Chlorine Atoms and Density Decide How Psychoactive Drugs Attenuate X-rays and Gamma Rays

When X-rays or gamma rays pass through matter, their fate is decided by three competing processes: photoelectric absorption at low energies, Compton scattering at intermediate energies, and pair production above 1.022 MeV. A new computational study published in Results in Physics has mapped how these processes play out across fifteen well-known psychoactive substances, from caffeine and nicotine to morphine, cocaine, diazepam, and LSD, and the results show that molecular composition leaves a clear and quantifiable fingerprint on radiation attenuation.

The study, carried out by Fatma Akyıldız, used the EpiXS software package together with the EPICS2017 photon cross-section library, which is based on ENDF/B-VIII.0, to calculate a comprehensive set of photon interaction parameters for each compound over the energy range of 0.015 to 15 MeV. The quantities evaluated included the mass attenuation coefficient, the linear attenuation coefficient, the half-value layer, the effective atomic number, the effective electron density, the equivalent atomic number, the total kerma coefficient, and both the exposure buildup factor and the energy absorption buildup factor. All fifteen substances were treated under identical computational conditions, allowing the effects of elemental composition and density to be separated cleanly.

The fifteen compounds were deliberately chosen to span different pharmacological classes and different elemental makeups. Amphetamine and nicotine are liquids composed almost entirely of carbon, hydrogen, and nitrogen. Caffeine, meprobamate, and the barbiturates pentobarbital and phenobarbital contain substantial oxygen fractions. Diazepam and chlordiazepoxide stand apart because they include chlorine, an element with a considerably higher atomic number than any other constituent in the dataset. Densities ranged from 0.936 g cm⁻³ for amphetamine to 1.56 g cm⁻³ for heroin, and the author notes that where directly traceable experimental densities were unavailable, database-derived adopted values were used rather than new measurements.

The most striking compositional effect appears at low photon energy. At 15 keV, diazepam shows the highest mass attenuation coefficient of the entire set, 3.0084 cm² g⁻¹, followed closely by chlordiazepoxide at 2.9168 cm² g⁻¹. These values are far above those of the chlorine-free compounds because even a modest chlorine fraction, roughly 12 percent by weight in diazepam and 11.8 percent in chlordiazepoxide, strongly enhances photoelectric absorption, which scales steeply with atomic number. Among the chlorine-free substances, those with higher oxygen or nitrogen content, such as meprobamate at 1.1282 cm² g⁻¹ and caffeine at 1.0782 cm² g⁻¹, outperform the carbon-rich compounds. Phencyclidine, or PCP, gives the lowest value at 0.7875 cm² g⁻¹, with amphetamine, methadone, and nicotine close behind.

This separation collapses rapidly as photon energy rises. In the Compton-dominated region around 1.5 MeV, the mass attenuation coefficients of all fifteen compounds fall within a narrow band from 0.05391 to 0.05706 cm² g⁻¹, and at 15 MeV they span only 0.01766 to 0.01854 cm² g⁻¹. The difference between the highest and lowest values shrinks from nearly a factor of four at 15 keV to just a few percent in the megaelectronvolt range, because Compton scattering depends mainly on electron density rather than atomic number. Once the mass coefficients converge, bulk density becomes the dominant discriminator of attenuation per unit thickness. Heroin, with the highest density in the series, then delivers the highest linear attenuation coefficient and the smallest half-value layer, 24.82 cm at 15 MeV, while low-density amphetamine shows the weakest performance with a half-value layer of 41.41 cm.

The effective atomic number tells the same story in a different language. At 15 keV, diazepam and chlordiazepoxide reach values of 11.180 and 11.001 respectively, while PCP and amphetamine sit at the bottom with 4.860 and 4.989. Around 1 MeV, all curves converge to a broad minimum as Compton scattering takes over, with values ranging from about 3.117 for PCP to 4.486 for diazepam. Above the pair-production threshold of 1.022 MeV, the effective atomic numbers climb again, reaching 3.476 to 5.089 at 15 MeV, because pair production carries a stronger dependence on atomic number than Compton scattering. The effective electron density follows a related but not identical pattern, since it counts electrons per unit mass and therefore weights both elemental fractions and atomic masses.

The total kerma coefficient, which quantifies the kinetic energy transferred from photons to secondary charged particles per unit mass, behaves differently from the attenuation coefficients altogether. Rather than declining monotonically with energy, KermaTot falls from its 15 keV maximum to a minimum near 50 to 80 keV and then rises steadily toward 15 MeV, where values reach approximately 3.28 to 3.48 × 10⁻¹¹ Gy·cm² per photon. At 15 keV, diazepam and chlordiazepoxide again lead with 6.4387 × 10⁻¹² and 6.2254 × 10⁻¹² Gy·cm² per photon, while PCP records the lowest value at 1.2716 × 10⁻¹² Gy·cm² per photon. This U-shaped profile reflects the increasing energy carried away by secondary charged particles at high photon energies even as the overall interaction probability drops.

The buildup factors reveal the most dramatic behavior in the entire study. Unlike the Beer–Lambert law, which counts only uncollided primary photons, buildup factors account for scattered photons that reach a point of interest after multiple interactions within the material. Using the geometric progression fitting method with equivalent atomic numbers interpolated from reference elements, the study calculated both exposure and energy absorption buildup factors at penetration depths from 1 to 40 mean free paths. At shallow depths the differences are modest, but they explode with penetration. At 80 keV and 40 mean free paths, PCP produces an energy absorption buildup factor of 1.182 × 10⁵ and an exposure buildup factor of 2.654 × 10⁵, meaning scattered photons multiply the dose by more than five orders of magnitude. Amphetamine follows at 1.069 × 10⁵, while chlordiazepoxide and diazepam remain more than an order of magnitude lower, at 3.752 × 10³ and 3.341 × 10³ respectively.

The physical explanation lies in the competition between absorption and scattering. PCP and amphetamine, being carbon-hydrogen-nitrogen-rich with low equivalent atomic numbers, absorb few low-energy photons photoelectrically, so photons survive to scatter repeatedly and accumulate. The chlorine-containing benzodiazepines, by contrast, remove a larger fraction of photons through photoelectric absorption before they can multiply. By 15 MeV and 40 mean free paths, the buildup factors converge to narrow ranges of roughly 7.8 to 8.2 for both quantities, and the ordering even reverses slightly, with diazepam highest and PCP lowest, showing that PCP’s dominance is confined to the low and intermediate energy region where Compton multiple scattering outpaces photoelectric removal.

The author emphasizes that these compounds are not proposed as practical radiation shielding materials, and that the findings are limited to theoretical calculations for idealized, homogeneous bulk forms defined by the adopted compositions and densities. Experimental verification, along with assessment of physical, chemical, and mechanical properties, would be needed before any shielding application could be considered. Nevertheless, the dataset offers a quantitative reference for computational photon transport, dosimetric evaluations, and radiation protection analyses involving pharmaceutical and organic media, and it demonstrates that no single parameter can capture the full photon interaction response of a material. Low-energy attenuation is governed by composition, thickness-dependent attenuation by density, and buildup by the interplay of absorption and multiple scattering across both energy and depth.

Subject of Research: Photon interaction parameters and radiation shielding characteristics of psychoactive substances

Article Title: Comparative investigation of photon interaction parameters in psychoactive substances: the role of molecular composition

Article References: Comparative investigation of photon interaction parameters in psychoactive substances: the role of molecular composition. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: radiation physics, photon attenuation, psychoactive substances, mass attenuation coefficient, buildup factors, effective atomic number, EpiXS, EPICS2017, diazepam, PCP, radiation shielding, dosimetry