Chris Rapley: the bent crystal spectrometer

Chris Rapley tells story of designing bent crystal spectrometer for NASA Solar Maximum Mission at UCL's Mullard Space Science Laboratory in the 1970s.

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Now, you know, for belt and braces reasons I’d also been developing position sensitive low energy proportional counters in the lab and these allowed you to divest yourself of the rocking mechanism in a Bragg Crystal Spectrometer. You could now curve the crystal and disperse and x-ray spectrum into these detectors. And that was exactly what was needed to study solar flares, or rapidly varying x-ray sources. So we had been collaborating with Lockheed Palo Alto Research Lab, I’d been over a couple of times. And they had flown a small trial Bent Crystal Spectrometer on their solar payload from White Sands, which they called Old Yella [ph]. The payload was called Old Yella [ph] ‘cause it was painted yellow. The first time it flew, the data were garbage. We couldn’t understand what the problem was. So I went over there and had a look and it – there were two things that were wrong. Firstly they’d wired up their digital electronics slightly wrong so it was missing coding things. They were incredibly embarrassed about that ‘cause they were a very professional outfit and they had not done their testing adequately. But secondly it turned out that from the sun, there was so much ultraviolet radiation coming into the detector, and it was an aluminium detector, it was generating lots of low energy pulses in the detector down at the sort of noise level. So the task before the house was to reduce the ultraviolet sensitivity of the detector by orders of magnitude whilst keeping the soft x-ray sensitivity, you know, fairly high. And so I – I have to say, I did something I was really quite proud of. I – I sat and thought about that. I was left pretty much on my own, with the help of their extremely competent technician, a guy called Kermit Smith [ph]. And with a combination of sputtering aluminium onto the back of these plastic – thin plastic polypropylene windows for the detector and a carbon diode coating, I did exactly that. I reduced the ultraviolet sensitivity, I think it was – I think it was by something like ten to the five. I mean, it was an extraordinary figure. I’ll have to go and look it up. But it was some enormous factor, with a loss of about a factor two or so in the x-ray sensitivity. And next time they flew it, it worked a dream. So that then led onto the next stage of my career, because Lockheed were bidding to put a very sophisticated x-ray spectrometer, a solar flare spectrometer, on the Solar Maximum mission, which was a NASA observatory mission, and NASA were really interested to have these Bent Crystal Spectrometers on because they provided a completely new – absolute new window on what was going on in these flares. And so essentially, as I say, I was commissioned to produce these things. And that’s what we did; we produced the detector systems and the whole of the Bent Crystal Spectrometers, an eight channel device, for the Solar Maximum missions.
  • Interviewee Chris Rapley
  • Duration 00:03:15
  • Copyright British Library Board
  • Interviewer Paul Merchant
  • Date of interview 4/27/2011
  • Shelfmark C1379/40

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