1. Ion signals with R134a and R134 in a parallel plate proportional counter Edwin Norbeck, J. E. Olson, Y. Onel University of Iowa For Sec. DH-3 of DNP06 Nashville, TN

2. E. Norbeck U. IowaDNP06 DH-3 Ion Signals2 OUTLINE Detector: Atmospheric pressure proportional counter made of two 1 cm 2 flat plates separated by 0.5 mm Gas: R134a, a heavy freon used in air conditioners a common detector gas with large electric dipole moment Operation: avalanche mode Signals: fast electron signal; slow, small ion signal Effect of dipole moment on signals Comments and Conclusions

3. E. Norbeck U. IowaDNP06 DH-3 Ion Signals3 DETECTOR Atmospheric pressure proportional counter made of two 1 cm 2 flat plates separated by 0.5 mm and about 2200 kV. PPAC (Parallel Plate Avalanche Counter) The minimum width of a PPAC signal coupled into 50  coax is the RC time constant with R = 50  and C being the capacity between the detector plates. For our small,1 cm 2 PPAC, RC = 0.09 ns. The 1 ns pulse width of the electron signal is determined by the electron transport time.

4. E. Norbeck U. IowaDNP06 DH-3 Ion Signals4 Signal into coax with no amplifier Signal observed directly with fast scope

5. E. Norbeck U. IowaDNP06 DH-3 Ion Signals5 R134A is the best gas we have found so far (after testing dozens of gasses and gas mixtures) Tetra-fluoro-ethane Does not harm ozone layer because it contains no chlorine Used in automobile air conditioners We found automobile grade as good as CP grade Excellent for anti-aging (use gold plated electrodes) Little hydrogen (get big signals from n-p) Large stopping power (MW = 102) Also used in: In other H.E.P. gas detectors As Čerenkov gas for particle I.D. F 3 CCH 2 F

6. E. Norbeck U. IowaDNP06 DH-3 Ion Signals6 OPERATION + + + + + + ++ + + + + + + + + + + - - - - - - - - - Anode Cathode Avalanche An electron gains enough energy between collisions to ionize another gas molecule. Most electrons formed close to anode, many within one mean free path. The ions travel most of the plate spacing, slowly, with many collisions.

7. E. Norbeck U. IowaDNP06 DH-3 Ion Signals7 R134A has a large electric dipole moment, about the same as water. Because of the large electron affinity of fluorine (it wants to have the electronic structure of the noble gas, neon) the (blue) fluorine atoms have a large negative charge. With three F atoms on one end and only one on the other, the left end is much more negative than the right end, hence a large dipole moment. To make the almost identical molecule, R134, with no dipole moment, move one F atom to the other end of the molecule. F 3 CCH 2 F

8. E. Norbeck U. IowaDNP06 DH-3 Ion Signals8 Tetra-fluoro-ethane F 3 CCH 2 F F 2 HCCHF 2 Large electric dipole moment (about the same as for water) Symmetric No dipole moment Fluorine atoms (blue) attract electrons and so are negative R134aR134

9. E. Norbeck U. IowaDNP06 DH-3 Ion Signals9 The performance of the detector is identical for the two gasses except for the ion transit time, which is longer for the gas with the large electric dipole moment.

10. E. Norbeck U. IowaDNP06 DH-3 Ion Signals10 Signal Shape 37 mV with 1.0ns width 1.8 ns for R134a 1.3 ns for R134 ions electrons

11. E. Norbeck U. IowaDNP06 DH-3 Ion Signals11 + - + Moving ion slowed by cloud of polar molecules Symmetric C 2 F 4 H 2 has normal ion speed

12. E. Norbeck U. IowaDNP06 DH-3 Ion Signals12 Comments and conclusions The long ion tail is so small that it can generally be ignored. The presence of ions in the gas does not interfere with a new event because they a confined to a small region around the particle track. R134a is often used as the major component of a mixed gas in RPCs (Resistive Plate Chambers, which are PPACs with the plates made of a material with a high resistance instead of metal) The only observable effect of the large dipole moment of R134a is the increased duration of the ion signal.