sign in sign up
<<
Home , Nuclear reaction

MM0900107

Jour. Myan. Aca

An Introduction to the IAEA -Gamma Atlas Ye Myint1, Sein Htoon2

Abstract NG-ATLAS is the abbreviation of Neutron-Gamma Atlas. have proved to be especially effective in producing nuclear transformations, and gamma is the informer of nucleus. In , observable features or data are determined externally such as, , spin and , where neutron and gamma plays dominant roles. (Neutron, gamma) (n,y) reactions of every are vital importance for nuclear physicists.

Introduction

Inside the nucleus, and neutrons are simply two different states of a single , the . When neutron leaves the.nucleus, it is an unstable particle of half life 11.8 minutes, into . It is not feasible to describe beta decay without the references to Dirac 's hole theory; it is the memorandum for his hundred-birth day . The role of neutrons in nuclear physics is so high and its radiative capture by elements (neutron, gamma) reaction i.e, (n,y ) or NG-ATLAS is presented. The NG-Atlas. data file contains nuclear reaction cross section data of induced capture reaction for targets up to Z =96 . It comprises more than seven hundred , if the half lives are above half of a day. Cross sections are listed separately for ground , first and second excited states .And if the isomers have half life longer than 0.5 days , they are also include as targets .So the information includes a total of about one thousand reactions on different channels and the incident neutron energy ranging fromlO"5 eV to 20 MeV. (10'5 eV neutron moves with about velocity of the sound, and it can only be produced after many collisions of neutron with hydrogen rich material and. 20 MeV neutron is relativistic.) Cross section for the production of ground states and also the excited states need to be considered, but we may neglect isomeric states of the nucleus. The data files are available from International Agency, Section, on line through the World Wide Web, or Internet address is www-nds.iaea.org/ . Also neutron reaction data library for nuclear activation and transmutation at intermediate are available cost free on request.

1. Department of Physics, Dagon University 2. Department of Physics, Yangon University 20 Jour. Myan. AcasL Am & Sc. 2005 Vol. HI. No. 3(i) Physics

Transmistfitfcm by neutron asid excited product nucleus

Neutrons have no charge, and so they are not subject to repulsive electrostatic forces in the vacinity of the positively charged nucleus, and therefore more likely to react or penetrate nuclei than charged like protons, deuterons and alphas. Not only are highly energetic neutrons capable of causing nuclear reactions, but slowly moving neutrons are extremely effective. Both slow and fast neutrons have their properties. Slow neutrons, as low as 1G"5 eV, can react with proton of hydrogen nucleus, with a cross section value of ten barns and turn into deuteron. But for the fast neutrons of greater 10 MeV, with that hydrogen nucleus proton, the reaction crosssection

1.8888+88** JEF-2,2

to- 1O

iO" 1Ol-t

e 1Q- 1O" e

» 1 to m m I io-«* r

1O" 1OS Enersry Cell) (The axes are in logarithmic scale) is of the order of millibam. [ 1 = 10"24cm2 ] . In nuclear physics , the concept of a nuclear cross section cannot be visualized as just the cross sectional area , or target area to the incident particle , instead it is the probability of the reaction . The experimental meaning of the cross section comes from its use as a measure of the number of nuclear events which occur Jour. Myan. Acad. Arts & Sc 2005 Vol. III. No. 3(i) Physics 21 under a given set of experimental conditions. A given nucleus can have widely different cross sections for different nuclear reactions and the values represent the relative probabilities of those reactions. When the cross section for a particular reaction is known, it is possible to predict the number of reactions that will take place when a sample of is exposed to a known of known irradiation time .This kind of information is needed in many practical problems., as in the manufacture of radionuclides , irradiation time for radiotherapy , etc . The most common nuclear process is this " (n,y )" ,or radiative capture of neutrons . If high yield of neutron flux is available, neutron handling is the first step experiment and neutron gamma atlas is essential file. Emitted gamma carries specific discrete energy and they can be specified like fingerprints are to human. H-l(n,T)H-2 1.8886+88** JEF-2.2

10"s 1 105 102 r 1 1

10 1 10

t r -. 1

1 ID" r C 0 2 ,-2 10" r 10

10-3 r 10i-3 e

r •= 10"

Nfc'Uw il"1 '^ Ml "

10-5 -• 10" 10"s 10s Energy (eU) 22 Jour. Myan. Acad. Arts & Sc. 2005 Vol. III. No. 3(i) Physics *

Before the development of the chain -reacting pile , or , neutron sources and are the main sources of neutron .The reactor neutron energy may exceed 100 MeV but the rest two are about 14 MeV at most .Handling of neutron , one important step is to slow down or high flux of thermal neutrons . For formation of product , information should be in hand first, and experiment follows along with precaution. In this NG-Atlas library file, more than five hundred target.nuclides for incident neutrons with energies up to 100 MeV, data are available. The target nuclides cover the range from hydrogen to polonium. All necessary informations or contents are given to the data user. Targets irradiated with neutrons and proton up to 100 MeV are also available from the files other than NG-Atlas, but protons of more than 10 MeV is not available at the moment. Theoretical data analysis can be started with the aid of these banks.Crbss sections for threshold reactions for production of product nucleus can also be considered. NG-Atlas for Nitrogen (n, gamma ),N14(n,y )N15 and H-l(n,y )H-2 are shown in the graph , as an example .The behavior of cross section for energy greater than 1 MeV show peaks or . For every nuclide and for neutron energy range of thermal to fast, accurate cross section values are recorded. Even with these data in hand, to perform a nuclear experiment, the following points should also borne in mind.

(i) The nuclear reaction to perform. (ii) The activity of the product nucleus and half life. (iii) How much energy is available or the number of energy points for the reaction, (iv) How the reaction cross section is obtained, (v) That the method is based on geometry, (vi) Level density considerations (number of excited level in one energy range) are taken into account with shell model or shell effects (vii) Total reaction cross section and inverse cross section calculations are also considered, (viii) Then, corrections for calculated cross section was performed for some reaction channels to achieve the agreement with available data. Jour. Myan. AcatL Arts & Sc 2005 Vol. III. No. 3(i) Physics 23

Conclusion Nuclear experiments are very expensive and time consuming, but scholars are to perform with every available information in hand, including NG-Atlas library and many other literature, data, experience, etc. Handling of neutron is also a must. Results can vary up to the factor of "10", but for random processes, they are in the acceptable region.

References Kaplan ,1. (1965 ) "Nuclear Physics" ( London: Addison-Wesley) McGee ,T . Rao, C.L. Saha, G.B . et al., (1970) Nucl. Phys, A150, 11 Michel, R. Brinkmann ,G . Weigel, H . et al., (1979) Nucl. Phys, A322, 40 Michel ,R. Peiffer ,F . Stuck ,R. (1985) Nucl. Phys, A441, 617