Round 1 results
Round 1 of Edition I closed on 7 October 2026. Every solution received before the deadline was marked against one mark scheme, out of 100 points. The qualifying score for the Final is 75 points, and the participants below reached it.
Qualified for the Final
The list is alphabetical, not a ranking — places are decided in the Final. Individual scores are not published: each participant received their own score by email, broken down by part.
- Abd Alhadi Alrai Syria
- Abdulaziz Alshaya Saudi Arabia
- Ahmed Adel Abdelghafar Fathy Rashwan Egypt
- Alima Oralkhanova Kazakhstan
- Angelina Chizhova Kazakhstan
- Angelina Kyrgyzbaeva Kyrgyzstan
- Annavy Woro Ramadhani Indonesia
- Apakayeva Zhanna Tagirovna Kazakhstan
- Arjun Vijay Prakash India
- Bayarkhuu Munkh-Egshiglen Mongolia
- Cindy Lim Shin Yee Malaysia
- Gnapowska Karolina Poland
- Humay Turkmenistan
- Iman Muhammad Hassan Nigeria
- Jidnyasa Bodave India
- Khalfani Xavier Kamil Susanto Indonesia
- Lê Duy Phúc Vietnam
- Lubna Saoud Syria
- Lum Sze Nga Canada
- Mahmudur Rahman India
- Neda Gholipour Iran
- Nersan Khalid Musaed Al-Zahrani Saudi Arabia
- Nguyen Vu Hoai An Vietnam
- Nirmala Shantanu Pande India
- Olivia Defne Doğan Türkiye
- Rubaidul Karim Chowdhury India
- Ruszinosz Ánthi Virág Hungary
- Sanan Gaflanzada Kazakhstan
- Suhib Malak Egypt
- Syifa Andini Indonesia
- Trần Ngọc Khả Tâm Vietnam
- Vaibhav Sagar Shukla India
- Vandson Devid Galdino de Almeida Brazil
- Victor Ramiro Padilla Valle Ecuador
- Vidyut Sundar India
- Zahra Mohammed Alali Saudi Arabia
What happens next
The Final is a single live online session on 15 November 2026 at 14:00 UTC, lasting 120 minutes, under webcam supervision. Qualifiers receive their invitation by email. A place is held once the US$10 entry fee is paid at ircho.org/pay — the payment is the confirmation, and the deadline is 8 November 2026.
Rounds & format Pay the entry fee
How the papers were marked
Part A was worth 20 points (2 per question, no negative marking), Part B 36 (12 per problem) and Part C 44 (C1 14, C2 14, C3 16).
- Method marks stood alone: a correct formula with the right values substituted earned its marks even where the arithmetic went wrong.
- A wrong number carried forward from an earlier sub-question was penalised once, at the point where the mistake was made, and not again downstream.
- A missing or wrong unit cost one point of that sub-question, never more than one.
- A described method with no numbers still earned the method marks, as the round rules promised.
If you believe your paper was marked wrongly, write to organisers@ircho.org before 5 November 2026, naming the question. After that date the Round 1 results are final.
Answers and mark allocation
These are the official answers, with the marks carried by each sub-question. Tolerance bands were applied throughout: a result inside the band scored full marks, and significant figures were not marked.
Part A — 20 points (10 × 2)
| Question | Answer |
|---|---|
| A1 | B — ²³⁴Th |
| A2 | A — Activity: becquerel (Bq) |
| A3 | A — 100 Bq |
| A4 | B — A neutron converts to a proton, emitting an electron and an antineutrino |
| A5 | D — Emits gamma radiation (140 keV) and no alpha or beta particles; half-life ≈ 6 h |
| A6 | B — Equal to the activity of the parent |
| A7 | D — β⁻ decay |
| A8 | C — +4.8 MeV (exothermic) |
| A9 | D — 11 460 years |
| A10 | C — 1.25 |
B1 — ⁹⁰Sr / ⁹⁰Y 12 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | ⁹⁰Sr → ⁹⁰Y + e⁻ + ν̄ₑ ; ⁹⁰Y → ⁹⁰Zr + e⁻ + ν̄ₑ (⁹⁰Zr stable) | 2 |
| (b) | Ratio of half-lives ≈ 3.9 × 10³ — secular equilibrium applies | 2 |
| (c) | A(⁹⁰Y) after 10 d = 46.3 kBq | 3 |
| (d) | Total activity after 20 d ≈ 99.7 kBq | 3 |
| (e) | 99 % of equilibrium after 425 h = 17.7 days | 2 |
B2 — Neutron activation of gold 12 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | ¹⁹⁷Au + n → ¹⁹⁸Au + γ — radiative capture (n,γ); A + 1, Z unchanged | 2 |
| (b) | N = 3.057 × 10²⁰ atoms | 2 |
| (c) | Saturation activity = 30.2 GBq | 3 |
| (d) | Saturation factor 0.2269; A at end of bombardment = 6.84 GBq | 3 |
| (e) | A after 5 d = 1.89 GBq | 2 |
B3 — Dose from a ¹³⁷Cs source 12 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | Air-kerma rate = 0.430 mGy/h at 30 cm | 4 |
| (b) | Dose in 5 min = 35.8 µGy | 3 |
| (c) | ≈ 35.8 µSv (w_R = 1), with a stated reason why the equality is approximate | 2 |
| (d) | ≈ 560 procedures per year against a 20 mSv limit | 3 |
C1 — Unknown beta emitter 14 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | Net count rates: 800, 692, 598, 517, 447, 386, 334, 289, 250, 206 | 3 |
| (b) | ln A against t is a straight line of slope −λ | 2 |
| (c) | λ = 0.0485 d⁻¹, half-life = 14.3 days | 4 |
| (d) | ³²P (14.27 d); ³³P and ³⁵S are ruled out by the half-life | 2 |
| (e) | Initial activity = 26.7 Bq/L | 3 |
C2 — Post-accident water 14 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | 364.5 keV → ¹³¹I ; 661.7 keV → ¹³⁷Cs, with both decay equations | 3 |
| (b) | ¹³¹I = 59.0 Bq/L ; ¹³⁷Cs = 126 Bq/L | 4 |
| (c) | Both exceed 10 Bq/L — by ≈ 5.9× and ≈ 12.6× | 2 |
| (d) | Time since release ≈ 8.8 days | 3 |
| (e) | ¹³⁷Cs is unchanged over that time; roughly half the ¹³¹I has decayed | 2 |
C3 — Two-component medical waste 16 points
| Sub-question | Answer | Marks |
|---|---|---|
| (a) | Net rates: 750, 574, 448, 293, 211, 168, 144, 129, 107, 95, 75, 60 | 3 |
| (b) | Not linear: a sum of two exponentials, steep early and flattening (concave up) | 2 |
| (c) | Long-lived tail: half-life 359 min, back-extrapolated intercept 151 cpm | 4 |
| (d) | After subtraction, short-lived half-life = 20.2 min | 4 |
| (e) | ¹¹C (20.4 min) at ≈ 600 cpm and ⁹⁹ᵐTc (360.4 min) at ≈ 151 cpm | 2 |
| (f) | R(t) = 600 e^(−0.0344 t) + 151 e^(−0.00193 t) counts/min | 1 |