# Lab 3 Neutron Activation Analysis – Thermal Flux Characterization

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```					CVEEN 5700 and CHFEN 5657 CVEEN 6700 and CHFEN 6657 Laboratory Handouts

Lab 3: Neutron Activation Analysis – Thermal Flux Characterization
Objectives: a. Determine the neutron flux and in the pneumatic irradiator (“PI or rabbit”) when the reactor is operated at 1 kW and 90 kW. b. Determine the element and mass of two unknown foils from neutron activation calculations. c. Determine the elements and weight percent of each for one unknown foil. Background: Irradiation of a stable element in a neutron field will generate radioactive isotopes by the process of neutron absorption. If the composition and mass of the stable element are known before irradiation, then the neutron flux can be calculated from the activities of the radioisotopes generated by neutron bombardment. Conversely if the neutron flux is known then the same process can be used to identify the amount of an unknown stable element before irradiation. This process can be used to identify material composition and weight percents of the original sample. Technical Approach: The neutron flux in the reactor can be measured by irradiating a weighed thin gold foil. The activity or number of decay gamma rays being emitted by a particular radionuclide is dependent on the disintegration rate, which is in turn directly proportional to the neutron flux in the reactor. Thus, by measuring the number of decay gamma rays, the neutron flux can be calculated. The equation that describes the irradiation process is as follows:
A0  N ti 1  e  ti .





(1)

Ao is the activity produced for the radionuclide of interest, N is the number of target atoms in the sample,  is the neutron flux,  is the cross section for the reaction that produces the radionuclide,  is the decay constant and ti is the irradiation time. After irradiation the radioactive atoms will decay follow the decay equation:
At  A0  e   td  .

(2)

At is the radioactivity of the isotope after a decay time of td. With the neutron flux now known, the respective element and mass of additional unknown foils can be identified. When a radioactive element decays it gives off a specific spectrum of radiation. The signature spectrum can be used to identify the radioactive isotope present in the sample. The activity or number of decay gamma rays being emitted by a particular radionuclide is dependent on the disintegration rate, which

CVEEN 5700 and CHFEN 5657 CVEEN 6700 and CHFEN 6657 Laboratory Handouts is in turn directly proportional to the amount of its parent isotope in the sample. Thus, by measuring the number of decay gamma rays, the amount of the parent element can be determined. The activity of the activated parent nuclides created during the irradiation is directly proportional to the neutron flux and the amount of stable elements present in the sample. The equations used in this lab can now be used to determine the mass of the unknown foils using the previously determined flux value. Remember to account for isotopic weight abundances, efficiency, and gamma-energy branching ratios when calculating the mass of your unknown foils.

Procedure: One gold foil and multiple unknown foils will be irradiated in the pneumatic irradiator (“PI or rabbit”) of the TRIGA reactor prior to lab. Upon removal from the reactor, the foils’ activities will be measured using the InSpector 1000 spectrometer system. The power level, irradiation start time and the end of irradiation times will be be noted next to the foil samples along with the mass of the gold foil. The Teaching Assistants will be available to handle the sample during this project and help with any questions. Note: The procedure for operation of the InSpector 1000 is in the Lab 2 Handout.

CVEEN 5700 and CHFEN 5657 CVEEN 6700 and CHFEN 6657 Laboratory Handouts

Gold Foil Data Mass of gold foil: ______ ± ______ grams Reactor power level: ______ kilowatts Date/time into reactor___:___:___ on __________ Date/time out of reactor___:___:___ on __________ Date/time counted ___:___:___ on __________ Date/time gamma spectrum saved ___:___:___ on __________ Foil # Data (Single Element)

Reactor power level: ______ kilowatts Date/time into reactor___:___:___ on __________ Date/time out of reactor___:___:___ on __________ Date/time counted ___:___:___ on __________ Date/time gamma spectrum saved ___:___:___ on __________ Foil # Data (Single Element)

Reactor power level: ______ kilowatts Date/time into reactor___:___:___ on __________ Date/time out of reactor___:___:___ on __________ Date/time counted ___:___:___ on __________ Date/time gamma spectrum saved ___:___:___ on __________ Foil # Data (Multi Element)

Reactor power level: ______ kilowatts Date/time into reactor___:___:___ on __________ Date/time out of reactor___:___:___ on __________ Date/time counted ___:___:___ on __________ Date/time gamma spectrum saved ___:___:___ on __________

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 views: 36 posted: 12/23/2009 language: English pages: 3
Jun Wang Dr
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