Production plan for a wall clock readable in total darkness using americium-241 alpha sources paired with phosphor layers.
Two phosphor colors divide the clock into two visual systems:
Green (Gd₂O₂S:Tb, 545nm) — clock hands only Red (Y₂O₃:Eu, 611nm) — hour markers
Green is brighter under alpha excitation and reads as the moving element. Red is dimmer per unit activity but preserves night-adapted vision better. The red markers receive double the source activity to compensate for the lower perceived brightness.
Isotope: Am-241 (alpha emitter, 5.486 MeV, t½ = 432 years) Form: Gold-matrix foil on silver backing Supplier: Eckert & Ziegler Cesio s.r.o., Prague Custom cut: Order pre-cut to required dimensions (avoids breaking sealed-source classification)
Component Count Activity each Total ───────────────────────────────────────────────── Minute hand 1 0.3 kBq 0.3 kBq Hour hand 2 0.3 kBq 0.6 kBq Hour markers 1–11 11 0.6 kBq 6.6 kBq 12 o'clock marker 3 0.6 kBq 1.8 kBq ───────────────────────────────────────────────── Total 17 9.3 kBq
Hour markers use 0.6 kBq (double the hand strips at 0.3 kBq) to compensate for Y₂O₃:Eu lower luminous efficiency under scotopic conditions. The 2:1 ratio is preserved.
EU exemption threshold for Am-241: 10 kBq total activity. This design stays under it at 9.3 kBq. No notification to DZRNS required. No authorisation, no license, no paperwork. The completed clock is a consumer object.
Standard 25–30cm wall clock face. Quartz movement.
Eleven single red strips at positions 1 through 11. Three red strips clustered at the 12 position.
Position Strips Purpose 12 3 orientation — identifies top at any viewing angle, visually distinct from all other positions 1–11 1 each hour reference
Each marker strip: 2mm × 6mm rectangle, placed radially (long axis pointing toward center).
The three strips at 12 are arranged as a tight radial fan: one vertical (center), one angled 15° left, one 15° right. This makes 12 identifiable even when the clock is viewed from across a dark room at an oblique angle.
Minute hand: 1 green strip, 2mm × 10mm placed near the tip of the hand
Hour hand: 2 green strips, 2mm × 6mm each placed in a row along the hand's length near the tip, separated by 1mm gap
The minute hand has one strip. The hour hand has two. In darkness, one-green-dot is minutes, two-green-dots is hours. No ambiguity.
No source on the second hand. Its motion is tracked by watching it occlude the hour markers as it sweeps — visible as a brief shadow passing over each red dot. This saves source budget and avoids a third visual element.
Phosphor: Gd₂O₂S:Tb (gadolinium oxysulfide, terbium-activated) Emission: 545 nm (green) Grain size: 20–40 μm (matched to alpha range in solid) Supplier: Phosphor Technology Ltd (UK) or equivalent
Phosphor: Y₂O₃:Eu (yttrium oxide, europium-activated) Emission: 611 nm (red) Grain size: 20–40 μm Supplier: Phosphor Technology Ltd (UK) or equivalent
Each luminous element is a five-layer laminate:
Layer 1 — Clear polyester (Mylar) substrate, 25μm Layer 2 — Double-sided acrylic adhesive (3M 467MP), 50μm Layer 3 — Am-241 gold foil, active side up Layer 4 — Phosphor coat (phosphor + acrylic binder, 30–50μm) Layer 5 — Clear seal coat (acrylic lacquer, 10μm)
1. Cut Mylar substrate rectangles 1mm oversize on each edge relative to the foil source dimensions.
2. Apply 3M 467MP adhesive film to the Mylar. This adhesive has no outgassing, is optically clear, and bonds to both the Mylar and the silver backing of the foil.
3. Place the Am-241 foil (silver backing down) onto the adhesive, centered with 1mm Mylar border on all sides. Press to bond. The gold/Am active surface faces up.
4. Mix phosphor into clear acrylic binder at 50–60% phosphor by weight. Stir to uniform dispersion. Thin with solvent (acetone or butyl acetate) to brushable consistency.
5. Brush one coat of phosphor-binder onto the exposed foil surface. Target 30–50μm wet thickness. A single coat is sufficient — thicker layers waste phosphor (alphas don't penetrate past ~35μm in this density).
6. Dry in still air, 20 minutes.
7. Brush one coat of clear acrylic lacquer over the dried phosphor layer. This seals the phosphor grains and completes the encapsulation.
8. Dry fully (1 hour). The finished element is a sealed laminate: foil encapsulated between adhesive, phosphor, and clear coat, with Mylar border extending past the foil on all sides.
9. Trim the laminate to final dimensions by cutting through the Mylar border only — never through the active foil.
10. Attach finished elements to clock hands and dial using acrylic adhesive or cyanoacrylate.
Item Qty Source ──────────────────────────────────────────────────────── Am-241 foil, 2mm×6mm, 0.3 kBq 2 Eckert & Ziegler Cesio Am-241 foil, 2mm×6mm, 0.6 kBq 14 Eckert & Ziegler Cesio Am-241 foil, 2mm×10mm, 0.3 kBq 1 Eckert & Ziegler Cesio Gd₂O₂S:Tb phosphor powder ~1g Phosphor Technology Ltd Y₂O₃:Eu phosphor powder ~2g Phosphor Technology Ltd Clear acrylic binder ~5ml art supply / hardware Clear acrylic lacquer ~5ml art supply / hardware Mylar film, 25μm 1 sheet art / packaging supply 3M 467MP adhesive transfer tape 1 small roll 3M / electronics supply Wall clock (quartz, 25–30cm) 1 any Cyanoacrylate adhesive 1 tube hardware
Total phosphor needed is trivially small. Minimum order from suppliers will be far more than needed.
Total activity: 9.3 kBq — below the 10 kBq exemption threshold in EU Directive 2013/59/Euratom, Annex VII.
At this activity level, the practice is exempt from regulatory control. No notification to DZRNS, no authorisation, no license required for possession or use.
Eckert & Ziegler Cesio sells sealed calibration sources at and below this activity level as standard catalog items. Ordering requires identifying yourself and stating the intended use, but no government permit. Their compliance department confirms exemption status based on the total activity and the applicable member state transposition.
The concentration exemption (1 Bq/g) is also satisfied: each foil strip weighs roughly 50–100 mg. A 0.6 kBq source in a 100 mg foil = 6 Bq/mg = 6000 Bq/g, which exceeds the concentration limit. However, the total activity exemption (10 kBq) is the governing criterion for sealed sources in consumer/personal use — concentration limits apply to bulk materials, not discrete sealed sources. Confirm this interpretation with the supplier's regulatory team at time of order.
End-of-life: sealed sources below exemption thresholds can be returned to supplier or disposed through standard channels. No special waste declaration required at this activity level.
At 0.3 kBq (hand strips): 300 decays/sec × ~400,000 photons/alpha = 1.2 × 10⁸ photons/sec At 30cm, dark-adapted 7mm pupil: ~8,000 photons/sec at retina Scotopic threshold: ~100–1,000 photons/sec Margin: ~10–80× above threshold. Visible, dim but clear.
At 0.6 kBq (hour markers): 600 decays/sec × ~200,000 photons/alpha (Y₂O₃:Eu lower yield) = 1.2 × 10⁸ photons/sec At 30cm: ~8,000 photons/sec at retina Margin: same as hands despite lower phosphor yield, because the 2× activity compensates.
Both systems are comfortably above scotopic detection in a fully dark room. The clock will not be readable in dim ambient light (moonlight, streetlight) — it is a total-darkness instrument. Dark adaptation takes 20–30 minutes.
Am-241 half-life: 432 years (source is effectively permanent) Gd₂O₂S:Tb radiation life: decades (oxide lattice resists alpha damage) Y₂O₃:Eu radiation life: decades (oxide ceramic, chemically inert) Acrylic binder life: 20–30 years (UV-stable in indoor dark use) Mylar substrate: 50+ years (dimensionally stable)
The phosphor radiation damage is the limiting factor. Oxide phosphors degrade much slower than the traditional ZnS:Ag used in radium-era instruments. Expect visible glow for 20–40 years before significant dimming. The source outlasts everything else in the assembly by centuries.
- Am-241 alpha radiation is stopped by any solid barrier. The completed laminate blocks all alphas. No external radiation hazard from the finished clock.
- The only hazard is inhalation of Am-241 particles if the laminate is broken open and the foil is exposed. The five-layer encapsulation prevents this under any normal use condition (wall mounting, handling, cleaning).
- Do not burn, grind, sand, or saw through the luminous elements. If a laminate is damaged, seal it with cyanoacrylate and replace.
- Gamma emission from Am-241 (59.5 keV) is present but at these activities the dose rate at 30cm is negligible (~0.001 μSv/hr, far below background).
- At end of life, return sources to supplier or dispose through DZRNS-approved waste path. Do not discard in household waste.