1. Gas Detectors and Neutron Detection
Tao Yi

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

3. 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

4. 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

5. 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。

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

7. 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

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

9. 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

10. Multiwire Proportional Chambers
Location
2mm ,20um

11. 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

12. 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)

13. 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

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

15. 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: v1
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: , 2005.

16. Thanks for your attention!