block is processed a line of data is written to the event file and the histogramming arrays are updated. One channel of input data may participate in several histograms and contours at the same time.