Ion signals with R134a and R134 in a parallel plate proportional counter

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. OUTLINE • Detector: Atmospheric pressure proportional counter made of two 1 cm2 flat plates separated by 0.5 mm • Gas: R134a, a heavy freon used in air conditionersa 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 DNP06 DH-3 Ion Signals

3. DETECTOR • Atmospheric pressure proportional counter made of two 1 cm2 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 W coax is the RC time constant with R = 50 W and C being the capacity between the detector plates. For our small,1 cm2 PPAC, RC = 0.09 ns. • The 1 ns pulse width of the electron signal is determined by the electron transport time. DNP06 DH-3 Ion Signals

4. Signal into coax with no amplifier Signal observed directly with fast scope DNP06 DH-3 Ion Signals

5. 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. F3CCH2F DNP06 DH-3 Ion Signals

6. - - + + + - + - + + + - + + - - + + + - + + + + - + + Anode Cathode OPERATION 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. DNP06 DH-3 Ion Signals

7. 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. F3CCH2F DNP06 DH-3 Ion Signals

8. Tetra-fluoro-ethane R134a R134 F3CCH2F F2HCCHF2 Large electric dipole moment (about the same as for water) Symmetric No dipole moment Fluorine atoms (blue) attract electrons and so are negative DNP06 DH-3 Ion Signals

9. 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. DNP06 DH-3 Ion Signals

10. Signal Shape ions 1.8 ns for R134a1.3 ns for R134 electrons 37 mV with 1.0ns width DNP06 DH-3 Ion Signals

11. Moving ion slowed by cloud of polar molecules + - + - + - + + - + - + - + - Symmetric C2F4H2 has normal ion speed DNP06 DH-3 Ion Signals

12. 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. DNP06 DH-3 Ion Signals