Medical Physicist · MAX 4000+

How MAX 4000+ fits the TG-51 / TRS-398 reference-dosimetry chain.

Five decision rules on why a reference-grade electrometer belongs on the physicist bench — sourced against the TRS-398 uncertainty budget, TG-51 addendum, and AERB SC/IR-1 dosimetry-service framework.

The electrometer is one term in a ≤ 3 % dose-delivery uncertainty budget

IAEA TRS-398 and AAPM TG-51 both target an overall dose-delivery uncertainty around 3 % combined. Electrometer accuracy is one contributor alongside ion-chamber calibration factor Ndw, beam-quality kQ, temperature-pressure PTP, polarity Ppol and recombination Pion. The electrometer needs to sit well inside 0.5 % over its scale so its contribution to the combined budget stays small. Generic multimeters that drift multiple percent between service intervals push the combined budget past 3 %.

Source: IAEA TRS-398 §7 — uncertainty budget; AAPM TG-51 addendum (McEwen et al., Med Phys 41(4):041501 2014).

Charge + current modes — the two integration read-outs both matter

Reference dosimetry integrates charge (nC) over a beam-on window and back-computes dose from Ndw × kQ. Charge mode is the standard TG-51 / TRS-398 read. Current mode (pA) is used for slow-dose in-vivo checks and low-dose-rate radionuclide sources where charge integration would take unreasonable time. A reference-grade electrometer runs both modes to the same calibration standard — MAX 4000+ swaps between them with a single control, keeping the calibration chain intact.

Source: IAEA TRS-398 §4 — reference dosimetry procedure; charge + current mode definitions.

Bias polarity — Ppol needs both signs at the same voltage

TRS-398 requires the polarity correction Ppol be measured by reading the chamber at ±V and averaging. The electrometer must supply a stable bias in both polarities at the same voltage magnitude (typically ±300 V for Farmer chambers, ±100 V for parallel-plate). MAX 4000+ carries selectable positive and negative bias with < 0.1 V setpoint drift, so the ±V pair used for Ppol is genuinely symmetric.

Source: IAEA TRS-398 §4.3.2 — Ppol determination requirements; McEwen 2014 addendum.

Traceability chain — NIST/PTB reference back to the primary standard

The electrometer's calibration factor is traceable back to a primary standards laboratory — NIST in the US or PTB in Germany, via an ADCL secondary lab. That traceability is not decorative: ISO/IEC 17025 accredited dosimetry services (including any hospital physicist audited by an ACPRO / AERB review) need the traceability chain documented per instrument. MAX 4000+ ships with a NIST-traceable calibration certificate; the recommended re-calibration interval is 2 years, matched to ADCL cycles.

Source: AAPM TG-51 addendum (2014) — traceability chain; AERB SC/IR-1 dosimetry-service accreditation requirements.

One reference electrometer, one Farmer chamber, one calibration factor

A single-linac / single-cobalt department only needs one reference-dosimetry electrometer + one Farmer chamber for TG-51 / TRS-398 output calibration. Their Ndw calibration factor is pair-specific — swapping either instrument forces re-calibration. Larger multi-machine departments typically stock a second unit as a QC-check backup, keeping the reference pair on the shelf between annual calibrations to preserve the traceability chain.

Source: AAPM TG-51 §6 — chamber + electrometer pairing rules; TRS-398 §4.2 for the equivalent framework.