Radiation length

Energy loss of electrons above 100 MeV is dominated by nuclear encounters producing bremsstrahlung. This process, characterized by the radiation length R within which most of the energy is lost, is independent of particle energy. 

Thus, the average stopping power is proportional to the initial energy except for corrections due to atomic collisions (electronic excitation) near 108 eV. For a medium of nuclear charge Ze and mass number A, the radiation length is given by (Bethe and Ashkin, 1953)... 

Bulb of different radiation length in an optimized compact housing... 

During the run, the 0.6 mm (1.8-10-3 radiation length) plexiglass converter (C) was periodically inserted in the channel before the spectrometric magnet. 

The calorimeter was set up at the H4 beamline of the Super Proton Synchrotron (SPS) at CERN and was tested with beams of protons, pions, electrons, and muons with energies between 15 and 350 GeV Plewnia et al 2004. Protons and pions could not be distinguished, they are treated as hadrons, as in the air shower experiment. To identify electrons, a lead plate (15 mm thick, corresponding to three radiation lengths) has been placed in front of the first layer of ionization chambers and the signal in this chambers was used to select primary electrons. Contaminations of muons and pions in the electron beam could be efficiently rejected. 

Neutron-protective properties of a material are determined by the radiation length Z of a neutron in medium—the mean length of the path over which its energy falls to Me of its valne as a resnlt of collisions. Thus, / is the effective thickness of the protective layer. Neutron radiation length is dependent on the concentration n of centers interacting with neutrons and cross section o of an elemental act of interaction ... 

Because the total cross section of the "B-n interaction is negligible if compared with that of the °B-n interaction, let us assume that a boron-containing material used for neutron shielding is B monoisotopic (otherwise, the calculated neutron radiation length should be divided by the relative concentration of °B). In this case, in the logarithmic scale, the energy dependence of the minimal radiation length is expressed by the linear function... 

The primary components of the GLAST pair conversion telescope are the solid state (silicon strip) tracker and the segmented Csl calorimeter. The tracker has 12 conversion layers (0.5 radiation lengths) plus a front veto... 

Broad Energy Range Low and High E Spectral Breaks or Continuations Broad Pion Bump at 70 MeV Reduce Radiation Lengths (Low Energies) Increase Area and Prevent Self Veto (High Energies)... 

There is a simple formula to estimate the radiation lengths of elements (Bichsel et al. 2002) ... 

Note that the radiation length cannot be defined for low-energy electrons as their rest mass We cannot disappear and the kinetic energy may not be large enough to cover the energy loss required. 

Contributions of various processes to the relative rate of energy loss for electrons and positrons in lead. On the left scale x is given in units of radiation length (Xo(Pb) = 6.37 g/cm ), whereas on the right one it is in g/cm ... 

The expected performance of the track reconstruction is shown in Fig. 2.4 for muons, pions and hadrons. The track reconstruction efficiency for high energy muons is about 99% and drops at 7 > 2.1 due to the reduced coverage of the forward pixel detector. For pions and hadrons the efficiency is in general lower because of interactions with the material in the tracker. The material budget of the CMS tracker in units of radiation length is presented in Fig. 2.5. 

The radiation length Xq is a characteristic of a material, related to the energy loss of high energy electrons in the material. 

Fig. 2.5 Material budget of the CMS tracker in units of radiation length Xo as a function of pseudorapidity divided into the contributions of the different subdetectors [4]...

Charged particle tracking and momentum measurement are done with a large central drift chamber. Because the charged particle momentum spectrum is soft, increased attention to reduction of the numVjer of radiation lengths presented by the structure, wires and gas of the drift chamber pays off handsomely in improved momentum resolution. 

A Fast RICH is the reference particle identification system. The R D on this technology has been pursued primarily, but not exclusively, by the European Fast RICH Collaboration. A Fast RICH with a liquid radiator and pixel readout can provide 4cr separation for particles from two-body B meson decays over most of the solid angle. An important concern here is whether a sufficient number of Cerenkov photons can be detected in a ring with adequate angular resolution, with a device having a 20 cm radial extent and 15% of a radiation length at 90°. 

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