1. Activities on straw tube simulation
PAVIA group
PANDA meeting, Jülich., 8 September 2009

2. Single straw simulation: remember that… -1
The position and number of electron clusters is sampled from the exponential distribution (25 cluster/cm in Argon).
The number of electrons belonging to a cluster is taken from both experimental and theoretical papers.
The energy lost is obtained from the mean energy spent for a ion pair (27 eV in Ar)
Their position is dispersed according to the GARFIELD diffusion curves.

3. Single straw simulation: remember that… -2
The arrival time of each electron on the wire is derived from the GARFIELD x-t curves.
The arrival of each electron gives rise to a charge, obtained by sampling from a Polya distribution with mean value given by the GARFIELD gain.
The total ADC signal is obtained by summing the charge over the number of electrons.
Gain: (GARFIELD)
sampled from the
Polya distribution
Electrons
on the wire
no electronics
response

4. The formation of the signal ...
Primary electrons arrival times

5. Recent improvements
All this single straw simulation was already performed.
Now, to adapt the existing simulations to the new detector
parameters, we have resumed some studies with GARFIELD
to make available correct distributions with our new parameters.
Until now, we have reproduced the drift velocity and the x-t curves
with different gas mixtures: Ar/CO2 80/20 % and 90/10%
with different voltages: DV = 2150, 2300, 2500 V
with magnetic field (B = 2 T).
For the TDR, it could be useful to continue a systematic work
with GARFIELD by testing the STT in different working conditions.

6. GARFIELD drift velocity: gas mixing studies
Ar/CO2 90/10%
Drift velocity
nearly constant
in a limited region
around the wire
Ar/CO2 80/20%
Drift velocity
never constant
Both cases:
p = 2.2 bar DV = 2150 V B = 2 T
tube diameter = 1 cm anode diameter = 20 mm

7. GARFIELD drift velocity: voltage studies
All cases: Ar/CO2 90/10% P = 2.2 bar B = 2 T
2150 V V V v
(cm/
ms) v
(cm/
ms) v
(cm/
ms)
r (cm) r (cm) r (cm)
An enhancement of the voltage
could slightly reduce the gap in the drift velocity

8. GARFIELD x-t curves for single electron
Drift faster in
Ar/CO2 90/10%
mixture
Ar/CO2 90/10%
B = 2 T

9. Wide dynamical range
“autofocus”

10. FT CF Signal threshold Fixed: above a static value
(ex. 5% of mean value of all the signals)
Constant fraction: a % of max of the signal
(ex. 5% of maximum value of each signal) FT CF
simulated white noise
time (ns)
Warning: this will lead to an offset

11. Single straw simulation: remember that…
The shape of the electrical signal is reproduced taking into account the (gaussian) response to each charge multiplication.
The time is given by a threshold on the impulse, to be determined.
The cumulative of the simulated time histogram gives the response curve r(t) of the tube, and so the spatial resolution curve.
Parallel illumination

12. Self-calibration
No differences between
FT and CF
Significant differences
between the two gas mixtures
Calibration: use a parallel beam and find the time response of the tube
(simulation of the calibration with cosmic rays ):

13. Offset correction during calibration

14. Offset correction during calibration

16. Spatial resolution curves
Fixed threshold:
Max 160 mm
Min about 60 mm
Constant fraction:
Max 110 mm
Min below 50 mm
Residuals: S |( xrec – xtrue )| / N
Curves obtained with
Ar/CO2 mixing
(no difference between
80/20 and 90/10 %)
B = 2 T
Experimental data
(P.Wintz, with B=0)
well reproduced
Mean value
(fixed threshold)

17. Fast simulation

18. ADC simulation
Problem with
GARFIELD

19. Gain measurements
© Peter Wintz

20. Rate measurements
© Peter Wintz

21. dE/dx performances, stt vs tpc
similar results with
1 bar pressure
Supposing
the particles
are hitting
22 tubes
tpc [Panda TPR]

22. Summary I (from GARFIELD)
With Ar/CO2 90/10 % gas mixture, compared to 80/20 % one,
the drift process is faster
the drift velocity is nearly constant
(at least over a limited region of about 200 mm around the anode wire)
A little enhancement of the drift velocity could also be obtained by increasing the voltage up to 2500 V.
The GAIN calculation presents some problem, not yet fully
understood

23. Summary II (from the simulation code)
Self calibration and offset correction is important for achieving good resolution
(mean value with fixed threshold: about 90 mm).
The gas mixture affects the time response
but does not affect the spatial resolution
The spatial resolution improves by determining the signal threshold by the constant fraction method