At line 4 added 13 lines. |
+ Single tube counters and monitors are handled through the EL737 counter box. This seems simple |
+ but the EL737 counter box has some quircks. |
+ |
+ * Quite some confusion comes from the fact that meaningful cables from monitors and counters are |
+ plugged into the counter box and thus mapped to an integer id in the counter box. These interger IDs are |
+ directly mapped into SICS. Replugging at the counter box is easy. With the effect that it is never |
+ clear which monitor is which in SICS. |
+ * Counting on preset monitor works only on the monitor which is plugged into channel 1 of the counter box. |
+ * The EL737 counter box also detects the NoBeam condition through a monitor threshold. |
+ |
+ The EL737 counter box has in interesting output: this is the TTL trigger signal if data acquisition |
+ is active or not. |
+ |
Line 7 was replaced by lines 20-87 |
- !! Time-Of-Flight Data Acquisition |
+ In order to understand neutron DAQ with area detectors let us follow the path of a detected neutron through |
+ the system. |
+ |
+ # The neutron causes a charge on one of the detector wires |
+ # After preamplification and some signal processing the neutron causes a pulse on some wire |
+ # An electronic box called the Multi Detector Interface (MDI) detects the pulse and encodes it into a |
+ message to be sent through a fibre optic link. The neutron event message sent consists of: |
+ ## A message header identifying the message type |
+ ## The status of various sync bits. These are external inputs to the MDI. An important sync bit is the |
+ input from the EL737 counter box if data acquisition is active or not. |
+ ## The position information of the detected neutron event. |
+ # The fibre optic link transports the neutron event message to the histogram memory. |
+ # The histogram memory (HM) is indeed a computer. In the case of SINQ a MEN A12 VME PowerPC board running |
+ realtime linux. This HM computer runs two main tasks: a server task is responsible for configuring the |
+ HM and for communicating with then outside world which consists mostly of SICS. This server task is |
+ actually a WWW-server. Then there is the histogramming task: this task continuously reads the fibre optic |
+ link and analyizes the neutron event messages. It checks if the events are OK and accumulates them in |
+ the appropriate bins according to the position information in an in memory representation of the detector. |
+ # SICS communicates with the HM, configures it, starts and stops DAQ and download and saves the data to file. |
+ |
+ Even with an area detector an EL737 single counter box is necessary. This box handles the monitors and |
+ through its TTL output triggers neutron data acquisition. |
+ |
+ Just another time, to make the triggering process crystal clear. When data acquisition becomes active, |
+ the EL737 counter box sets its TTL output. This is fed into the MDI and is represented is one of the sync |
+ bits of the neutron event message. The histogramming task tests both the event type and the presence |
+ of the sync bit against a mask. Only matching neutron events with appropriate sync bits are histogrammed. |
+ |
+ The question may arise why this is so important. Now, SINQ is an unreliable source compared to a reactor. |
+ The beam may go down for milliseconds to days. But when the beam is down, the detector and electronics |
+ continue to produce noise. You do not want this noise on your data. Thus DAQ needs to be interrupted by |
+ way of the threshold in the counter box and the sync bit. |
+ |
+ |
+ !! Time-Of-Flight Neutron Data Acquisition |
+ |
+ In time-of-flight (TOF) neutron data acquisition not only the positional information of a neutron event is |
+ measured but also the time it needed to cover a certain distance before it arrived at the detector. This is |
+ a measure for the energy (speed) of the neutron. In order to do this, a starting point in time is needed |
+ when all neutrons started to travel towards the detector. At pulsed neutron sources this starting point in |
+ time is provided by the source. At SINQ it is provided by the chopper. This is a fast rotating disk which |
+ cuts the neutron beam into pulses by way of slits in the disk. |
+ |
+ TOF neutron DAQ is very similar to area detector data acquisition: |
+ # After some suitable electronics preprocessing the neutron event arrives at the MDI in a position encoded |
+ input channel. |
+ # The MDI maintains a counter which counts time. The time is the time since the last reset from the pulse |
+ generator. At SINQ, this reset signal is delivered by the chopper. |
+ # The MDI encodes the header, sync bits, time stamp and position information in a neutron event message. |
+ # The neutron event message is forwarded via the fibre optic link to the HM |
+ # The HM histograms the neutron event according to position and time stamp. |
+ |
+ Besides the normal sync inputs there are additional ones. Most notably an input from the chopper which |
+ indicates if the chopper is running at the desired speed and phase. If any of this is not the case, then |
+ the neutron event is useless. |
+ |
+ Another complication comes from the fact that most choppers have two slits (for symmetry) but deliver only |
+ one signal per chopper revolution. The signal needs to be duplicated. This is the purpose of the Emmenegger |
+ electronics. This is a rather obscure bit of hardware. Mr Emmenegger has left PSI a long time ago and |
+ no one knows any details about it anymore. |
+ |
+ Yet another complication comes from the fact that the data width for the time stamp is only 20 bits. In order |
+ to increase the available time resolution a delay time is subtracted in the MDI. This delay time is the |
+ time which the fastest interesting neutrons need to travel from the chopper to the detector. The delay |
+ time is set in the MDI by way of a RS232 connection from SICS. Unfortunately the implementation of the |
+ RS232 interface is dubious. RS232 is connected via a fibre optic link. Thus there is a fibre optic |
+ to RS232 converter. Some of those converters work, some not. One has to try out several ones when this |
+ link needs to be replaced until a converter is found which works. |
At line 8 added 16 lines. |
+ Summing it up, the MDI in the TOF case needs the following inputs: |
+ # The position encoded inputs from the detector |
+ # A time reset signal from the chopper via the Emmenegger electronics |
+ # A sync input from the EL737 counter box which indicates wether DAQ is active or not |
+ # A sync input from the chopper which tells if the chopper is OK |
+ # A RS232 input to set the delay time from SICS |
+ |
+ Time-of-flight brings with it yet another complication: the spectrum of the incoming neutron beam is not |
+ flat. I.e. for different neutron energies there are different source intensities. In order to arrive at |
+ absolute (or relative) intensities in reduced data the scientist has to normalize against the source |
+ spectrum. To this purpose the monitors are passed into the MDI too and histogrammed as well. How this is |
+ done differs from TOF instrument to TOF instrument: At AMOR this data is handled as a different detector |
+ bank, on others the monitors are appended to the real data as extra detectors. |
+ |
+ |
+ |
At line 10 added 42 lines. |
+ CCD cameras are like digital cameras in that incoming photons increment a charge in a detector pixel. |
+ After exposure, the charge map of the detector is read out. CCD detect photons: neutrons are detected by |
+ looking at scintillator screens which essentially converts neutrons into light. This explanation serves |
+ to give the background for the fact that the electronic noise suppression which works for normal |
+ detectors at the neutron event level does not work for CCDs. Thus one needs to rerun the |
+ exposure if SINQ goes down to long during an initial exposure. SICS checks for this and does the |
+ reexposure when necesary. |
+ |
+ Now, commercial CCDs come from companies living in the dark ages IT wise. Which means that they |
+ usually get deliverd with some windows system to run them. The better ones come with a Linux API |
+ in a library. This is what we use. On top of the library a litte WWW-server, ccdwww, is run which |
+ allows SICS to expose images and download data, configure the camera and such. This WWW-server is run |
+ at a separate computer. For a good reason, these CCD cameras require a special hardware connection. |
+ Which often fails. With a different computer the CCD camera computer can be restarted without |
+ having to take down the rest of the data acquisition system. |
+ |
+ |
+ !! Specialities at different Instruments |
+ |
+ ! AMOR |
+ |
+ At AMOR, the Emmenegger electronics died. Fortunately, the data for a full revolution of the chopper |
+ could be fitted into the 20 bit time stamps. Thus there are actually two pulses in the neutron event data. |
+ This is corrected for by a special histogramming process on the HM computer for AMOR which merges |
+ the two pulses. This makes the HM configuration for AMOR a little complicated. |
+ |
+ ! POLDI |
+ |
+ POLDI works with frame overlap. This means that the data from different neutron pulses overlap in the |
+ data. POLDI has a very complex chopper: the disk is split into four quadrants with each 8 slits. |
+ And there is only one reset signal from the chopper per revolution. Fortunately all data fits into the |
+ 20 bit time stamp. The raw data from the POLDI HM contains data from all four quadrants after each other |
+ in time. SICS sums the four quadrants together again. |
+ |
+ |
+ ! HRPT |
+ |
+ HRPT is the first instrument having the second generation data acquisition electronics. Namely the |
+ prototype. It replaces the MDI and front level electronics. This electronics has an own TCP/IP address |
+ and its own console. No documentation available, naturally. |
+ |
+ |