1. Performance of timing-RPC prototypes with relativistic heavy ions
C. Paradela1, Y. Ayyad1, J. Benlliure1, E. Casarejos2, I. Durán1, N. Montes1, J.R. Pereira
1. Universidad de Santiago de Compostela, Spain
2. Universidad de Vigo, Spain 1

2. Introduction
The use of RPCs was proposed within the R3B experiment of FAIR for building a time-of-flight wall for relativistic heavy ions (iTOF).
Large-area tRPCs present excellent time resolution for minimun ionization particles (~ 50 ps s), but their performance with heavy ions is almost unknown. 2

3. Introduction
The use of RPCs was proposed within the R3B experiment of FAIR for building a time-of-flight wall for relativistic heavy ions (iTOF).
Large-area tRPCs present excellent time resolution for minimun ionization particles (~ 50 ps s), but their performance with heavy ions is almost unknown.
iTOF main requirements:
30 ps for the full iTOF (50 ps for one RPC)
Efficiency for heavy ions close to 100 % [1,2].
In this work, we complete the R&D studies providing the time resolution obtained with double- gap and single-gap prototypes.
[1] Y. Ayyad et al., Nucl. Instrum. Meth. 661, S141
[2] E. Casarejos et al. Nucl. Instrum Meth. in press 3

4. RPC prototypes 12 cm 40 cm Small-size tRPC concept:
- Symmetric double-gap stack (300 mm nylon).
- Single-strip (2 cm self-adhesive copper tape).
- HV applied to the inner strip while outer ones are grounded.
- Own-developed amplification electronics placed at both edges and specific DAQ.
- Isobutane-free gas mixture: 90% R134a, 10% SF6
Protoptype test: Two tRPCs separated for a few cm, placed inside a gas-tight aluminum box.
Medium-size tRPC concept:
- Single gap RPC.
- Multi-strip (2 mm interstrip space).
- Improved amplification electronics.
- No gas container. Tightness assured by gluing the glass perimeter.
- A pair of RPCs coupled in one stack.
12 cm
40 cm 4

5. Electronics & DAQ 40 MHz clock QDC FEE TACQUILA
RPC signal amplification:
- Pre-amplifier card (Transistor+MAXIM chips) with adapted impedance placed next to the strip.
- FOPI FEE card as part of a TACQUILA in a VME-based acquisition.
TACQUILA provides the high-accurate time information (<15 ps intrinsic resolution) and a piggy-back QDC card, the charge information.
FOPI DAQ optimised for small signals. Amplification stage saturates for hundreds of mV signal.
A coarse calibration of TACQUILA QDC shows a saturation effect for large signals (>100 mV) due to the FOPI FEE.
QDC
FEE
TACQUILA 5

6. Test with electron beams
Bunches of 10 MeV electrons at ELSA (Bruyeres-le-Chatel):
Low rate: 5 Hz
2 mm diameter beam spot.
10 ps FWHM bunch length
Beam intensity modulated within 2 orders of magnitude.
Trigger and time reference provided by a fast-timing plastic scintillator.
Energy deposition (and charge signal) per bunch comparable to a heavy ion, but in a “large” spot.
Time resolutions below 100 ps for all the beam intensities.
Improved time resolution with beam intensity (up to 30 ps). 6

7. Test with an Uranium beam
Small prototype at FRS (1 GeV/u 238U)
Beam spot of a few cm diameter
Ion rates down to several tens of ions/s (10-30 Hz/cm2).
Self-triggered setup in parasitic mode.
Data analysis:
Tails in RPC position removed to clean the scattered particles.
70 ps time resolution obtained from time-of-flight between both RPCs.
Comparable resolution to electron results when considering same conditions (same signal charge and method calculation) 7

8. Test with a Xenon beam
Last prototype test with the medium size single-gap RPC in GSI Cave C.
500 MeV/u 136Xe beam in dedicated mode.
Measurements performed at very low rate (<10 Hz/cm2).
Time resolution performance studied using only one strip in each RPC. Independent trigger with two plastics in coincidence.
Double slewing correction performed for each time-of-flight.
Preliminary analysis confirms the excellent time resolution obtained for previous prototypes. Best results (46 ps at 1.5 ) around 2900 V where efficiency is larger than 95 %.
Large 3 tails (~10%) (important contribution of secondary reactions in different layers) and strong dependence with the HV. 8

9. Rate effects Rate effects
Splitting the statistics in two rate ranges: 2 Hz/cm2 and 5 Hz/cm2, we can already observe important differences:
The time resolution significantly worsens at the higher rate.
Signal charge decreases at higher rate.
As expected the rate effect with heavy ions is much more important than for MIPs (5 Hz/cm2 vs. 1 kHz/cm2).
2 Hz/cm2
5 Hz/cm2
2 Hz/cm2
5 Hz/cm2
5 Hz/

10. Streamers contribution
No direct evidence of streamers with TACQUILA- QDC information.
But, they were observed on- line and in previous tests.
Their contribution to the time resolution confirms that observation.
Avalanche signals too large for FEE (charge saturation).

11. Conclusions
Two different type of tRPCs have been tested in beams for studying their time resolution performances.
Resolutions close to 40 ps have been obtained for reduce ion rates (a few Hz/cm2), but they are worse for increasing rates.
Time resolution also worsens drastically for the high voltage region where streamers are expected (above 100 kV/cm).
For a HV plateau about 2.9 kV, the single-gap RPC prototype we have tested fulfill the main requirements: 95 % efficiency and 50 ps  for one detector at less than 10 Hz/cm2.
Tests with short electron bunches are a good option to characterize the RPC electronics performance in similar charge conditions than ions, but cleaner beam conditions. 11