edited by: RON DELORENZO Mlddie Gearg~aCollege Cochran, Georgia 31014
Neutrography: Particles versus Waves Ronald DeLorenzo Middle Georgia College Cochran, GA 31014 The following discussion is designed to give your students more insight into electromagnetic radiation (such as X-rays and gamma rays) and particle radiation (such as alpha particles and neutrons). We will conclude this discussion with a recent application to illustrate how neutron radiography (neutrography) can be used to give scientists, historians, and artists a clearer victure of man's distant vast. Neutrograph; is the technique of taking a picture of an obiect using a beam of neutrons. A varticle beam of neutrons can be used to take a picture of &object in much the same way that pictures are taken with X-rays. Neutron pictures are taken by placing an object to be photographed between a neutron source1 and a sensitive photographic film.2 Regions of higher neutron absorption in the object cause less hlackening of the film than do regions of lower neutron absorption.
' Neutrons are produced only in nuclear reactions. One possible source of neutrons is a nuclear reactor which produces neutrons by fission reactions. The detection of a neutron depends upon observing a charged particle or gamma ray from a neutron-induced reaction. Special photographic emulsions can be used to detect neutrons.
The result is a neutron picture that is referred to as a neutroeranh. Consider the elements hydrogen, carbon, and lead. Hydroeen and carbon are of oarticular interest because these are " elements of which we are composed primarily. Skin, protein, carbohvdrates. enzvmes, amino acids. and manv other components of living or once living organisms are hydrocarbon derivatives. (Hydrocarbons are compounds made up of just hydrogen and carhon.) Plastics, such as polyethylene, are also hvdrocarhons or hvdrocarbou derivatives. -If neutrons were fired through a plastic shield (a hydrocarbon made up of hydrogen and carbon atoms) and a lead shield (a bunch of lead atoms), which shield would the neu-
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m i s feature presents a collection of descriptive applications. and analogies designed lo help students understand some of mi dinicun conceplr i r q & n t y encomtered n cnem ~ l r )Contr8wl m i IMI& 11 prO(luC.9 a areatar a.m . c a l m an0 rnowleoce - oi .oolitical, religious, economic, historical, and scientific aspects of life are encouraged Ron DeLorenm is an Associate Professor in the Department of Chemistry at Middle G e ~ c l i aGollese. He is also C a m ~ u s coordinatoriar the c6mputing facilities and teilLh~san onginm ng m u s e in FORlRAh he rece md hrs BS from St Jorm s Awers w n 1963 and the MS and PhD n Pn.scal-Inorganic Chemistry from Lowell Tech-
Chemistry" p&lished by D.C. Heath and C o m l ~ n ywhich &nphasires his appl8catms and analogles approach in teaching
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trans ~ e n e t r a t emore easilv? Most students think that the neutrons would get through hydrogen and carbon atoms more easilv than thev would eet throunh lead atoms. hut this is not correct. A collection ofsolid carion and/or hidrogen atoms contains more nuclei and fewer electrons per unit volume than a collection of lead atoms as indicated hy the following calculations.
- 3.31 X loz2nuclei
A neutron is a oarticle with a mass aooroximately 2000 (like a 2000-lh car hitting a 1-lh cat). Also, the neutron is neutral, so i t does not electrically interact with charged particles such as electrons and protons. Neutrons would zip through a lead sample more easily than a hydrocarbon sample because a unit volume of lead contains fewer nuclei and more electrons than a unit volume of hydrocarhon. The only way that a neutron would be stopped from penetrating a lead sample is if the neutron collided with a lead nucleus. Remember. however, that the nucleus of an atom occuoies very thirds of an inch. Therefore, neutrons can penetrate a lead sample very easily. Why would neutrons have trouble getting through a hydrocarbon sample (a collection of hydrogen and carbon atoms)? We know that electrons cannot stop neutrons because electrons are too light and their negative charges do not interact with neutral neutrons. Onlv a head-on collision with a nucleus wonld stop a neutron. In a hydrocarbon sample there are manv more nuclei oer unit volume relative to the lead sample. There is a greater chance that the neutron would collide with a nucleus in a hydrocarhon sample and he stopped. Why does the lead sample stop X-rays? An X-ray can be thought of as a wave with positive and negative properties. The electrical part of the X-ray fluctuates (changes) regularly between positive (+) and negative (-1 values. You can think of an X-ray as a wave that sometimes acts as if it were positively charged and sometimes acts as if it were negatively charged. The fluctuating charge of an X-ray interacts with lead hecause lead has a high electron concentration (large number of electrons per unit volume). Electrons are negative and they will interact with the X-ray's positively charged electrical
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component (attraction) and with the X-ray's negatively charged electrical component (repulsion).It is because of these interactions that the X-ray has difficulty penetrating a lead shield. Why would X-rays get through a hydrocarbon sample with relative ease? X-rays can get through a hydrocarhon sample easier than they can get through a lead sample becanse a hydrocarbon sample contains fewer electrons per unit volume, i.e., there are fewer electrons with which the X-ray can interact in a hydrocarbon sample than there are in a lead sample. In summary, X-rays are stopped by heavier elements (such as lead) that have higher electron concentrations (more electrons per unit volume). X-rays can more easily pass through lighter elements (such as carbon) that have higher nuclei concentrations (more nuclei per unit volume). When physicians want to check for a possible broken bone, they use X-rays. Bone contains calcium. A collection of calcium atoms has a higher electron concentration (greater number of electrons per unit volume) than a sample of skin (hydrocarbon derivative made up primarily of lighter elements such as hydrogen, carbon, and nitrogen). X-rays can pmetrate skin more easily than hone. When a bone is bombarded with a stream of X-rays, usually denser parts of the hone absorb (stop) more X-rays. After passing through the hone, the Xrays strike a sensitive film. The film shows the denser parts of the bone as lighter areas, the less dense parts as darker areas. Neutrography would be used to look a t a tumor (a thick hvrlrncarhon derivative) inside hone because neutrons would --, -~ Iw stopped hy rhe tLnwr inn~dcu honrsince thr tumur i. primrarilv -h\.dn,ern nnd ciirhn. iwth a.f ahi,.h have relallvrl\. ...----, ~ ~~~n " hirh nuclei concentrations. he bone material (calcium) has a lower nuclei concentration than the tumor and would he less visible than the tumor to a neutron beam. Whv doesn't the skin interfere with seeinn - the tumor if both the tumor and skin are hydrocarhon derivatives? Both neutron and X-ray intensities can he varied. The intensity of the neutron beam can he set so that it penetrates the thinner skin laver but is stoooed bv the thicker tumor in the bone. he United ~t'atesis considering developing and perfecting a neutron bomb. The neutron bomb kills people but does not destrov hnildines and cities. Whv is this so? A high inteniity neutron b e a k wonld interfere with the higher nuclei concentration of bodv tissue but would slio through buildings and other structuial components of a city which are made of iron, concrete, etc., i.e., material with a lower nuclei concentration. An ancient Chinese urn which was buried with a Chinese nobleman more than 2000 years ago was recently unearthed. The urn nresentlv resides at the Los Aneeles - County Museum of Art. Scientists, artists, and museum curetors were interested in learnine how the urn was made and what it was made of. A neutrograph (picture using neutrography) showed the urn to he mostly bronze. The legs of the urn were hollow and were packed with clay, straw, and scraps of wood. Without damaging the urn, no other method could have revealed the contents of the hollow bronze legs. ~~
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