Gas Detectors and Neutron Detection Tao Yi

Outline Gas Detector Neutron Detection Energy loss Proportional Counters MWPC Neutron Detection Classification Artificial Source

Energy loss Detection of charged particles only e.m. interaction dominate process：incoherent Coulomb interactions (incoming & medium) average differential energy loss (loss per unit length) due to Coulomb interactions Bethe-Bloch formula

Energy loss Bethe-Bloch formula: with I： Effective ionization potential， for Z > 12, I ≈ (10±1eV)×Z When E↗，(relativistic effect) transverse electric field ↗，screening effect↗ ⇒ energy loss↘ for high energy particles： Particle data Group. Euro.Phys.J.,C,1998.3 NA：Avogadro常数；re：Bohr radius(electron)； 入射：z；介质：A、Z； x=Xp，called reduced medium thickness； I：depend on the material of the medium (Z) 图片：能损vs 入射粒子动量 入射粒子的横向电场、原子核外电子的屏蔽效应 δ term：Density Effect

Energy loss decrease ~ (1/β2) minimal region --minimum at βγ≈3.2 increase ~ 2lnβγ (βγ > 4) increase ~ lnβγ --density effect constant tail --Fermi plateau Particle data Group. Euro.Phys.J.,C,1998.3 explanation for fermi plateau：入射粒子将很大部分能量传递给少数几个电子，这些电子又从介质层中逸出，很少的能量沉积下来。

Energy loss distribution thin material --non Gaussian Landau distribution λ: normalized deviation F.Sauli δE: actual loss, mp:most probable

Proportional Counters E(r): depend on the geometry A cylindrical coaxial ~ 1/r A spherical homocentric ~ 1/r2 Avalanche: A single primary electron proceeds towards the anode, inregions of increasingly high fields, experiencing ionizing collisions; due to the lateral diffusion, a drop-like avalanche, surrounding the wire, developes. Electrons are collected in a very short time (1 nsec or so) and a cloud of positive ions is left, slowly migrating towards the cathode. Time development of an avalanche in a proportional counter

Proportional Counters Signal character contribution of electrons to the total signal is very small typical values a = 10μm b = 10mm λ = 1μm

Proportional Counters Recombination before collection Ionization chamber Q = Ne Proportional counter M ~ > 104 VT：voltage threshold Geiger-Miller counter I:复合占主导；II:复合损失可忽略，收集Q；III：2个阶段，普通正比区&有限正比区；电压过高，引起其他过程破坏正比收集

Multiwire Proportional Chambers Location 2mm ,20um

Multiwire Proportional Chambers 每个阳极平面的信号只给出一维坐标，两个正交的阳极平面才给出x-y信息

Neutron Classification Energy range Interaction forms with nucleus slow neutron 0~1keV elastic scattering (n,n) neutron capture (n,γ) intermediate-energy neutron 1keV~0.5MeV fast neutron 0.5~10MeV inelastic scattering (n,n') nuclear reaction (n,α) or (n,p) express neutron 10~50MeV all of above and (n,2p)、(n,pn)

average energy of neutron/MeV neutron gain /(106·s-1·Ci-1) Neutron Source Isotope source 1.(α,n) type: Source name T1/2 average energy of neutron/MeV neutron gain /(106·s-1·Ci-1) γ background 138.38d 4.2 2.3~3.0 <0.1 1600a 3.9~4.7 10.0~17.1 60 24100a 4.5~5.0 1.5~2.7 <1 432.2a 5.0 2.2~2.7

Neutron Source Isotope Source 2.(γ,n)type 3.spontaneous fission type Accelerator souce ……

References F.Sauli, Principles of Operation of Multiwire Proportional and Drift Chambers, CERN 77-09 Yigang Xie et al., Particle Detectors and Data Procurement, Science Press, 2003 I.Giomataris et al., A novel large-volume Spherical Detector with Proportional Amplification read-out, arXiv: 0807.2802v1 B.X.Zhang and Z.Zhang, Nuclear Radiation Physics and Observation, Harbin Engineering University Press, 2011 M.J.Carson et al., Simulations of neutron background in a time projection chamber relevant to dark matter searches, Nucl.Instrum.Meth.A546:509-522, 2005.

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