Neutron Activation Analysis

Neutron Activation Analysis

• NAA is a non-destructive technique. The sample is placed inside a region of high neutron flux (like the core of a nuclear reactor). The bath the sample in neutrons. The nuclei in the sample will absorb some of the neutron, leaving unstable radioactive elements which decay with characteristic gamma ray emissions. Use of a high precision gamma detector to acquire the spectrum will allow one to determine the distribution of all trace elements in a single shot

Forensic bullets, paint, glass, metals, gunshot residue swabs

Gamma Detector

NuclearReactor Core

The prepared sample and standard sample are placed in a “detector” one at a time.

• The detector system counts and records gamma radiation emissions for a period of time.

• Time varies, but is

usually in the range of 5 minutes to an hour.

Specialized software analyzes radiation peaks. Peak data is correlated to specific elements for identification and quantification.

Computer data is compared to a nuclide chart to evaluate the results.

9

Neutron induced radioactivity• Irradiate sodium (Na23) sample with neutrons– Neutron captured to give Na24 in excited state– Jumps to lowest state of Na24—> emits gamma-ray(s)

(photon)– With 1/2 life 15 hours:

• Na24 emits electron to become Mg24 (beta decay)• And another gamma (photon)

• Electrons —> autoradiography• Gammas —> NAA• Similar story for many (not all) elements

10

Nuclear vs. atomic spectra• Electronic transitions in atoms

– Spectral lines in the visible (~500 nm), ultra-violet (~10-400 nm), X-rays (~0.01-10 nm)

– Photon energies ~ 2 eV, ~ 3-100 eV, ~ 100 eV -100 keV

• Neutron or proton transitions in nuclei– Spectral lines at even shorter wavelengths– Photon energies ~ 100 keV-??? MeV = gamma rays

• Nuclear emits gamma ray(s) characteristic of particular isotope (nuclide)

11

e

e

Gamma-ray spectrometer

Q

Voltage supply

Germanium crystal

e hh

he

+

—

display

Multichannel pulse-height analyzer

gammaray

energy

cou

nts

12

Preferential nuclide selectionactivity

irradiationtime

short1/2-life

long1/2-life

short long

activity

delaytimeshort long

short1/2-life

long1/2-life

13

Advantages of NAA

• Simultaneous analysis for many elements

• Chemical preparation rarely necessary• Non-destructive testing• Reliable quantitative results• Very high sensitivity (for some

elements)

14

Sensitivity for different elements

19 elements not even listed!

235

23221242

73

1 atom in a million-million in a 1 gram sample

1 atom in a hundred thousand in a 1 gram sample

Bottom line— great analysis technique—IF you’ve got the right problem

10-12 g

Gamma Ray Energy Spectrum from NAA Study

Gamma-ray spectrum showing several short-lived elements measured in a sample of pottery irradiated for 5 seconds, decayed for 25 minutes, and counted for 12 minutes with an HPGe detector.

Gamma Spectra from a Sample of Pottery Irradiated for 5 Seconds, Decayed for 25 Minutes, and Counted for 12 Minutes with an HPGe

Detector.

Gamma Spectra from a Sample of Pottery Irradiated for 24 Hours, Decayed for 9 Days, and Counted for 30 Minutes on a HPGe Detector.

Ujian

• 100 menit• Closed book• Soal 5 buah• Materi sd NAA• PR ada di elisa menghitung sistem kristal

dan unit sel dari data XRD dikumpulkan saat ujian.

Uses of Neutron Activation Analysis

• NAA can be used to determine the presence of a variety of elements in a sample.

Examples include:

• Mercury levels in tuna fish• Aluminum levels in finger nails• Gold levels in hair• Zinc levels in soil• Arsenic levels in plants

(Photos courtesy of the Laboratory Facility at the University of Florida Training Reactor)