## Master Isotope Table — NIST SRD-128 Kα₁ (Z = 61–80)
| Z | Elem | Known | Stable | Unstable | Pred. | Gap | Nuclear γ (keV) | NIST Kα₁ (keV) | Context |
|---:|:---:|---:|---:|---:|---:|---:|---:|---:|---|
| 61 | Pm | 39 | 0 | 39 | 93 | 54 | — | 38.707 | No stable isotopes; XRF line |
| 62 | Sm | 41 | 7 | 34 | 97 | 56 | 333.0 | 40.093 | ¹⁵³Sm therapy + XRF |
| 63 | Eu | 40 | 2 | 38 | 95 | 55 | 121.8 / 344.3 | 41.507 | ¹⁵²Eu calibration + XRF |
| 64 | Gd | 41 | 7 | 34 | 97 | 56 | 103.2 | 42.950 | ¹⁵³Gd tracer + XRF |
| 65 | Tb | 39 | 1 | 38 | 93 | 54 | 298.6 | 44.422 | ¹⁶⁰Tb therapy + XRF |
| 66 | Dy | 40 | 7 | 33 | 95 | 55 | — | 45.924 | Stable; XRF Kα₁ |
| 67 | Ho | 39 | 1 | 38 | 93 | 54 | 133.0 | 47.456 | ¹⁶⁶Ho therapy + XRF |
| 68 | Er | 40 | 6 | 34 | 95 | 55 | — | 49.017 | Stable; XRF Kα₁ |
| 69 | Tm | 39 | 1 | 38 | 93 | 54 | 88.0 | 50.609 | ¹⁷⁰Tm calibration + XRF |
| 70 | Yb | 41 | 7 | 34 | 97 | 56 | — | 52.231 | Stable; XRF Kα₁ |
| 71 | Lu | 40 | 1 | 39 | 95 | 55 | 113.0 / 208.4 | 53.884 | ¹⁷⁷Lu therapy + XRF |
| 72 | Hf | 36 | 5 | 31 | 85 | 49 | 482.2 | 55.568 | ¹⁸¹Hf tracer + XRF |
| 73 | Ta | 37 | 1 | 36 | 88 | 51 | 67.7 / 1221.4 / 1231 | 57.284 | ¹⁸²Ta γ emitter + XRF |
| 74 | W | 35 | 5 | 30 | 83 | 48 | 685.8 | 59.318 | ¹⁸⁷W tracer + XRF |
| 75 | Re | 39 | 1 | 38 | 93 | 54 | 137.2 | 61.140 | ¹⁸⁶Re therapy + XRF |
| 76 | Os | 35 | 7 | 28 | 83 | 48 | 129.4 | 63.087 | Stable + XRF |
| 77 | Ir | 34 | 2 | 32 | 81 | 47 | 316.5 / 468.1 / 604.7 | 65.145 | ¹⁹²Ir industrial γ + XRF |
| 78 | Pt | 35 | 6 | 29 | 83 | 48 | 99.0 | 66.832 | ¹⁹⁵ᵐPt (lab) + XRF |
| 79 | Au | 36 | 1 | 35 | 85 | 49 | 411.8 | 68.804 | ¹⁹⁸Au tracer + XRF |
| 80 | Hg | 38 | 7 | 31 | 90 | 52 | 279.2 | 70.819 | ²⁰³Hg environmental + XRF |
Say “Next” and I’ll deliver Z = 81–100.