RCA 931A & 931B  
Written by Accutron on 2017-05-09  

Devices included in this entry:

RCA 931A nine-stage photomultiplier
Sylvania 931A nine-stage photomultiplier
RCA 931B nine-stage photomultiplier


The RCA 931A is the first commercially successful photomultiplier tube, and the first implementation of the popular nine-stage electrostatically focused circular dynode cage structure. Developed in a time before computer simulation, the electrostatic geometry of the 931A dynode cage was determined by placing mechanical models of the electrodes beneath a stretched rubber membrane, with the height of each electrode set to correspond with its electrical potential. Small balls were then placed on the membrane and allowed to roll along the simulated electron path.

The 931A features a S-4 spectral response and a peak sensitivity of 400nm. The 931B is an improved device, with a higher photocathode quantum efficiency and anode sensitivity, and a lower dark current than the 931A.

RCA 931A Datasheet (PDF)

We have just returned from Sparta, Tennessee, a prime viewing location for the Great American Eclipse. Our site was a gravel turnaround along Rt. 136, about 350m north of the eclipse centerline. There were a few puffy clouds that gave us some concern, but the sky was completely clear for the main event. I managed to capture sunspots right after C1, and I got some good photos of totality, complete with streamers and prominences. I also discovered that autofocus is not fond of the diamond ring effect. The photographs were captured freehand with my Canon Powershot SX530HS, and I used a piece of gold-plated #12 welding glass as a filter during the partial phase. We experienced 2m 39.4s of totality, only a couple seconds shy of the absolute maximum at Hopkinsville, Kentucky.

Beyond the eclipse itself, the most striking feature of the event was the complete shutdown of everything else near and during totality. All of the traffic on the nearby freeway just stopped, and it grew completely silent.

This article is part of the [Detection & Imaging Tubes] exhibit.

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