Radiochemistry: Radioactivity, Carbon Dating, Neutron Activation

A nucleus with too many or too few neutrons relative to its proton count is unstable. It rearranges itself by ejecting a particle or a photon, and the process is what radiochemistry counts. Three decay modes dominate the casework-relevant isotopes.

Alpha decay ejects a helium-4 nucleus (two protons and two neutrons) from a heavy nucleus. The atomic number drops by two and the mass number by four. Alpha particles are heavy and slow, deposit energy densely along a short track, and stop in a few centimetres of air or in a sheet of paper. Forensic exposure to alphas almost always happens by inhalation or ingestion of an alpha-emitting source (polonium-210 was the Litvinenko case in London in 2006); the danger is internal, not external.

Beta decay ejects an electron from the nucleus, the result of a neutron converting to a proton. The atomic number rises by one, the mass number is unchanged, and an antineutrino carries off some of the energy. Beta particles are lighter and faster than alphas, penetrate a few millimetres of tissue, and are blocked by a centimetre of plastic. Carbon-14 (5,730 year half-life), tritium-3 (12.3 year half-life) and phosphorus-32 (14.3 day half-life) are all pure beta emitters and all appear in forensic and medical work.

Gamma decay is the relaxation of an excited nucleus by emission of a high-energy photon, usually following an alpha or beta decay that leaves the daughter nucleus in an excited state. Atomic number and mass number are both unchanged. Gamma photons penetrate centimetres of lead and are the longest-range of the three; they are also the easiest to do spectroscopy on, because the photon energy is sharply defined and is the fingerprint of the emitting nucleus.

The activity of a radioactive sample is A = lambda × N, where lambda is the decay constant and N is the number of radioactive atoms present. Activity decays exponentially as A(t) = A0 exp(minus lambda t), and the half-life t1/2 = ln 2 / lambda is the time for activity to drop by half. The SI unit of activity is the becquerel (Bq), defined as one disintegration per second. The older curie (Ci) is 3.7 × 10^10 Bq, and the microcurie (37 kBq) is still the common unit for sealed sources in Indian university teaching labs licensed by AERB.

The detector landscape in radiochemistry sorts into three physical families: gas-filled detectors, scintillation detectors and semiconductor detectors. Each family has its own resolution, sensitivity and price point, and each lab settles on a particular mix for its workload.

The Geiger-Muller tube is the founding gas detector. A cylindrical metal tube filled with argon plus a quenching gas (methane or a halogen) at low pressure holds a thin central anode wire at about 400 to 900 volts. An ionising particle entering through the thin mica or aluminised-mylar window ionises gas atoms; the freed electrons accelerate towards the anode and trigger a Townsend avalanche that produces a pulse large enough to count with a simple amplifier. The tube is cheap, robust, and sensitive to alpha, beta and gamma. The fundamental limitation is that every pulse looks the same regardless of the energy of the incoming particle; a GM counter reports disintegrations per second but cannot identify the isotope producing them. That is fine for radiation safety surveying and for crude alpha/beta/gamma triage; it is not enough for spectroscopy. Indian forensic radiation surveys at scene (a suspected radioactive contamination event, a missing source recovery) still rely on GM-based handheld monitors of the Nucleonix RM-7000 or Polimaster type.

Proportional counters share the gas geometry but operate at a lower voltage where the avalanche size stays proportional to the initial ionisation, so the pulse height carries energy information. Useful for low-energy X-rays and for alpha counting where you need to separate alphas from beta and gamma background.

Scintillation detectors split into three workhorses. Sodium iodide doped with thallium (NaI(Tl)) crystals coupled to a photomultiplier tube are the standard gamma-spectroscopy detector at hospital nuclear medicine departments and at any lab that wants a basic gamma spectrum without the cryogenic complexity of HPGe. Resolution at the 662 keV gamma line of caesium-137 is about 7 percent, which is enough to separate caesium from cobalt-60 but not enough to resolve the close-lying lines of a complex NAA spectrum.

Liquid scintillation counters dissolve the sample in a scintillation cocktail (a hydrocarbon solvent like toluene or diisopropylnaphthalene plus a fluor like PPO) inside a glass or plastic vial. A beta particle from carbon-14 or tritium dissolved in the cocktail loses its energy to the fluor molecules and produces visible-light flashes. Two photomultiplier tubes in coincidence read the flashes and reject single-tube noise as background. Liquid scintillation is the historical workhorse for radiocarbon dating, for tritium environmental monitoring and for the medical isotopes used in biology. Modern Hidex 300SL and PerkinElmer Tri-Carb units sit at BARC, at BSIP Lucknow and at the AERB approved radiation laboratories.

Plastic scintillators are fast and cheap but lower resolution; useful for personnel monitoring and for portal radiation monitors at port and airport checkpoints.

Semiconductor detectors use a reverse-biased diode (germanium or silicon) where ionising radiation creates electron-hole pairs that the bias field collects as a charge pulse. Because the energy required to create a pair is only about 3 eV (against 100 eV for a typical scintillator), the resolution is far better. HPGe (high-purity germanium) cooled to liquid-nitrogen temperatures gives 1.8 keV resolution at the 1,332 keV cobalt-60 line and is the reference gamma spectrometer for neutron activation analysis. Si(Li) silicon detectors do low-energy X-ray and beta. Silicon drift detectors (SDD) are the modern replacement for Si(Li) in energy-dispersive X-ray spectroscopy on electron microscopes.

The counting electronics behind any of these detectors run in two basic modes. Total counting integrates all pulses above a discriminator threshold and reports a count rate; useful for activity measurement once the isotope is known. Spectroscopy sorts pulses by energy through a multi-channel analyser and reports a spectrum of counts versus energy; required for isotope identification when more than one emitter is present. Coincidence counting accepts pulses only when two detectors fire within a narrow time window, which suppresses background and isolates pair-emitting decays.

The radiocarbon clock starts in the upper atmosphere. Cosmic-ray neutrons collide with atmospheric nitrogen-14 and convert it to carbon-14 by the reaction N-14 + n → C-14 + p. The resulting carbon-14 oxidises to CO₂, mixes through the atmosphere on a timescale of years, and enters the biosphere through photosynthesis. The steady-state ratio of carbon-14 to ordinary carbon-12 in the atmosphere is approximately 1.2 × 10^-12, which is the same ratio that ends up in every living plant and in every animal that eats the plant. When the organism dies, carbon uptake stops, the carbon-14 in its tissue decays with a half-life of 5,730 years, and the ratio drops as a clock.

The dating equation is straightforward. The age t since death is t = (t1/2 / ln 2) × ln(N0/N), where N0 is the carbon-14 abundance at death (taken as the modern reference value) and N is the carbon-14 abundance now. For a 5,000-year-old sample, the ratio has dropped to about 55 percent of modern; for a 25,000-year-old sample, to about 5 percent; for a 50,000-year-old sample, to about 0.2 percent, which is where the radiocarbon clock runs out against detector background.

Three measurement methods historically competed. Conventional liquid scintillation counting needs a gram or more of carbon, converts the sample to benzene through a wet-chemistry chain, and counts the carbon-14 beta decays in a low-background scintillation counter for several days to several weeks. Gas proportional counting needs similar sample sizes and converts the carbon to methane or CO₂ for counting. Both are slow, both are insensitive, both struggle below 30,000 years.

Accelerator mass spectrometry (AMS) is the modern method and the only one that matters for forensic casework. The sample is graphitised (typically combusted to CO₂, then reduced to elemental graphite over an iron catalyst) and pressed into an aluminium target holder. A caesium sputter ion source generates negative carbon ions from the graphite; a low-energy mass spectrometer selects mass 14; a tandem accelerator stripping foil destroys molecular interferences (especially the troublesome N-14 hydride and C-13 hydride); a high-energy mass spectrometer separates the surviving atomic carbon-14 ions; and a gas-ionisation or silicon-strip detector counts them individually. The carbon-12 and carbon-13 ions are measured in Faraday cups in the same run for the ratio. AMS needs only about a milligram of carbon, completes a measurement in tens of minutes (plus sample preparation), and extends the practical dating range to about 50,000 years.

Forensic applications of radiocarbon concentrate in three areas. Questioned-document dating reads the paper itself or the ink (if the binder contains modern enough organic carbon to extract). A claimed 1925 manuscript that AMS-dates to 1985 settles the authenticity question without further argument. Antiquity authentication reads pigments, parchments, textiles and wooden frames of disputed artworks against the claimed period. The Shroud of Turin AMS dating in 1988 (three independent labs returning a median date of approximately 1325 AD against a claimed first-century provenance) is the standard reference for the technique. Wildlife forensics reads ivory and rhino horn against the 1989 CITES ban; pre-ban material is legal and post-ban material is contraband, and AMS resolves the question on a single shaving of horn or ivory.

Indian forensic AMS access has historically meant sending samples abroad to Beta Analytic in Miami, the Oxford Radiocarbon Accelerator Unit, or the ANSTO facility in Australia. The Inter-University Accelerator Centre (IUAC) in New Delhi commissioned an AMS facility based on a 500 keV pelletron in the mid-2010s (circa 2015), and the Birbal Sahni Institute of Palaeosciences in Lucknow runs a separate AMS for palaeoclimate and archaeological work. PRL Ahmedabad and the National Geophysical Research Institute Hyderabad have related radioisotope and noble-gas mass spectrometry capability. Most CFSL questioned-document cases that require AMS still route through one of these three Indian facilities or, for capacity reasons, through an overseas commercial lab.

Neutron activation analysis (NAA) puts a sample inside the neutron flux of a nuclear research reactor. Stable nuclei in the sample capture a neutron and become unstable; the resulting radioactive isotopes decay by gamma emission, and the gamma energies and intensities identify and quantitate the parent elements. The neutron flux at BARC's Dhruva reactor is in the 10^14 neutrons per square centimetre per second range, which is enough to activate trace levels of more than 70 elements in a sample of a few milligrams.

The basic reaction is (n, gamma) capture: stable isotope X(A) plus a thermal neutron becomes X(A+1) plus a gamma. Many product isotopes are radioactive and decay by beta-minus emission to an excited daughter, which then de-excites by emitting a gamma photon of characteristic energy. Manganese-55 captures a neutron to become manganese-56, which beta-decays to iron-56 with prompt gamma emission at 847 keV; that 847 keV line is the manganese fingerprint in any activated sample. Arsenic-75 captures to arsenic-76, with gammas at 559 keV and 657 keV; mercury-198 captures to mercury-199m and 197 keV gamma; gold-197 to gold-198 and 412 keV.

The instrumental variants split by chemistry and by timing. Instrumental NAA (INAA) irradiates and counts without any chemical separation; the multi-element spectrum is unscrambled by the HPGe resolution and by gamma-line and half-life cross-matching. Radiochemical NAA (RNAA) separates the element of interest chemically before counting, which gains sensitivity by removing the gamma background of the matrix. Prompt-gamma NAA (PGAA) reads the prompt gamma emitted during the capture itself, requires the sample to sit inside the reactor with a gamma detector outside, and is used for light elements (hydrogen, boron, nitrogen, silicon) that do not produce useful delayed activity. Delayed-gamma NAA (DGNAA) is the routine post-irradiation counting that handles the bulk of casework.

The forensic-relevant strengths of NAA are three. First, it is multi-element in a single run: a single irradiation plus a single HPGe count yields a panel of 30 to 60 elements at trace levels. Second, it is genuinely non-destructive for many sample types: a hair shaft, a paint chip, a glass fragment or a soil grain returns to the analyst essentially unchanged after the activity has decayed away. Third, it reaches parts-per-billion sensitivity on many elements without preconcentration, which is rare in any other multi-element technique.

The Indian forensic-NAA workload concentrates in a few well-defined applications. Chronic heavy-metal exposure analysis on hair shafts is the classical case: arsenic, antimony, mercury, gold and thallium incorporate into growing hair in proportion to the blood concentration, and a longitudinal scan along the shaft (in 1 cm segments, against a typical growth rate of 1 cm per month) reconstructs an exposure history over months to years. BARC and NCCCM Hyderabad have run NAA on hair samples for several historic Indian poisoning investigations and for environmental arsenic-exposure cohorts in West Bengal. Glass fragment comparison reads trace elements at parts-per-million precision and pairs well with refractive-index measurement for glass-from-clothing cases. Soil profiling reads the rare-earth element pattern, which is regional-geology specific. Ink dating reads trace metals whose formulation has shifted across decades and helps date historical documents in conjunction with optical microscopy. Drug-origin tracing reads trace metals whose pattern reflects the geography of the precursor synthesis. Authentication of historical manuscripts reads pigment composition without destroying the artefact.

The map of Indian radiochemistry facilities relevant to forensic work is short enough to commit to memory. Bhabha Atomic Research Centre (BARC) at Trombay in Mumbai runs the Dhruva reactor (100 MW thermal, commissioned 1985, the highest-flux research reactor in the country) and historically the Apsara reactor (1 MW, the first Asian research reactor, originally commissioned 1956, modernised and recommissioned as Apsara-U in 2018). The Cirus reactor (40 MW, Canadian-supplied) operated from 1960 to 2010 and was decommissioned. The BARC Radiochemistry Division provides NAA services on a project basis to research and forensic users; samples are sealed in high-purity polyethylene capsules, irradiated in the reactor pneumatic-transfer system, cooled for a defined period, and counted on the HPGe bench.

Tata Institute of Fundamental Research (TIFR) Mumbai operates the 14 UD pelletron accelerator and runs low-energy nuclear physics. The Variable Energy Cyclotron Centre (VECC) in Kolkata operates a K-130 cyclotron and is building the K-500 superconducting cyclotron. The Raja Ramanna Centre for Advanced Technology (RRCAT) in Indore runs the Indus-2 synchrotron (2.5 GeV) with X-ray absorption and X-ray diffraction beamlines; the Bhabhatron telecobalt unit (a BARC-developed cobalt-60 teletherapy machine) is the Indian-made radiation therapy device used at regional cancer centres.

The Inter-University Accelerator Centre (IUAC) in New Delhi houses a 15 UD pelletron and, more recently, a dedicated 500 keV pelletron AMS facility for radiocarbon dating. Birbal Sahni Institute of Palaeosciences (BSIP) in Lucknow runs an AMS for palaeobotanical and Quaternary geology dating; the same instrument has occasionally serviced forensic questioned-document cases. The Physical Research Laboratory (PRL) Ahmedabad runs noble-gas mass spectrometry and radioisotope geochronology. The National Centre for Compositional Characterisation of Materials (NCCCM) in Hyderabad, a DAE laboratory, runs NAA and other trace-analysis services on a charge basis.

The limitations of Indian forensic radiochemistry are not the chemistry but the logistics. Reactor time at Dhruva is allocated by project rather than by case, with turnaround measured in weeks. AMS samples at IUAC or BSIP queue against the host institution's own research programme. Sample submission requires advance approval, sealed packaging under prescribed conditions, and documented chain of custody compatible with both the receiving institution's radiation safety protocols and the originating investigation's evidentiary requirements. The cost per sample sits in the tens of thousands of rupees for routine NAA and the lakhs of rupees for AMS dating, which limits the technique to cases where the question is large enough to justify the spend.