Diff: NeutronDAQ

Search SINQ Wiki:

- Main Page
- Search SINQ Wiki
- Sample Environment
- Probenumg. Intern
- Troubleshooting SICS

This Page
- Page Info
- Printer Friendly

Referenced by

Wiki Info
- Unused pages
- Undefined pages
- RecentChanges
- Page Index
- System Info
- JSPWiki Docu
- SandBox
- OneMinuteWiki
- Create a New Page

JSPWiki v2.0.52

Difference between version 2 and version 1:
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 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.

Back to NeutronDAQ, or to the Page History.