CLYC Dual Gamma-Neutron Scintillator Lab

1. Cubic Elpasolite Crystallography & Bridgman Growth

Cesium Lithium Yttrium Chloride ($\text{Cs}_2\text{LiYCl}_6$, CLYC) belongs to the elpasolite quaternary halide family, crystallizing in the face-centered cubic space group $Fm\bar{3}m$ ($a = 10.51\,\text{Å}$, $Z = 4$) with a solid density of $\rho = 3.31\,\text{g/cm}^3$.

The fundamental engineering advantage of CLYC over hexagonal halide scintillators (such as $\text{LaBr}_3$) is its optical and mechanical isotropy ($\Delta n = 0$). Because cubic lattices expand uniformly across all three dimensions ($\alpha = 24 \times 10^{-6}\,\text{K}^{-1}$), single crystal boules can be solidified in multi-zone vertical Bridgman furnaces at $640^\circ\text{C}$ inside carbon-coated quartz ampoules and cooled with virtually zero anisotropic thermal stress, yielding high crystal extraction yields.

2. Dual-Mode Gamma vs. Neutron Scintillation Kinetics

CLYC features a dual scintillation emission mechanism that enables unambiguous particle identification:

• Gamma Interaction: Gamma photons generate high-velocity secondary electrons that excite $\text{Ce}^{3+}$ activator ions ($5d \to 4f$ transition at $373\,\text{nm}$) with an emission decay constant of $\tau_\gamma \approx 50\text{--}100\,\text{ns}$. CLYC also exhibits sub-nanosecond Core-Valence Luminescence (CVL) around $300\,\text{nm}$ for picosecond coincidence timing.
• Thermal Neutron Capture: Neutrons interact with Lithium-6 via the exothermic capture reaction:
  $$^6\text{Li} + n_{\text{th}} \to \alpha (2.05\,\text{MeV}) + {}^3\text{H} (2.73\,\text{MeV}) + 4.78\,\text{MeV}$$
  The high stopping power ($dE/dx$) of the alpha and triton produces a localized dense plasma of self-trapped excitons, transferring energy slowly to $\text{Ce}^{3+}$ centers with a prolonged decay constant of $\tau_n \approx 500\text{--}1000\,\text{ns}$.

3. Pulse Shape Discrimination (PSD) & FOM

Because the tail of a neutron scintillation pulse contains a vastly higher fraction of late light than a gamma pulse, digital multi-channel analyzers compute the charge-comparison ratio:

The separation quality is measured by the Figure of Merit ($\text{FOM}$):

An $\text{FOM} > 2.0$ guarantees complete separation with a gamma rejection ratio exceeding $10^{-6}$, virtually eliminating false-positive neutron alarms in security screening.

4. Helium-3 Replacement & Fast Neutron Detection

CLYC is the primary solid-state replacement for Helium-3 ($^3\text{He}$) proportional counters. With 95% $^6\text{Li}$ enrichment, a $2"\times 2"$ CLYC cylinder achieves higher thermal neutron detection efficiency than a standard $2"\times 12"$ $^3\text{He}$ tube at 4 atmospheres, packed into a fraction of the physical volume.

Furthermore, fast neutrons interact with Chlorine-35 via the $^{35}\text{Cl}(n,p)^{35}\text{S}$ reaction ($Q = +615\,\text{keV}$). The light generated by the emitted proton is directly proportional to incident neutron energy ($E_p = E_n + 615\,\text{keV}$), enabling direct fast neutron spectroscopy without cumbersome paraffin moderators.