This preview shows page 1-2-19-20 out of 20 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 20 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Delayed Neutron Activation Analysis Wen-fai Fong Dr. Ila Pillalamarri IAP 12.091Introduction to NAA  NAA is a quantitative chemical method based on nuclear activation of a sample  It focuses on radioactivity of the nuclides formed  Used in two main cases  Radioactive nuclei emitting radiation (delayed time measurements are used)  Stable elements possessing undesired radiation, which is emitted immediately (prompt measurements are used)Overview of Delayed Neutrons  Interactions of fissionable nuclei may produce radioactive products  Consequently, more than one neutron may be emitted by these products  Timing of a delayed neutron reaction is governed by rate of β-decayDelayed Neutron Emission in Fission  A material is irradiated  May give way to one or more radioactive components  These radioactive components may have the capability to emit neutrons  If they do, they are referred to as delayed neutron precursors (DNPs)More on DNPs  DNPs are radionuclides that have the capability to emit neutrons  Generally have few neutrons in excess of a fully-occupied, closed neutron shell  Due to low binding energy of the loose neutrons, greater probability that those neutrons will be emitted  Those nuclides that cannot emit neutrons undergo subsequent β-decay insteadEmission of Delayed Neutrons  DNPs emit ‘late’ neutrons which are referred to as delayedneutrons  The β-decay of the radionuclide with high decay energy fills excited states  These states possess greater energy than the neutron binding energy of the nuclide  Excited states “de-excite” themselves by emitting neutronsExamples of DNPs  Some of the more common examples of DNPs  85As  87Br 135Sb  138IDelayed Neutrons on a Larger Scale  In a given nuclear reaction, delayed neutrons make up less than 1% of total neutron emission  30% of delayed neutrons are emitted less than 1 second after fission reaction  This type of emission is rarely used for non-fissionable materials  Most common fissionable materials are Uranium, Thorium, PlutoniumExperimental Setup  Irradiation source  Provides light, or energy, to induce a nuclear reaction  Detection System  Moderators (paraffin, polyethylene, water)  Shielding from gamma rays  Neutron detectorsNeutron Detectors  Common ones are 3B-BF3 or 3He detectors  Substances have been chosen based on their sensitivity to impulses generated by neutron emission  Are tubes placed in a cylinder filled with moderatorExperimental Setup (cont.)  Sample chamber is at the center of a cylinder  Cylinder surrounded by shielding to protect sample from interference of gamma rays  Discriminator (single-channel analyzer) may be placed on the outsideUse of Delayed Neutrons in Detecting Trace Elements  Method may be used to determine concentration of fissionable isotopic mixtures of one element  May also be used for multi-element mixtures of two fissionable elementsMethodology  Irradiation of the material  Reaction produces DNPs  Amount of radioactive atoms present is detected by the system  Amount of radiation depends on number of atoms activated  Number of atoms activated is proportional to number of atoms in the target sample  Can analyze the elemental contents of sampleBasic Steps  Nuclear activation of the material by irradiation  Count the neutrons emitted  Identify the type of radiation emitted  Determine its energy or half-life  This may allow researcher to identify the element in the sample and its quantityCommon Sources of Error  Sampling errors  Measuring the weight of the sample  Timing of irradiation  Counting system  Data reduction  Low efficiency of the detectorsImportance of Delayed Neutrons in Nuclear Reactors  Time delay is controlled by the half life of the β-decay precursors (DNPs)  In nuclear reactor, nuclear reaction must be controlled  Time delay in delayed neutron emission provides ample time for control  Substantially increases average time between generations of neutronsImportance of Delayed Neutrons in Nuclear Reactors  Consequently, delayed neutrons can control the reactor’s period  Even true in fast reactors, where reactors deal mainly with fast neutrons  Also, amount of decay heat after reactor shutdown is determined by the delay timeOther Applications  Nuclear physics  Delayed neutron emission used as a tool for studying the fission process and molecular interactions  Astrophysics  Used in rapid-neutron capture, which is frequently used to estimate the age of the galaxy or universeConcluding Remarks  NAA has long been known as a reliable method for detecting major, minor, trace or bulk elements in samples  Delayed neutron emission provides one more option  Allows for control in nuclear reactors  More research should be done on its application in other fieldsMain References  Alfassi, Zeev B. Activation Analysis. Boca Raton: CRC Press, 1990.  Das, S. “The Importance of DelayedNeutrons in Nuclear Research – A Review.”  Friedlander, Gerhart. Nuclear and Radiochemistry. New York: John Wiley &Sons, 1981.  Kruger, Paul. Principles of ActivationAnalysis. New York: John Wiley &


View Full Document

MIT 12 091 - Delayed Neutron Activation Analysis

Download Delayed Neutron Activation Analysis
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Delayed Neutron Activation Analysis and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Delayed Neutron Activation Analysis 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?