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Ionization. Measuring Ions A beam of charged particles will ionize gas. –Particle energy E –Chamber area A An applied field will cause ions and electrons.

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Presentation on theme: "Ionization. Measuring Ions A beam of charged particles will ionize gas. –Particle energy E –Chamber area A An applied field will cause ions and electrons."— Presentation transcript:

1 Ionization

2 Measuring Ions A beam of charged particles will ionize gas. –Particle energy E –Chamber area A An applied field will cause ions and electrons to separate and move to charged plates. –Applied voltage V –Measured current I I V   A E

3 Saturation Ion – electron pairs created will recombine to form neutral atoms. –High field needed to collect all pairs –V > V 0 Uniform particle beam creates constant current. –Saturation current I 0 I0I0 V0V0 I V Ion recombination Saturation

4 Saturation Current A uniform beam is defined by fluence rate and energy. –Intensity is the product The number of ions depends on the gas. –Ionization energy W The saturation current is proportional to intensity Energy per area per time: The number of ion pairs N is: The saturation current is:

5 Ionization Energy W values measure the average energy expended per ion pair. –Electrons uniform with energy –Protons above 10 keV similar to electrons W for heavy ions increases at low energy. –Excitation instead of ionization GasW  W  (eV/ion pair) He4342 H 2 3636 O 2 3331 CO 2 3633 CH 4 2927 C 2 H 4 2826 Air3634

6 Electrometer Typical Problem A good electrometer can measure a current of 10 -16 A. What is the corresponding rate of energy absorption in a parallel-plate ionization chamber with W = 30 eV/ip? Answer The energy rate is related to the intensity. –(10 -16 C/s)(30 eV)/(1.6 x 10 -19 C) = 1.88 x 10 4 eV/s –Equivalent to one 18.8 keV particle per second

7 Smoke Detector Many household smoke detectors are ionization chambers. –Electric field from a battery – 241 Am alpha source (.5 mg) Smoke interrupts saturation current through recombination.

8 Liquid Argon Liquid noble gases can be used in ionization chambers. –Liquid argon, krypton, xenon An applied field of 1.1 MV/m used to suppress scintillation in liquid Ar. Focus on an example from the Dzero electromagnetic calorimeter. Liquid argon parameters –Density 1.41 g/cm 3 –Boiling point 87 K –W value 23.6 ev/ion pair

9 Uranium Cell Uranium plates are alternated with readout pads. –Separated by liquid argon Readout pads are 5-layer printed circuit boards. –Outer readout pads –Inner layer readout wires –Ground planes to reduce crosstalk –Resistive coat at 2.5 kV 4.0 mm 2.3 mm 4.3 mm depleted uranium liquid Ar gaps readout pad

10 Shower Production Uranium acts as an absorber. –Density 19.05 g/cm3 –Interaction primarily in uranium –4 cm for electromagnetic Shower particles ionize liquid argon in the gaps. –Measured on circuit board pads depleted uranium incident particle

11 Sampling Calorimeter The energy loss in the uranium is much greater than in the argon. Ionization is a sample of the particles in the shower. –Readout signal is proportional to a sample of the shower energy. A sampling calorimeter loses some resolution due to statistics. –Gains in flexibility to construct optimal layer thicknesses

12 High Voltage Response

13 Pulse Mode Ionization signals are read out as individual pulses. –Cell is a capacitor C D –Total charge dQ proportional to ionization Charge sensitive preamp integrates current pulse to get charge. –Measure as voltage change   i in v out CDCD CFCF

14 Pedestal Integrating the charge means selecting a sample time and initial voltage. –2.4  s –Subtract the baseline voltage The distribution with no input signal is the pedestal. –Asymmetric due to uranium noise

15 Cell Noise The pedestal is not constant. –Variation of the pedestal contributes to statistical error. –Depends on cell capacitance –High voltage on picks up uranium noise

16 Resolution Total energy for a particle is due to a sum of N channels. Resolution varies with total energy Signal variance S 2 depends on a number of sources of error. –Statistical channel error  –Channel crosstalk error c

17 Electromagnetic Calibration Electrons of known energy are used to calibrate the cells. –Initial digital counts A j –Cell calibration  j –Tower calibration  –Offset for other material  Compare beam momentum to measured energy for various energies.

18 Beam Response

19 Measured Resolution Energy resolution is measured as a fraction  /E. –From mean and standard deviation fit to Gaussian Resolution is fit to a quadratic as a function of momentum –Channel-to-channel variation C –Statistical sampling S –Energy-independent noise N Fit results: –C = 0.003 ± 0.003 –S = 0.157 ± 0.006 –N = 0.29 ± 0.03 Quoted resolution:

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