1. GRETINA experiments with fast beams at NSCL
Dirk Weisshaar, NSCL @

2. Layout of a fast-beam experiment at NSCL…
…involving the S800 spectrograph
Reaction target
mg/cm2
Identification and
beam transport
80-100MeV/nucleon
pps
Reaction product identification S800 spectrograph
A1900 fragment separator
Production target
Beam energy from cyclotrons MeV/nucleon
Length ~80m (260 ft) (linear distance)

3. (Find) GRETINA in S3

4. GRETINA/S800 data acquisition
NSCL DAQ framework:
Run Control
Online Analysis
(s800) event data traffic
start/stop
start/stop
S800 data
S800 data
online data
GRETINA
DAQ + GEB *
S800 DAQ
data sample
GRETINA +
S800 data
Online analysis
Storage of production data
on GRETINA disk array
*Gretina Event Builder

5. GRETINA/S800 triggering/synchronization
Downscaled, 12.5MHz
GRETINA
clock
Gamma-ray trigger
S800 focal plane trigger
~250ns latency
GRETINA
DAQ
S800 Trigger Logic
S800-GRETINA
Coincidence window: 600ns
Trigger signal issued ~900ns after particle detection in S800
S800 DAQ
Important note:
If GRETINA runs into
deadtime, no busy signal or the like is provided.
Trigger live out

6. GRETINA timing GRETINA leading edge timing GRETINA ‘t0’ timing: 600 ns
This timing quality is available as prompt signal (~200 ns latency) for a trigger logic.
GRETINA ‘t0’ timing:
This timing quality is available after decomposition, i.e. in the data analysis.
Coincidence with 1.3 MeV γ ray
of 60Co source
energy
energy
600 ns
600 ns

7. T0 timing quality at low energies
Ge-Ge MeV:
14ns
100keV
200keV
~40%
~15%
Coincidence with 1.3 MeV γ ray
of 60Co source
300keV
400keV
400 ns
~5%
<5%
T0 comes from decomposition fit. This means that a significant amount of low-energy events were fitted badly by decomp. On the other hand, for events > 400keV fits do well in the regard.

8. Singles efficiency: Treat GRETINA as independent crystals
(and ignore scattering and sum-peak effects)

9. I love low-activity sources!
Here, 60Co at 0.3uCi does <0.2% randoms (time gated)
This is as close as you get to a ‘single gamma GEANT’
(Note: angular correlation does have few % impact)
Gretina gated on 1332keV in LaBr:Ce
Thought on tracking:
Assume average detector fold 5 and 10kHz per detector, i.e. crystal livetime 0.9 (otherwise energy pile-up).
 Only 60% chance that all 5 detectors deliver ‘good’ information, especially interaction points for tracking. In other words on 40% of the events, tracking works on an incomplete data set
LaBr:Ce

10. P/T calorimeter and add-back
Calorimeter for 1173keV P/T = 0.394
Calorimeter for 1836keV P/T = 0.355 SE
511 DE
‘wrong’ fep subtraction
Note: 1.53/1.42 = 93%, i.e. addback recovers 90+% of calorimeter efficiency. (at low multiplicity)

11. Doppler-shift reconstruction
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

12. FWHM after Doppler-shift reconstruction
full GRETINA
FWHM at 1.78MeV:
16keV, 0.9%
Measured FWHM of 0.9%
translates (naively) in a spatial resolution of σ=1.8mm
Energy [kev]

13. FWHM correlated with angle
Doppler Broadening is driven by the uncertainty of:
spatial resolution of γ detection
velocity of emitter
direction of emitter
beam spot size
Measured FWHM vs. angle
defines an effective angle resolution of 14 mrad
 σ ≈ 1.2 mm
FWHM [keV] of 1779keV line Δθ Δβ
Beam spot
Polar angle [rad]

14. Comparison with SeGA
Meaning: ‘Good’ Doppler-correction we did already before tracking

15. That’s GRETINA’s advantage over SeGA
64Ge γ-γ, produced from 65Ge on 9Be at v/c=0.4
plain singles
tracked*
* Tracked and provided
by I-Yang Lee
Reduction of Compton background by tracking allows – for the first time – gamma spectroscopy with fast beams with spectral quality comparable to arrays with anti-Compton shield, i.e. GammaSphere.

16. Spectral quality using add-back6+
3991 4+
2754 2+
1368 0+
24Mg

17. Remarks
Tracking arrays are beautiful devices for in-beam spectroscopy with fast beams!
FWHM of the Doppler-reconstructed peaks is dominated by the position resolution (thin target)  Nice testing ground of decomposition (and tracking)
‘Alternative’ (and simple) methods like add-back give already good results in those low-multiplicity data