中子与物质的相互作用

（2）中子的性质 质量：mn＝1.008665u＝939.565300MeV/c2 自旋：sn＝1/2, 费米子 电荷：0，中性粒子 磁矩：n＝－1.913042N 中子寿命：发生－衰变的半衰期T1/2=10.60min
3 中子与物质的相互作用
中子与物质的相互作用实质上是中子与物质 的靶核的相互作用。
第四讲
射线与物质的相互作用（4）
Radiation Interactions with Matter
Baidu Nhomakorabea
中子与物质的相互作用
1 Neturon Source
A. Spontaneous Fission 自发裂变源 1) The most common spontaneous fission source is 252Cf; 2) Half-time:2.65y; 3) the dominant decay mechanism is α decay; 4) the energy spectrum of neutrons is plotted in fig. 1-10.
9 4 2 1
Be h Be n
8 4 1 0 1 1 1 0
Q 1.666MeV Q 2.226MeV
H h H n
Notes:
1) The main disadvantage of photoneutron sources: It is needed that very large gamma-ray activities must be used in order to produce neutron sources of attractive intensity. one gamma ray (105 or 106) → one neutron 2) Some of the more common gamma-ray emitters are 226Ra, 124Sb, 72Ga, 140La and 24Na.
2 中子的分类与性质
（1）中子的分类 1) 慢中子：0～1KeV。包括冷中子、热 中子、超热中子、共振中子。
热中子：与吸收物质原子处于热平衡状态， 能量为0.0253eV，中子速度~2.2×103m/s.
2) 中能中子：1KeV～0.5MeV。 3) 快中子：0.5MeV～10MeV。 4) 特快中子：>10MeV。
（1） 中子的散射 1) 弹性散射 (n,n)
出射粒子仍为中子、剩余核仍为靶核。 出射中子的动能： T T n n 反冲核的动能：
C. Photoneutron Sources
Some radioisotope gamma-ray emitters can also be used to produce neutrons when combined with an appropriate target material. The resulting photoneutron sources are based on supplying sufficient excitation energy to a target by absorption of gamma-ray photon to allow the emission of free neutron. Only two target nuclei, 9Be and 2H, are of any practical signification for radioisotope source.
D． Reactions From Accelerated Charged Particle (“neutron generator”)
Because alpha particles are the only heavy charged particles with low z conventiently available from radioisotopes, reactions involving incident protons, deuterons, and so on, must rely on artifically accelerated particles. Two of the most common reaction of this type used to produce neutrons are:
dN 1/ 2 E / T E e dE
B. Radioisotope (α, n) Sources同位素源 Several different target materials can lead to (α, n) reactions for the alpha particle energies which are readily available in radioactive decay. 1)The maximum neutron yield is obtained with Be as the target:
4 2
Be C n
9 4 12 6 1 0
It’s Q-value:5.71MeV.
2) The 239Pu/Be sources is probably the most widely used of the (α, n) isotopic neutron sources, see table 1-6.
D—D D—T Notes :
2 1
2 1
H H He n
2 1 3 2 1 0
3 1 4 2 1 0
3.26MeV (Q-value)
H H He n 17.6MeV (Q-value)
1) 1 mA D (accelerated potentiao 100~300keV) → 109n/s (D-D), 1011n/s(D-T). 2) Other neutron generators (higher energy): 9Be(d,n), 7Li(p,n), 3H(p,n)