API researchers study physics and technical issues of the large arrays of position-sensitive neutron detectors for neutron scattering and nuclear safeguards applications.

Modern cold neutron scattering instruments require large area arrays of pixilated detectors for sensing low energy neutrons. Detectors should have high neutron detection efficiency, low gamma-ray sensitivity, making gaseous chambers filled with ³He the natural choice. Dead area on the surface of the array should be minimized to reduce gaps in the energy momentum coverage of the neutron scattering instrument. Geometry of the detector surface should reduce uncertainty in the arrival time of cold neutrons and its contribution in instrument resolution.

The gaseous neutron detector filled with ³He provides high efficiency, low gamma-ray sensitivity, low background, and good discrimination capabilities. Position-sensitive versions of the detector are 2-D flat window multiwire proportional chambers (MWPC), and arrays of a linear position sensitive counters (LPSD). The Iarocci tube with rectangular cross-section is the type of LPSD which is attractive for the detection of cold neutrons.

Gaseous radiation detectors are complex scientific and engineering systems. API scientists and engineers perform computational and experimental projects in the detector development.

Modeling and optimization of various parameters of the detector array is carried out using the following simulation tools:

• Mechanical design
• 3-D engineering packages (SolidWorks, AutoCAD) with stress and pressure FEA analysis options and direct output for the CNC machine
• Electromagnetic properties
• MAXWELL simulates frequency and time domain electromagnetic fields in complex 3-D structures
• Ion and electron processes in gases
• SRIM/TRIM: Monte Carlo transport code to simuate stopping and range of ions in matter and gas mixtures
• HEED computes energy loss of fast charged particles in gases, taking delta electrons and multiple scattering into account, and simulates absorption of photons through photo-ionization
• MAGBOLTZ: Boltzmann transport of electrons in gases to calculate drift, diffusion, gain and attachment of electrons in gases with applied electric and magnetic fields at arbitrary angles
• GARFIELD: the complex code for simulation of gaseous detectors
• Readout electronic circuits
• SPICE: simulation of output signals, noise analysis, optimization of anode resistance, readout circuits
• Nuclear reactions
• Monte Carlo Neutrons and Particles (MCNP)

The technical issues such as longeviety of the detector system and aging of the components are solved by application of proper vacuum and gas handling techniques.