1. Benchmarking Tracking by Doppler Reconstruction
Dirk Weisshaar, NSCL @
Or:
What’s the best way finding the coordinates of the first interaction of a gamma ray?

2. Gamma spectroscopy with fast beam
γ rays of 28Si
at v/c = 0.38 in
GRETINA
S800
36Ar
28Si θ Ge
v/c ≈ 0.4
ϴ [rad] in GRETINA
energy [keV] (laboratory frame)
(Obvious) requirements for a spectrometer:
Doppler-shift correction
 Spatial resolution
Lorentz Boost
 Detection efficiency at forward angle
…and GRETINA is a perfect match
beam

3. Key question for Doppler reconstruction
What are the coordinates of the first interaction of the gamma ray?
In the past (and for the lazy folks):
Use the Main Interaction, which is the interaction point with the highest energy deposition.
In the days of tracking: Determine the track of the gamma ray and therefore the first interaction in the track.
How I track:
Go through all possible tracks/permutation of the interaction points reported in a crystal. Find track with smallest figure-of-merit FoM
with angle θen computed from energies and Compton formula and θvec computed from coordinates.

4. Some experimental results
Doppler-reconstructed spectra using
MAIN INTERACTION
(black) or
TRACKED 1st INTERACTION
(magenta)
275keV
1779keV

5. 1779keV, tracked and main interaction
Mpt: main interaction point
Trkpt: tracked first interaction point
For events with Mpt ≠ Trkpt
using FIRST TRACKED INTERACTION
using MAIN INTERACTION

6. 1779keV, tracked and main interaction
Mpt ≠ Trkpt Mpt used
Mpt = Trkpt
Mpt ≠ Trkpt Trkpt used

7. diffFoM = FoMMpt – FoMTrk (always ≥ 0)
A FoMMpt can be found as the smallest FoM for all possible tracks with the Main Interaction point fixed as first interaction point. With FoMTrk being the FoM result from the tracking we define:
diffFoM = FoMMpt – FoMTrk (always ≥ 0)
Mpt used
Trkpt used
using MAIN INTERACTION
using FIRST TRACKED INTERACTION
diffFoM
diffFoM

8. Resolution and diffFoM
0.2 – 1.0
1.0 – 2.0
2.0 – 4.0
> 4.0
Trkpt used
Mpt used
Mpt
Trkpt
For small values of diffFoM and for large values: Use Main Interaction Point!

9. Do we learn anything from the FoM value?
0 < diffFoM < 0.2
0.2 – 1.0
1.0 – 2.0
2.0 – 4.0
> 4.0
Trkpt used
Mpt
FoMTrk
Trkpt
FoMTrk

10. A new hope ….(still 1779keV)
Consider those events for which number of hit segments equals number of interaction points, i.e. events for which GRETINA decomposition put only ONE interaction per segment.
Mpt used
#Pts = #Seg
Trkpt used #Pts = #Seg
using MAIN INTERACTION
FIRST TRACKED INTERACTION
diffFoM
diffFoM

11. The case where Tracking beats Mpt
For events where GRETINA decomp put ONE interaction per segment AND for
diffFoM in ‘mid-range’, the tracked 1st interaction leads to a better result for the Doppler reconstruction.
Mpt used
#Pts = #Seg
Trkpt used #Pts = #Seg
diffFoM
diffFoM
diffFOM in [0.1:1]
Main interaction better
diffFOM in [1:3]
Tracked interaction better
diffFOM in [3:10]
Main interaction better

12. Trkpt used #Pts = #Seg diffFoM [1:3]
diffFoM/FoM
I couldn’t find any helpful meaning of the absolute value of the FoM for the Doppler correction. But how about diffFoM/FoM, as ‘relative improvement from Mpt to Trkpt’?
Mpt used
#Pts = #Seg
Trkpt used #Pts = #Seg
diffFoM/FoM
diffFoM/FoM
Mpt used
#Pts = #Seg
diffFoM [1:3]
Trkpt used #Pts = #Seg diffFoM [1:3]
diffFoM/FoM
diffFoM/FoM

13. What does the simulation say?
SIMULATION ucgretina, 1779keV, 6mm packing, NO SMEARING
W=#pts
W=#segs
W=1
main interaction
Large diffFoM Mpt better
Tracked 1st interaction
17keV
Mpt≠Trkpt Mpt used
Mpt≠Trkpt
Trkpt used
Tracked 1st
main interaction

14. Conclusion
As of now, for performing the Doppler-shift correction the use of the Main Interaction Point as first interaction point leads to better peak shapes and resolution then using the Tracked First Interaction Point from a state-of-the-art tracking algorithm purely based on the Compton-energy formula.
Furthermore:
Fast beam data with GRETINA is an excellent way to investigate position resolution and the ability of tracking algorithms to identify the first interaction point of a gamma-ray track. Fast beam data shown is available to everyone interested in it. Also the raw data (traces), allowing to do ‘decomp’ again (other fit, other basis, …)

15. Discussion
Many more slides

16. n Reconstructed 1779keV, diffFoM with different weights W=#pts W=#segs
main interaction
Tracked 1st interaction

17. Reconstructed 1779keV, gated on events with (Num interaction == Num Segments)
W=#pts
W=#segs
W=1
Mpt
Trkpt
Main≠Tracked Main used
Main≠Tracked
Tracked used
Tracked 1st

18. n Normalized by peak amp Normalized by peak amp Log scale Linear scale
Experimental spectra:
Black: all events, main interaction used (100%, FWHM 15.1keV, ~84000cts)
Magenta: main interaction = tracked interaction (50%, FWHM 14.4keV, ~42000cts)
Red: main = tracked, #interaction=#segments (11%, FWHM 13.3keV)
Normalized by total counts in [1500:2000]
Normalized by peak amp
Linear scale
Normalized by peak amp
Log scale
Not normalized

19. n Comparison simulation with ‘realistic’ position resolution.
Black: simulated, ALL event and main interaction used
Red: simulated, as black, but position smearing of sigma=1.8mm used Blue: In-beam data, events with #interaction=#hit segment, tracked=main interaction.

20. n 19Ne, beta .3870 3.3%, 272keV NNDC: 275.1 21Ne, beta .3566

21. n 23Na, beta .3586 23Na, beta .3580 1.5%, 439.7keV 1.3%, 627.7keV
NNDC: 440.0
23Na, beta .3580
1.3%, 627.7keV
NNDC: 627.5

22. n 30P, beta .3671 1.6%, 705.0keV NNDC: 708.7 26Al, beta .3716

23. n 26Al, beta .3730 1.1%, 1011.2keV NNDC: 1011.7 24Mg, beta .3714

24. n 28Si, beta .3722 0.9%, 1779.1keV NNDC:1779.0 24Mg, beta .3740

25. n 28Si, beta .3746 0.9%, 2839.5keV NNDC:2838.3 28Si, beta .3763

26. n
16O, beta .375
1.6%, 6116keV
NNDC: 6129

27. Decomp (Mpt) vs segment based