Critical Grazing Angle: Reactive Anti-Positron Armor

 

「What kind of countermeasure did you use against the positron cannon? Please tell me.」

Arima seemed eager to know the details. Thinking there was no reason to hide it, Kou answered.

「I used something called Reactive Anti-Positron Armor. It slightly reflects and deflects the positron beam.」

「Deflects it?」

「It’s a property of positrons, apparently. Positrons repel the positive charge in atomic nuclei of matter. On Earth, there was a device called Total Reflection High-Energy Positron Diffraction—TRHEPD—which utilized that property. When a positron beam is directed at a surface at a shallow angle below a certain threshold, it bounces off like a stone skipping across water. That angle is called the critical grazing angle.」

Kou spoke, recalling what he’d learned about positrons from Kinukawa and Astraea.

The Total Reflection High-Energy Positron Diffraction Kou described utilizes the property of the critical grazing angle, reflecting positrons off materials. From the traces left by the reflection, it’s used in nanoscale research to analyze the atomic arrangement of surface structures.

「A specific angle at which positrons are reflected—almost parallel to the surface, right?」

「Exactly. The critical grazing angle varies depending on the energy involved and the material, but it’s generally less than two degrees. The material we used for the armor was nanoceramics, with an angle of about one degree.」

「One degree of inclination?」

「One degree. But even an angle as small as one degree can cause significant displacement over distance, right? From 100 meters away, a one-degree angle creates a 174-centimeter shift. At a kilometer, the shift is 17.4 meters. The enemy was 10 kilometers away. If we can deflect it a few kilometers ahead, that’s more than enough.」

「So, when the battle started and the armor exploded, it wasn’t due to a direct hit, but because the detection system responded to the laser used to aim the positron cannon?」

「It’s a feat only possible with the processing power of the MCS. The armor exploded upon detecting the laser irradiation. The positron cannon has the drawback of requiring a two-stage process. Unless you create a perfect vacuum, it’s practically unusable within the atmosphere.」

「You knew the slight weakness of the positron cannon… And you exploited it.」

Arima couldn’t help but murmur in admiration. As long as there’s a transition time from laser targeting to positron emission, countermeasures are possible. Even predictive deployment could have achieved a similar effect.

「The reactive anti-positron armor scattered a massive number of tiny, needle-like elongated materials at just one degree of inclination, forming a vast conical shape. The positron beam, when fired toward the Silhouette, hits those media, changes its angle, and gets deflected away.」

「That’s quite the idea you came up with.」

「It wasn’t my achievement alone. The accumulated knowledge acquired during the development of the D-Rifle, the experience from assembling Arthur’s positron cannon, Abel-san teaching me about Britain’s bouncing bomb, and the ingenuity of Arges and Steropes, and their wisdom cultivated since the beginning of the Pioneering era. Without all of these, we wouldn’t have been able to develop this countermeasure.」

「Britain again… I remember now. The bouncing bomb developed by Barnes Wallis, a former colleague of the engineer who built the Panjandrum, used the very same principle as stone skipping.」

Even Asia seemed to recognize the bomb being referenced. Barnes Wallis was also known as the visionary behind the bunker buster, and was the creator of the Tallboy and Grand Slam, leaving his mark in history. The angle of water surface reflection is said to be around twenty degrees.

「I discussed it with Arges and Steropes. Just in case an enemy Another Level Silhouette appeared and used a weapon similar to Arthur’s positron cannon. The idea we came to wasn’t to withstand the attack but to deflect it like a skipping stone. That concept became our foundation. Without those two sages from the Pioneering era this would’ve been impossible.」

「Aren’t those two, robots? You are quite peculiar, referring to them as if they were people, Kou-san.」

「There’s no better way to describe them than ‘the two sages’.」

Arges and Steropes were expressive and full of emotion. For Kou, they were no different than any other human.

「To think that something so innovative started with a bouncing bomb… There really are a lot of people on Earth who come up with strange ideas.」

「Yeah.」

Kou didn’t deny Arima’s comment. Not just limited to weaponry, nor to Japan or Britain—the people who came from Earth were full of eccentricities. There was no way to refute that.

「I heard about the structure of the D-Rifle from Flack. It’s an assault weapon that fires molten metal projectiles like a spear, right? The projectiles are designed to strike with molten metal so they can’t be deflected.」

「That’s right. When materials collide, the stress imparted differs depending on the substance. There’s a similar principle—an interface called a shock impedance boundary. Earth once had armor that made use of this. It’s difficult to deflect every direct-hit shell, so different materials were layered to deflect just one shot—essentially disposable armor. The principle is different, but the operational philosophy is similar. This kind of armor is effective only once. After deflecting the first shot, I needed to settle things before they could fire multiple positron shots.」

「This kind of armor… it’s the height of inefficiency, isn’t it? Or rather, is this what Outis meant by ‘the end reached after challenging the irrational and mastering inefficiency’?」

「You heard that? That’s part of my dark history.」

Kou laughed involuntarily. It may have already been obvious, but it was supposed to be a secret beyond Trireme’s personnel. Hearing it put into words again was a little embarrassing.

「You’re not even trying to hide your real identity anymore. When are you planning to reveal your real name?」

Asia smiled as well. The atmosphere had grown gentle.

「Just a little longer… I want to keep being Arima a little longer.」

Seeing as he couldn’t hide it any longer, the boy replied cheerfully.

「Let me ask a question too, Arima. Can I assume I’ve overcome the trial?」

「It’s just beginning. Not here—your destination itself is the trial.」

「I see… So it’s about Libya after all?」

「Yes.」

Kou’s objective was to reclaim the main core of Libya. The boy had seen right through him.

「After defeating Yōnātl Tepatztl, you acquired some of the components you needed to repair an Another Level Silhouette. That should be sufficient for the time being.」

「Whether it’s enough, I can’t say. But we’re leaving planet Libya soon. Or… are you going to keep watch over this planet?」

「Not at all. That’s not my role.」

The boy chuckled softly.

「That should be enough, don’t you think? We might not be close friends, but we are old acquaintances—like distant relatives, in a way. Right, Typhon?」

「What a shame. I thought our relationship was quite amicable.」

「If that’s how you feel, then I’m glad too.」

The boy responded to Asia with a charming, disarming smile that made it impossible to dislike him.

AN:

This time, the content was written with reference to materials such as the 2014 press release by the Japan Atomic Energy Agency, particularly focusing on the structure of Total Reflection High-Energy Positron Diffraction.

When looking into TRHEPD, various studies show that for metals like aluminum, the critical angle is below 0.3 degrees, while materials like silicon or graphene tend to fall below 2 degrees. As Arima mentioned, even lasers can be reflected this way. The main issue is that this kind of use makes the armor a consumable, making its cost-effectiveness terrible.

And now, a bit more on the British quirkiness—here’s an anecdote:

Barnes Wallis: 「Let’s make the hard bomb spin with gyroscopic effect! That would make it burrow underground! Oh, you have to make it by hand since it’s complicated.」

British factory: 「Are you kidding me? What’s the unit cost of this thing!?」

Barnes Wallis: 「It’s a little costly so if it fails, we’ll bring it home. Oh, it weighs ten tons so prepare something big to transport it.」

British maintenance crew: 「There’s no plane that can carry this! Guess we’ll have to build a dedicated bomber!」

After the war, Wallis became a passionate researcher of variable-sweep wings, contributing to developments that eventually led to the F-111. He’s considered one of the top three iconic figures representing British military engineering. Yes—there are three of them like him in the British sphere…