Hexogen
Hexogen the boiling crytal - 1969
Alain Filhol - 1969 - unpublished results DOI: 10.13140/RG.2.1.1008.0161- Crystallography lab. of Prof. R. Gay, University of Bordeaux I, France.
Hexogen is a secondary explosive which is easy to crystallize.
It is also known as RDX, Cyclonite, 1,3,5-trinito-1,3,5-trinitro-1,3,5-triazine, 1,3,5-trinito-1,3,5-triazacyclohexane, etc.
This page [6] is about my very first research project "Behaviour of hexogen crystals under radiation".
My newbie experiments used what was available at the lab at zero expense (X-ray tube, ultraviolet lamp, 16 mm camera, liquid nitrogen, etc.) I designed and built the setup by myself. I presented the movies you will find here at a project progress meeting at the CEA-CESTA (France). A Professor from an other lab discouraged me to continue in that direction and my supervisor did not say a word. It is true that this had little to do with crystallography so I found something else to do on hexogen [4].
No effort has been made so far to compare the results below with more recent works on similar topics.
MP4 movie, duration 6'27"
X-ray setup
a: movie camera, b: magnifying optics, c: X-ray tube, d: light, e: sample chamber with lead shielding, f: stop motion timer
Ultraviolet setup
a: Stop motion timer, b: light, c: sample under the microscope, d: mercury vapour lamp with a quartz focusing optic pointing at the crystal surface, e: movie camera
Irradiations with 30 keV X-rays at room temperature
Experimental conditions
- X-ray source : a standard tungsten tube (30 keV, 10 mA).
- Both the hexogen and oxalic acid being almost transparent to X-rays, the bulk of the sample is irradiated unlike what happens with ultraviolet rays.
- Sample: hexogen single crystals a few millimeter wide
- Dose rate: 130 Mrad in 6 days
- Image recording: 10 photos per hour.
Comments
- At the end of the experiment, simply touching the crystal made it collapse into powder.
Ultraviolet light at room temperature
Experimental conditions
- Source of ultraviolet light: a standard black light lamp (Wood's lamp) from which the glass filter was removed. The lamp was placed in a metal housing to avoid hazards to skin or eyes. The beam was focused on the sample through quartz lenses.
With ultra-violet rays the dose rate is much higher than with X-rays but the radiolysis is superficial (a few tens of microns). This is very important to understand the experiment. - Samples: hexogen single crystals 6 mm wide. The exposed face is (120)
- Duration of irradiations: 12 hours
- Dose rate: it was never estimated since the project was canceled before.
- Shooting: 68 images per hour (1 image each 53 seconds)
- Magnified views: the microscope field of vision was 0.15 mm wide.
Description
- Very rapidely "bubbles" forms at the surface of the hexogen crystal and burst. Each "bubble" persists for at least 15'.
- This give the impression of a "boiling" material, however, the surface of the crystal is not molten because its melting point is too high (TF = 201,5ºC) and the UV lamp does not heat up that much.
- The bubbles forms on what seems to be the emergence of lattice defects.
- When the irradiation is stopped, boiling continues for several hours (delayed radiolysis) to finally leave the crystal surface almost intact apart from small dots (small craters?)
- If the irradiation period is very long, the crystal ejects small grains (white powder) as far as 2 or 3 cm from itself (i.e. 5 times its diameter). The movie shows such tiny particles ejected from the sample surface.
Ultraviolet light at low temperature
Experimental conditions
- They are identical to that at room temperature except for the temperature.
- The hexogen crystal is placed in contact with a piece of brass which is cooled down with liquid nitrogen.
- Temperature is measured with a thermocouple thermometer,
Description
- The low temperature irradiation leaves the crystal visually intact (transparent) except for a visible yellowing.
- When irradiation is stopped the yellowing remains as long as the temperature is kept low enough.
- Then the crystal is warmed slowly. "Bubbles" appear at about -70ºC and these are similar to "bubbles" observed during of after the UV irradiation at room temperature.
Test sample: oxalic acid
Experimental conditions
- X-ray source: a standard tungsten tube (30 keV, 10 mA)
- Sample: oxalic acid [(COOH)2, 2H2O], single crystals a few millimeter wide
- Dose rate: 100 Mrad in 4 days
- Image recording: 10 photos per hour.
Description
- This material was selected because big crystals were available and because it contains no nitro groups.
- I used it as test sample for the setup and I also wanted to see if the behavior of hexogen under irradiation was either similar or substantially different from others. This was a 1st of a planed set of tests that was canceled.
MP4 movie, duration 1'46"
Discussion
Radiolysis of hexogen
In 1969 my knowledge about the radiolysis of hexogen was almost zero. The data below probably came form an internal CEA-DAM report or private communications from Jean Cherville's team [1] since papers and reports dates from 1971.
In my records I noted the following:
The yield and nature of the final radiolysis products are nearly independent of the radiation used (UV (254 mµ), XR (9 et 150 keV), γ (60Co et 137Cs), ē (4.5 MeV), n (14 MeV), p (207 et 600 MeV)). The fact that the radiochemical yield is the same for n and γ radiations indicates that the recoil of nuclei is not the dominant phenomenon in the radiolysis mechanism for this material.
Irradiation is accompanied by the formation of decomposition products (liquids and gases). The main gases listed are:
Temp H2O N2 CO2 N2O 300 K 18% 43% 17% 7% 77 K 18% <17% 40% 10%
Nature of bubbles
The fact that the formation of bubbles persists for several hours after irradiation and they appear to -70ºC in the absence of radiation, all this confirms that there is no melting of the surface crystal du to UV heating. Delayed radiolysis is described in [2].
Formation mechanism
In 1969, I hazarded two explanations for the bubbles:
- the radiolysis gases exfoliate crystal layers. This would suggest that the broken bonds are within plans parallel to (120).
- the bubbles do not come from a melting of the surface but from radiolysis gases pushing out liquids through lattice defects. Which liquids ? H2O is not a good candidate since bubbles exist as low as -70ºC.
The first mechanism is more consistent with small white particles being projected all around the crystal.
Why do bubbles appear at -70ºC?
N2O is the only radiolysis product in the list above with a boiling point close to -70ºC, however it represents only 10% of gases emitted by the irradiated material.
Why a yellowing of the crystal under irradiation at low temperature ?
- This is not explained by the colorless N2O.
- Nitrogen peroxide NO2 is a radiolysis product known to be trapped inside the lattice [2] but its high melting point (TB=21ºC) is not in favor with its direct contribution to bubbles.
- The irradiation of hexogen also produces free nitroxide radicals (C2N0•) [2] . The yellowing of the crystal could be due to the later since they give a yellow color to Tanol, a stable free nitroxide radical [3].
The appearance of bubbles at -70ºC while the irradiation is stopped for long is in favor of radiolysis products such as NO2 or (C2N0•) reacting with the material. The mechanism of the delayed radiolysis process is postulated by [2,3].
References
[1] Cherville J., "Physical chemistry and the gamma radiolysis of hexogen (in French)", CEA-R--4146, 1971. <http://www.osti.gov/scitech/biblio/4025522>
[2] Cherville J. et al., "Décomposition retardée de la trinitro-1-3-5 hexahydro s-triazine (hexogène) après irradiation γ", Int. J. Radiat. Phys. Ch. 3, (1971), 457–465.
[3] Lemaire H., Rey P., Rassat A., de Combarieu and Michel J.C., Molecular Phys. 14 (1968) 201.
"Le phénomène le plus important consiste en une décomposition retardée qui dure plusieurs jours. Cette décomposition est attribuée à l'effet sur l'hexogène du peroxyde d'azote piégé lors de l'irradiation."
which can be loosely translated as:
"The most important phenomenon is a delayed decomposition which last several days. It is attributed to the effect on the hexogen of the nitrogen peroxide trapped during the irradiation."
[4] "Contribution à l'étude de la molécule d'hexogène dans le cristal et à l'état libre ou en solution" (Contribution to the study of the hexogen molecule in the crystal and in the free state or in solution.), A. Filhol, Univ. Bordeaux I, 25 June 1971, Doctor-Engineer thesis nº 142.
[5] This page is also available here <https://www.researchgate.net/publication/277284655_Hexogen_RDX_the_boiling_crystal>