Using Magboltz

Introduction

LINUX PC version uses DRAND48 double precision random no generator (can also use RNDM2 from CERNLIB with the same precision).

The program allows anisotropic elastic and inelastic scattering, reference: NIM A 421 (1999) 234-240 The gas data base list below shows those x-sections which contain anisotropic scattering data.

PostScript plots of the database x-sections can be obtained on:- http://cern.ch/magboltz/cross.

This version allows spatial gradients to be included in the solution for the Townsend gain and attachment coeficients. The program automatically gives a solution with spatial gradients for both time of flight (TOF), pulsed townsend (PT) and steady state townsend (SST) parameters. The nomenclature is similar to Sakai et al. J. Phys. D10 (1977) 1035. The simulation of avalanche gain detectors at high field requires the use of SST Townsend parameters. The program automatically updates the common blocks /CTOWNS/ and /CTWNER/ with the SST parameters if the spatial gradients are greater than: | alpha - attachment | = 60/cm at NTP. where alpha is the Townsend coefficient and attachment the attachment coefficient. (For smaller values of | alpha - attachment | < 60/cm, the change of the gain or attachment is typically less than 3 % for the solution without spatial gradients).

Estimates of the Penning effect at high field can be obtained by inspection of the detailed collision frequencies for excited states in the gas mixtures. Penning effects can occur between excited states in the gas mixture which are higher in energy than the lowest ionisation potential in the mixture. Previous results in Argon hydrocarbon mixtures show a transfer efficiency of about 25 % from excited states in Argon to ionisation of hydrocarbons.

The program should always converge to a solution. the error on the integration should scale with the square root of the number of collisions (parameter NMAX). The output should always be checked to ensure that the number of collisions in the last energy bin is small, any value less than 500 should give reasonable systematic errors if the number of collisions is greater than 500 then the integration energy range should be increased.

The program is limited in precision by the statistical accuracy of the results. It is possible to obtain a statistical accuracy of better than 0.1 % on the drift velocity and 1 % on the diffusion coeficients in most counting gas mixtures in about 30 seconds of computing time on a PC, Alpha or workstation. At high field when the Townsend coeficient is included in the spatial gradient the computation time may be required to increase to a few minutes. When velocity vectors are small such as the case with small Lorentz angles the parameter NMAX will need to be increased to 20 or more.

Geometry

The results of the calculation are loaded into common blocks:

COMMON/VEL/WX,WY,WZ
COMMON/VELERR/DWX,DWY,DWZ
COMMON/DIFLAB/DIFXX,DIFYY,DIFZZ,DIFYZ,DIFXY,DIFXZ
COMMON/DIFERB/DXXER,DYYER,DZZER,DYZER,DXYER,DXZER
COMMON/DIFVEL/DIFLN,DIFTR  
COMMON/DIFERL/DFLER,DFTER
COMMON/CTOWNS/ALPHA,ATT    
COMMON/CTWNER/ALPER,ATTER

• WX,WY,WZ are the drift velocity vectors
• DIFXX,DIFYY,DIFZZ,DIFYZ,DIFXY,DIFXZ are the values of the diffusion tensor in the cartesian coordinate system. Note: off-diagonal elements are defined so that the coefficients are equal, DIFXY=DIFYX, DIFXZ=DIFZX and DIFYZ=DIFZY.
• DIFLN,DIFTR,DIFXX are the diffusion coefficients in the coordinate system aligned along the drift direction (it is only calculated for the case where the magnetic field is at 90° to the electric field). If there is no magnetic field the values DIFLN and DIFTR represent the longitudinal and transverse diffusion.

• velocity: cm/sec
• diffusion: cm**2/sec
• ALPHA: 1/cm
• ATT: 1/cm
• calculation errors: % of value.

INPUT CARDS

First card

• NGAS: number of gases in mixture;
• NMAX: number of real collisions (multiple of 10**7), use a value between 2 and 5 for inelastic gas to obtain 1 % accuracy, use a value above 10 for better than 0.5 % accuracy and a value of at least 10 for pure elastic gases like Argon;
• EFINAL: upper limit of the electron energy in electron Volts, if EFINAL = 0.0, program automatically calculates upper integration energy limit.

Second card

• NGAS1 etc.: gas number identifiers (between 1 and 80) see gas list below for identifying numbers.

Third card

• FRAC1 etc.: percentage fraction of gas1 etc.;
• TEMP: temperature of gas in centigrade;
• TORR: pressure of gas in Torr.

Fourth card

• EFIELD: electric field in Volt/cm;
• BMAG: magnitude of the magnetic field in kilogauss;
• BTHETA: angle between the electric and magnetic fields in degrees.

Card 4*N+1

Gas numbers

Author