CERN Program Library Long Writeup W5013 - CERNLIB ...

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2.2 Recommendations The full energy loss fluctuations or the restricted fluctuations complemented by the production of δ-rays give equivalent results if the number of δ-rays generated is sufficient, e.g. a few tens along the full trajectory of the particle in the medium. For a relativistic particle, the number of delta-rays produced per cm can be estimated integrating the cross section provided in [55]: dN dx ≈ D ρZ Zinc 2 2 A DRCUT where (1) D 0.307 MeV cm 2 g −1 ; ρ density of the medium; Z, A atomic number and atomic weight of the medium; Z inc charge of the particle; DRCUT energy threshold for emitted delta-rays. This formula holds for electrons/positrons as well as for other particles. The number of δ-rays must be sufficient to reproduce the statistical fluctuations in the energy loss, but not too large, as in this case a large amount of time could be spent in tracking them without a corresponding improvement of the energy loss distribution. On the other hand, the distribution of the energy lost by a particle does not necessarily correspond to the distribution of the energy deposited by the same particle in the medium traversed. This is particularly true for light or thin materials where δ-rays can escape into the next material. It is the responsibility of the user to estimate the number of δ-rays per cm that are needed, according to the considerations above. Then the correct value for DRCUT should be set with the help of GDRPRT. Itis recommended to use the same value for DCUTE and DCUTM. As the GEANT tables for cross-sections are not generated below EKMIN, DRCUT cannot be smaller than EKMIN. The value of EKMIN can be changed via the ERAN data record, and its default value is 10 keV. In a light material like gas, the number of emitted δ-rays for DRCUT =10keV may not be sufficient to ensure a correct energy loss distribution. If the user is mainly interested in the energy lost by the particle, no discrete δ-ray should be produced setting ILOSS =2(see [PHYS332]). In case the user is more interested in a precise simulation of the energy deposited, then the explicit δ-ray generation should be tried. As far as thin materials are concerned (∆ < 50), the default in GEANT is to use the Urbán model. Comparison with experimental data have shown that this model gives very good results, and it is considerably faster than both the ASHO and the PAI models. Users should understand well their setup and their results before they try with different models, which, in principle, should give very similar results. 2.3 Default values in GEANT In order to avoid double counting, there is an automatic protection in the code: if ILOSS=2, IDRAY is set to 0 and the generation of δ-rays is disabled. The following table summarises the different cases: LOSS=2 LOSS=1 (default) IDRAY 0 1 DCUTE 10 4 GeV CUTELE DCUTM 10 4 GeV CUTELE Where LOSS= 1means that the restricted fluctuation is activated and LOSS= 2means that the complete Landau/Vavilov/Gauss fluctuations are used in the region ∆ ≥ 50. PHYS333 – 4 270
Geant 3.21 GEANT User’s Guide PHYS334 Origin : I.Gavrilenko, P.Nevski, K.Lassila-Perini Submitted: 11.03.94 Revision : Revised: 11.03.94 Documentation : K.Lassila-Perini 1 Subroutines Models for energy loss fluctuations in thin layers CALL GSTINI GSTINI initializes the tables that are used for sampling of the energy loss, if the user has set the flag STRA = 1. It calls the subroutine GSTXIN to integrate an expression for the number of collisions and GSTTAB for preparing the tables for different Lorentz factors. It is called from GPHYSI at initialization time. CALL GSTXIN GSTXIN computes the values needed for the sampling tables. It uses the functions GOSCIN to integrate over the photoelectric cross-sections and the function GKOKRI to compute the real part of the complex dielectric constant. GSTXIN is called from GSTINI. VALUE = GOSCIN (EIN1EV,EIN2EV) GOSCIN integrates the parameterization of the photoelectric cross-sections from energy EIN1EV to energy EIN2EV. It is called from GSTXIN. VALUE = GKOKRI (E,EMINEV,EMAXEV) GKOKRI computes the real part of the complex dieletric constant which is needed in the preparation of energy loss sampling tables. E in the energy for which the values are being tabulated, EMINEV and EMAXEV are the integration limits. GKOKRI is called from GSTXIN. CALL GSTTAB (GAM,NT,EN,FN) GSTTAB prepares the energy loss sampling tables for different Lorentz factors. The input GAM is the Lorentz factor, the output values are: NT the lenght of the table, EN the energy array and FN the cumulative probabilty array. It calls GXGINT to integrate over the probability function GSTDN to get the value of FN. GSTTAB is called from GSTINI. VALUE = GXGINT (EXT,A,B,EPS) GXGINT performs the Gaussian integration from A to B over the function EXT with accuracy EPS. It is called from GSTTAB to integrate over the probability function GSTDN. VALUE = GSTDN (LGE) GSTDN is the function which gives the probability for a collision with energy transfer LGE. It is integrated to get the cumulative probabilty function to be used in the sampling of the energy loss. CALL GSTREN (GAMMA,ECUT,STEP) GSTREN computes the energy loss for a particle of Lorentz factor GAMMA in a step of length STEP if the user has set the flag STRA = 1. ECUT is the cut for the δ-ray production. GSTREN is called from GFLUCT. 271 PHYS334 – 1
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CERN Program Library Long Writeup W
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Chapter 1: Catalog of Geant section
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IOPA500 KINE001 KINE100 KINE199 KIN
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Geant 3.21 GEANT User’s Guide AAA
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GUDIGI GUOUT GTRIGC UGLAST GLAST GU
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SUBROUTINE UGEOM + (’TGT ’,2,
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LQ(JHEAD-1) user link IQ(JHEAD+1) I
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