How an HPGe Detector Works From Gamma Ray to Collected Charge EP2

One 662 keV gamma ray, fully absorbed in germanium, creates about 223,500 electron–hole pairs — a charge packet of just ~36 femtocoulombs. ⚛️ In Episode 2 of Atomic Academy, we freeze that single gamma ray at the crystal surface and follow its energy step by step: interaction, ionization cascade, charge drift, and the fundamental statistics that limit how sharp an HPGe peak can ever be. ◀️ Missed Episode 1 (the complete spectrometer)? Start here: [PASTE EPISODE 1 LINK] 📺 Full HPGe Gamma Spectrometry series: [PASTE PLAYLIST LINK] 🔔 Subscribe — Episode 3 goes inside the cryocooler. ✅ WHAT YOU'LL LEARN • Valence band, conduction band & the ~0.7 eV germanium band gap — without the textbook headache • How pure is "high-purity"? (~10¹⁰ impurities/cm³, zone refining, p-type vs n-type) • Reverse bias & depletion: how the active volume is created, d ∝ √(V/N) • Photoelectric absorption vs Compton scattering vs pair production — and what each does to your spectrum • The ionization cascade: N = E / 2.96 eV, phonons, and why the pair energy exceeds the band gap • Shockley–Ramo: why the signal starts while charges are still travelling • The Fano factor and the full resolution budget, with worked numbers at 662 & 1332 keV • Coaxial vs planar vs well geometries, dead layers, and charge trapping 🧮 KEY RELATIONS IN THIS EPISODE • Pairs created: N = E_dep / ε, ε ≈ 2.96 eV (Ge @ 77 K) • Statistical peak width: ΔE(FWHM) = 2.355·√(F·ε·E), F ≈ 0.1 (estimate) • Total resolution: ΔE² = ΔE_Fano² + ΔE_electronics² + ΔE_collection² ⏱️ CHAPTERS 0:00 Opening — Freeze One Gamma Ray 0:05 One Gamma Ray, One Question 0:52 A Semiconductor Ready to Listen 2:09 Reverse Bias & the Depleted Crystal 3:06 Three Ways a Gamma Ray Interacts 4:22 The Ionization Cascade 5:39 Drift: Signal Before Arrival 6:54 What Limits Resolution? 8:16 Geometry, Dead Layers & Trapping 9:45 Summary & Next Episode ⚠️ ACCURACY & TRANSPARENCY • Every calculated/illustrative value is labelled on screen (Fano factor F ≈ 0.1 is the common estimate; literature values ≈ 0.06–0.13). No measured spectra are presented as data in this episode. • Contact thicknesses (~0.5 mm Li-diffused / ≲1 µm implanted) are conventional-design examples and vary by model. • Educational content only — bias and temperature procedures are detector-specific; always follow the manufacturer's manual and local radiation-safety rules. 📚 REFERENCES & FURTHER READING • ORTEC — Review of the Physics of Semiconductor Detectors; Semiconductor Photon Detector overview (ε = 2.96 eV, F ≈ 0.1, saturation velocity): https://www.ortec-online.com • Mirion — SEGe coaxial Ge detectors (net impurity ~10¹⁰ cm⁻³, contact examples): https://www.mirion.com • Knoll, G.F. — Radiation Detection and Measurement (depletion, Fano statistics, charge induction) • Spieler, H. (LBNL) — Introduction to Radiation Detectors and Electronics (intrinsic resolution) ⚛️ ABOUT ATOMIC ACADEMY Atomic Academy explains how real nuclear instruments work — cinematic visuals, honest physics, and practical engineering, one instrument at a time. #HPGe #GammaSpectroscopy #SemiconductorPhysics #NuclearEngineering #RadiationDetection