The sgdevtid task determines whether the signals in an occurrence in the SGD constitute a valid event, and computes the event energy and the 3-dimensional coordinates of its first interaction.
The input file to sgdevtid is an SFF (second FITS file) with variable-length array columns where each row contains an occurrence with multiple signals recorded at a given time. The output event file, specified by the parameter 'outfile', has a different structure and does not contain columns with variable-length arrays. Each row in the output file corresponds to a reconstructed event, and the columns CAMERAX, CAMERAY, CAMERAZ, and PI contain the reconstructed position and energy of that event along with additional columns containing information related to the number and the type of interactions (see below).
The sgdevtid task has many parameters that are used in the event reconstruction. These have been optimized and it is advised to use their default settings.
The algorithm uses the different signals in each occurrence to reconstruct an event in a two-step process. The first step is to merge the signals according to their location and whether or not they are consistent with a fluorescence X-ray. The second step is to analyze the remaining signals, and determine whether the sequence is consistent with an event. If the algorithm fails, the value of PI is set to NULL and writes diagnostic columns with the reason why the occurrence could not be reconstructed. Below is a more detailed description of the algorithm.
The merging of the different signals depends on what is considered the maximum distance for these signals to be physically related. This critical distance in turn depends on the event geometry. The task first merges the multiple adjacent signals (charge sharing) present in a single detector layer. The parameter 'd10' defines the meaning of "adjacent" and specifies the critical distance for the merging to occur. The next step of the task consists of merging the fluorescence signals into the originating signal. This step depends critically on the event configuration. If both the originating and fluorescence signals are in the same CdTe layer, the critical distance is specified by the parameter 'd1a1a'; if not, the relevant parameter is called 'd1a1b'. If the originating signal is in CdTe, but the fluorescence signals are detected in a Si layer, the critical distance is now specified by the parameter 'd1a2'. If both are in the Si layers, the relevant parameter for the merging becomes 'd1a3'. These parameters are hidden in the task and values recommended by the instrument team are assigned by default.
At this point (one step/merger), the task reconstruction has reached the level of a "hit". If there is only one hit, the process is completed. Otherwise, the task now reconstructs the different hits until the process converges on a single most-likely sequence of hits.
The following step depends on the number of reconstructed hits. If there are more than 4 hits, the entire occurrence, considered consistent with a cosmic ray, is rejected. If there are between 2 and 4 hits, all the possible permutations for the sequence of hits are tried and all sequences with non-physical Compton scattering (F test) are rejected. If only one sequence remains, it is deemed to be the actual reconstructed event. If there is more than one sequence at the end of the first test, subsequent tests are performed. If the sequences have inconsistent Compton scattering angles and geometric scattering angles (for 3 or 4 hits; G test), the sequences are rejected. Uncertainties in geometric scattering angles are computed using the method specified by the parameter 'delgmethod' (either set to 'analytic' or 'corner'). The maximum acceptable discrepancy between geometric and Compton scattering angle, is given by the uncertainty in scattering angle (computed with the method specified above), multiplied by the value of the parameter 'b' (set to 1.0 by default). Again, if only one sequence remains, the process is completed and the sequence is declared the reconstructed event. If all sequences are rejected, the task calculates the escape energy, the unabsorbed part of the energy of a photon that is able to exit the camera after detection, and executes the previous F and G tests (for 3 or 4 hits) again, correcting for that effect.
A calibration file specified by the 'probseqfile' parameter, allows the task to abandon the low-probability sequences. The probability thresholds for 2, 3, and 4 hits are given by parameters 'probaccept2', 'probaccept3', and 'probaccept4', respectively.
The last step is to rank the remaining sequences by a figure of merit (FOM). The FOM is an angular resolution measure comparing the calculated kinematic scattering angle to the geometrical scattering angle, which is calculated by assuming that the incident direction is the line-of-sight. The FOM can be tuned using the offset parameters 'paraoffset0', 'paraoffset1', 'paraoffset2', and the weighting parameters 'weight0', 'weight1', 'weight2', and 'weight3'. The highest-ranking sequence is then declared the reconstructed event. The FOM formulae are given below.
M is the number of hits, G[k,n] is the difference between the kinematic angle and the geometric angle for hit n, and Prob[k] is read from the calibration file specified by the 'probseqfile' parameter.
Reconstruction results for each occurrence are in the output file specified by the 'outfile' parameter. Columns copied from the input SFF are TIME, OCCURRENCE_ID, CATEGORY, FLAG_LCHKMIO, FLAG_CCBUSY, FLAG_HITPAT_CC, FLAG_HITPAT, FLAG_FASTBGO, FLAG_SEU, FLAG_LCHK, FLAG_CALMODE, FLAG_TRIGPAT, FLAG_TRIG, LIVETIME, PROC_STATUS, and STATUS.
Output file columns populated by the reconstruction are as follows:
The bit flags in the RECO_STATUS (reconstruction status) column are as follows:
Bit Description ------(Occurrence mode identification)------------------------------- 0 Mode is Am 241 1 Mode is PSEUDO 2 Mode is CALMODE 3 Mode is READALL 4 Mode is NOTSURE ------(Event reconstruction skipped for this occurrence)------------- 5 Bad PROC_STATUS 6 Mode is READALL or CALMODE and SKIPRECO option is YES 7 Occurrence has no signals 8 FASTBGO or HITPAT flag is set and REJECTBGO option is YES ------(Event reconstruction outcome)--------------------------------- 9 Event could not be reconstructed Bit Description ------(Occurrence mode identification)------------------------------- 0 not used 1 Mode is PSEUDO 2 Mode is CALMODE 3 Mode is READALL 4 Mode is NOTSURE ------(Event reconstruction skipped for this occurrence)------------- 5 Bad PROC_STATUS 6 Mode is READALL or CALMODE and SKIPRECO option is YES 7 Occurrence has no signals 8 FASTBGO or HITPAT flag is set and REJECTBGO option is YES 9 Occurrence has too many signals to reconstruct 10 All signals in the occurrence are below threshold ------(Event could not be reconstructed)----------------------------- 11 Signal cluster has too many signals 12 Signal cluster has invalid shape 13 Too many signals in a group within Si layers 14 Occurrence has too many hits to reconstruct 15 All hits are bad by F test for Compton scattering (2 or fewer hits) 16 All hits are bad by F test for Compton scattering (3 or more hits) 17 All hits are bad by G test for Compton scattering 18 All hits are bad because they have low probability (2 or fewer hits) 19 All hits are bad because they have low probability (3 or more hits) 20 Singularity in escape energy computation ------(Trivial reconstruction)--------------------------------------- 21 One signal above threshold, and one signal in the occurrence 22 One signal above threshold, and multiple signals in the occurrence ------(Complex reconstruction succeeded)----------------------------- ------( Reconstruction by merging signals into one hit)------------- 23 One hit is left after merging signal clusters 24 One hit is left after merging CdTe fluorescence within a CdTe layer 25 One hit is left after merging CdTe fluorescence from a different CdTe layer 26 One hit is left after merging CdTe fluorescence from Si layer 27 One hit is left after merging scattered electron energy from Si layer to Si layer ------( Reconstruction by evaluating possible hit sequences)-------- 28 One hit sequence is permitted by F test (cosine of scattering angle valid) 29 One hit sequence is permitted by G test (scattering angle consistent with energies) 30 One hit sequence is permitted by probability threshold 31 One hit sequence is left after tie-breaking based on figure of merit (FOM) 32 not used ------(Reason for escape energy calculation, if one was conducted)-------- 33 Because all sequences initially ruled out by F test 34 Because all sequences initially ruled out by G test 35 Because all sequences initially ruled out by probability test ------(Whether escape energy was calculated)------------------------- 36 Escape energy calculation was done 37 not used 38 not used 39 not used
sgdevtid infile=SGD_FFF.fits outfile=SGD_output.fits remapfile=CALDB fluorefile=CALDB badpixfile=CALDB probseqfile=CALDB \
probfovfile=CALDB
2. Reconstruct events using some user-defined parameters to override the defaults, output any additional information, and expand output to include one row per signal.
sgdevtid infile=SGD_FFF.fits outfile=SGD_output.fits remapfile=CALDB fluorefile=CALDB badpixfile=CALDB probseqfile=CALDB \
probfovfile=CALDB d1a_1a=9.5 prob_accept2=0.6 paraoffset0=2.6 weight2=0.9 outtracefile=SGD_extra_output.fits \
clobber=yes