Particle Physics Booklet - Particle Data Group - Lawrence Berkeley ...

ΛI/ρ (cm)
50
40
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28. Detectors at accelerators 253
Fe Cu Ru Pd W Au Pb U
0
0
20 30 40 50 60 70 80 90 100
Z
Figure 28.21: Nuclear interaction length λI /ρ (circles) and
radiation length X0/ρ (+’s) in cm for the chemical elements with
Z>20 and λI < 50 cm.
extends detector lifetime. The higher mobility of electrons makes them
less sensitive to carrier lifetime than holes, so detector configurations that
emphasize the electron contribution to the charge signal are advantageous,
e.g., n + strips or pixels on a p-substrate. The occupancy of the defect
charge states is strongly temperature dependent; competing processes
can increase or decrease the required operating voltage. It is critical
to choose the operating temperature judiciously (−10 to 0◦Cintypical collider detectors) and limit warm-up periods during maintenance. For
a more detailed summary see Ref. 117 and and the web-sites of the
ROSE and RD50 collaborations at http://RD48.web.cern.ch/rd48 and
http://RD50.web.cern.ch/rd50. Materials engineering, e.g., introducing
oxygen interstitials, can improve certain aspects and is under investigation.
At high fluences diamond is an alternative, but operates as an insulator
rather than a reverse-biased diode.
28.9. Calorimeters
A calorimeter is designed to measure the energy deposition and its
direction for a contained electromagnetic (EM) or hadronic shower. The
characteristic interaction distance for an electromagnetic interaction is
the radiation length X0, which ranges from 13.8 g cm −2 in iron to 6.0 g
cm −2 in uranium.* Similarly, the characteristic nuclear interaction length
λ I varies from 132.1 g cm −2 (Fe) to 209 g cm −2 (U). † In either case,
a calorimeter must be many interaction lengths deep, where “many” is
determined by physical size, cost, and other factors. EM calorimeters tend
to be 15–30 X0 deep, while hadronic calorimeters are usually compromised
at 5–8 λ I. In real experiments there is likely to be an EM calorimeter
in front of the hadronic section, which in turn has less sampling density
in the back, so the hadronic cascade occurs in a succession of different
structures.
* X0 = 120 g cm −2 Z −2/3 to better than 5% for Z>23.
† λI =37.8 gcm −2 A 0.312 to within 0.8% for Z>15.
See pdg.lbl.gov/AtomicNuclearProperties for actual values.
ΛI
X0
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X0/ρ (cm)