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The prospect of Radiographic imaging with cosmic-ray muons: 

The prospect of Radiographic imaging with cosmic-ray muons 2003.10.20 报告人： 刘循序 指导老师： 叶沿林 应 军

课题背景: 

课题背景 集装箱走私的严峻形势： 在当今社会中，集装箱运输成为现代货物运输的一种十分重要的方式。但集装箱运输的货物较为隐蔽，难以完全检查。利用集装箱走私货物已经成为国际社会的一大公害。中国也不能避免此问题，出现了多起大案，严重干扰了正常经济秩序。 手工开箱检查有三种方式：彻底查验、抽查和集装箱查验 目前我国海关对集装箱大部分采用查验的方式检查。

目前应对措施：: 

目前应对措施： 欧美发展了集装箱检测系统，主要有以下几种类型： 加速器型集装箱检测系统 以电子直线加速器或静电加速器发射的X射线作为集装箱检测的射线源，目前正式投入运行的集装箱检测系统大都属于此类型（如：德国海曼公司、法国施伦贝谢公司、美国EG&G等，以及清华大学在1995年研究成功了的9MeV电子直线加速器系统样机） 缺点：辐射量大，须重重保护，成本极高，占地面积大。价格昂贵（2千万美金/台）

Slide4: 

X光机型透射+反散射集装箱检测系统 美国AS&E公司开发成功的系统。采用450KeV的X光机作为射线源，获取两个视图：透射视图和散射视图。 缺点：反散射光子的能量不超过220KeV，穿透能力有限，在集装箱检测应用受到很大限制。价格$1,000,000 U.S

Slide5: 

放射性同位素型集装箱检测系统。 以放射性同位素作为射线源，获取集装箱及内部货物图像的系统。 如：清华大学Co-60系统，采用超高压列阵电离室作为射线探测器，γ光子平均能量为1.25MeV，穿透能力相当于5MeV的加速器射线源。（清华大学1990.10 刘锡明 博士论文）

The report from Nature: 

The report from Nature Natural background particles could be exploited to detect concealed nuclear materials. Compared to X-ray radiography, it has irresistible charm. The experiment in lab has shown its’ feasibility and exactness of revert graph. NATURE | VOL 422 | 20 MARCH 2003 Konstantin N. Borozdin, Gary E. Hogan, Los Alamos National Laboratory, Los Alamos,

The limitations of X-ray: 

The limitations of X-ray an inability to penetrate dense objects the need for multiple projections to resolve three-dimensional structure health risks from radiation

Here we will show：: 

Here we will show： The natural background muons, which are generated by cosmic rays and are highly penetrating, can be used for radiographic imaging of medium-to-large, dense objects. And with a reasonably short exposure time. This inexpensive and harmless technique may offer a useful alternative for detecting dense materials

The principle of X-ray radiography: 

The principle of X-ray radiography In X-ray radiography, the intensity of an image pixel is determined by the attenuation of the incident beam caused by absorption and scattering — the maximum mean free path for photons is about 25 g cm_2 for all materials, corresponding to less than 2cm of lead.

Slide10: 

Hence, for thicker objects, it is better to use a different type of radiography that is based on the interaction of charged particles with matter by multiple Coulomb scattering. The many small interactions add up to yield an angular deviation that roughly follows a gaussian distribution

with the width, 0, related to the scatteringmaterial through its radiation length, L0, asfollows:: 

with the width, 0, related to the scattering material through its radiation length, L0, as follows: where p is the particle’s momentum in MeV c-1 and c is its velocity. (Hagiwara, K. et al. Phys. Rev. D 66, 01001 (2002).)

Slide12: 

The radiation length decreases rapidly as the atomic number of a material increases, and 0 increases accordingly: in a layer 10 cm thick, a 3-GeV muon will scatter with an angle of 2.3 milliradians in water, 11 milliradians in iron and 20 milliradians in lead. By tracking the scattering angles of individual particles, the scattering material can be mapped.

Is it true possible?The authors argue that:: 

Is it true possible? The authors argue that: The new technique relies on the scattering of atmospheric muons produced by primary cosmic rays. Muons are the most numerous cosmic-ray particles at sea level, moving at a rate of about 10,000 /m^2 min in horizontal detectors. [Grieder, P.K.F. Cosmic Rays at Earth (Elsevier Science,Amsterdam, 2001).]

Slide14: 

These particles are highly penetrating: a typical cosmic-ray muon of energy 3 GeV will penetrate more than 1,000 g cm^2 (10m of water, for example).

The Researcher’s Experiment: 

The Researcher’s Experiment To demonstrate the concept of muon radiography, we developed a small-scale experimental system with four drift chamber detectors spaced 27 cm apart. Each detector has an active area of 60*60 cm2 and records particle tracks at two positions in each of two orthogonal coordinates.

Slide16: 

The upper pair of detectors records the tracks of incident muons, and the lower pair records the scattered tracks. A tungsten cylinder was used as a test object, supported by a plastic plate and steel support beams. The tungsten is clearly visible in the reconstructed image, and the steel support beams are also evident

Slide17: 

Figure 1 Radiographic imaging with muons of a test object (left) and the reconstructed image of its Monte Carlo simulation (right).The test object is a tungsten cylinder (radius, 5.5 cm; height, 5.7 cm) on a plastic (35601 cm3) plate with two steel support rails.The tungsten cylinder and the iron in the rails are clearly visible in both the experiment and simulation reconstructions. Inset, the widths of the scattering distributions for tracks passing through the tungsten target are very similar for the experimental and simulated data.

The researcher’s conclusion: 

The researcher’s conclusion Author also developed a Monte Carlo simulation code that generated cosmic-ray muons and propagated them through a test volume. and the scatter angles of the simulated muons from the different materials (tungsten, lexan and steel) are consistent with the measured angles. Simulation of larger, more complex objects demonstrates that we can reliably detect a 10*10*10 cm3 uranium object inside a large metal container full of sheep in 1 min of exposure

The muons sourse: 

The muons sourse Cosmic rays are mostly protons coming from outer space, but there are others species too, such as heavier nuclei, electrons, photons, etc. When these particles reach the atmosphere, at heights of about 20 km, they can collide with nuclei producing many secondary particles, many of which are high – energy pions that interact and decay as they travel downwards giving rise to showers. Charged pions (we are only interested in charged particles which we can detect) decay, mostly, into muons and neutrinos with a lifetime of about 10^–8 s. Most muons are produced at heights of about 15 to 20 km:

Slide20: 

Fig. 2 Muon energy spectrum at seal level

One detector in CERN-PS : 

One detector in CERN-PS At the far site, the detector consists of two identical modules to exploit the space available in the existing experimental hall. This configuration also helps in the assessment of the systematic errors arising from the angular dependence of the contamination. The aim of the experiment is to measure the ratio / (as a function of energy and integrated over all energies) with high statistical significance for oscillation probabilities down to 10-3 for .

One detector in CERN-PS: 

One detector in CERN-PS All three detector modules have the same simple structure, made of a fine grained tracking calorimeter that constitutes the neutrino target, followed by a tail catcher and a muon catcher. The fully active tracking calorimeter is made out of a succession of 300 layers of plastic scintillator and streamer tube planes, for a total mass of . The detector at the site, made out of two modules, will have a total mass of . In the calorimeter, particle trajectories and energy loss are both sampled every .

One detector in CERN-PS: 

One detector in CERN-PS The tail catcher and the muon catcher have the same structure as the tracking calorimeter, with coarser granularity obtained with the addition of a suitable passive absorber. The dominant factor in the choice of detector type is the relatively short time scale foreseen for the experiment that does not allow for extensive detector R & D.

清华大学Co-60探测系统比较: 

清华大学Co-60探测系统比较 清华大学Co-60探测系统的原理 γ射线和物质相互作用有光电效应、康普敦效应和电子对效应。宏观效果而言，穿过一定厚度的物质后，体现为一部分的吸收，其他部分不变的通过。

Slide25: 

原理图

Slide26: 

探测装置示意图：

Slide27: 

清华大学Co-60集装箱探测系统采用准直的窄束单能γ射线束，忽略散射情况，有： 阵列探测器说明：采用超高压气体阵列电离室，共512个探测单元，用来测量γ射线穿透集装箱后的射线强度。呈扇形排列在以放射源为圆心的弧线上，并与经准直器准直的射线束精确对准。

我们的结论: 

我们的结论 Radiographic imaging with cosmic-ray muons （宇宙线粒子探测集装箱成像） 探测技术有国际领先的原理和不可比拟的优势（三维成像，零放射性污染，成本低，理论可靠）。目前国际上没有投入实践的报道。如果能研究成功，将会使我国处于探测技术的前沿，并且开拓一个新的产业。前景极为广阔。 当然了，作为本科生科研项目，我们可能只能走出一小步，但相信不停的坚持下去，会有一个令人信服的成果。