OBTAINING EYES-FREE AND HANDS-FREE RADIATION DETECTION.  Eyes-Free and Hands-Free radiation detection is important for a First Responder searching for victims.  An easy method to achieve this (and/or quantitative radiation dosimetry) is to mount a radiation detector on the First Responder's body or head.

To allow uninhibited radiation detection, and help of victims, the radiation detector should be small, lightweight, and portable.  Our SF/PMT detector meets these conditions.  It provides an 'eye' which can 'see' radiation.

To achieve fast detection of radiation without adding eye, hand, or time limitation, the radiation detector should give a millisecond 'alert' to the First Responder.   Our SF/PMT detector is ideally suited to this task.

If the SF is mounted on the First Responder's body or head, a further advantage results.  The SF/PMT detector can show the 'direction' of radiation with body or head turning.  Partial shielding may be put around the SF to aid in determining this direction.  (No harm will occur at the first radiation alert, because the radiation level is too low for health-harm at that point.)

Partial radiation shielding can be achieved using any material and a calculated thickness.  (C.f., American Institute of Physics Handbook, pp 8-93 to 8-98, McGraw-Hill, 1963).  Example: 0.35" lead provides ~ 20% shielding for 1 MeV gamma rays (a typical 'health-threat' radiation).

BODY/HEAD MOUNTED RADIATION DETECTORS: ADVANTAGES AND REQUIREMENTS.  Advantages: The main advantage of a Body/Head Mounted Radiation Detector is that it leaves the eyes and hands of a human free for other activities.  For a 'First Responder', it is especially important that he/she can concentrate on finding and helping victims, in the event of a catastrophe, without distraction, limitation, or delay.

Requirements: The main requirement of a Body/Head Mounted Radiation Detector is a response time that is fast (e.g. no longer than 1 millisecond).  This is so that no uncertainty or added delay is introduced for a First Responder quickly moving thru rubble.  Other requirements for a body/head mounted radiation detecting device is that it be small, lightweight, and portable (battery-operated).

The SF/PMT radiation detecting device described here has, uniquely, all of these features.

A useful feature for body or head-mounted radiation detectors is a SF/PMT mounting method which is easily accommodated.  This can be achieved by placing the SF/PMT detector, batteries, and all other components in a 'pouch' with a belt clip, or attached to a strap of adjustable length to encircle the First Responder' waist, and allow adjustment for size differences.  Audio-detection of the radiation can be provided by a small speaker 'clipped' to clothing near the ear.  The small speaker would be connected by wire to the components in the 'pouch'. For head or helmet mounting of the SF/PMT detector, it will be useful to provide some form of adapter to obviate the frontal slope of the helmet.  An adapter could be made of high density foam rubber covered with heavy duty vinyl to which the strap would be attached.
 

CONSTRUCTING THE SF/PMT RADIATION DETECTOR

Start with 5 mm x 5 mm (square cross-section) polymer, scintillating fiber (SF).  (Square cross- section SFs have a higher efficiency for 'light-piping' than round cross-section SFs.)  This fiber should have 2 coating layers of a material with an index of refraction which is smaller than that of the polymer SF.  These coatings are useful to achieve 'light-piping' which is not affected by contact of the SF with its supporting environment (and subsequent uncertainties due to contact).  These SFs, with coatings, can be purchased from numerous suppliers throughout the world.

Cut a 1" to 5" length of the coated SF for use as the SF-component in the SF/PMT detector.

Make a transition piece, 3 to 15 mm thick, so that the SF can be joined to the Photo-Multiplier- Tube (PMT).  This transition piece should bolt onto the PMT for reliable and secure positioning.  The transition piece, therefore, should have clearance holes which match, in location, with the tapped mounting holes found at the PMT light-input.  This allows attachment of the transition piece to the PMT. PMTs can be purchased from numerous suppliers (e.g., Bicron/Saint Gobain).

The transition piece should also be made to have a square hole of a size that matches the cross- section of the SF.  This hole should be aligned with the light input hole of the PMT.

The transition piece should be a soft material to allow easy machining.  Aluminum is an example of a suitable material.   A square 'broach' is a suitable way to 'punch' a square hole in a soft material containing a round hole already drilled in it.  This is a standard method used by machinists.

The 1" to 5" long, 5 mm x 5 mm transverse, coated SF is pressed into the transition piece by hand.  This should lead to a secure, snug fitting of the coated SF into the PMT.

The SF is then covered to prevent ambient light from reaching it.  A suitable cover is aluminum tubing.  Square, thin-wall, aluminum tubing, 1" on edge, is a standard size which should also allow the SF/PMT transition piece to fit within the tubing.  The joint between tubing and PMT, and the opposite end of the tubing, can be made 'light-tight' with a wrapping of opaque tape.

However, the SF can be covered with any material which prevents visible light from reaching the SF and PMT, and provides robust support.

When radiation passes through the scintillating fiber,  the interaction of the radiation with the SF leads to the production of visible light in the SF which is 'guided' by light-piping of the SF to the PMT.  The light causes an output voltage from the PMT.

The PMT provides an electrical output signal which is linearly related to the to the radiation dose- rate absorbed in the SF.  After amplification, the electrical signal, now from a low impedance, can be sent along many feet of conventional cable to a distant location where the 'pulse' amplitude can be measured to provide a linear measure of the radiation dose-rate striking the SF.

However, for 'eyes-free' and 'hands-free' audio-detection of radiation, the output of the PMT should go to a voltage-to-audio-frequency converter or to a LED.  The audio alert has the advantage of also giving a semi-quantitative measure (via 'pitch') of the radiation dose rate.

Battery-operated PMT's with a volume of only 2 cubic inches, can provide millivolt pulses from millisecond radiation pulses striking the SF which are too weak to harm the human body.  This makes the SF/PMT detector a fast, small, portable, and reliable detector of radiation even at radiation levels below those that can cause harm.

The unique features of the SF/PMT detector for radiation health-harm alert include that it can provide a fast 'alert' signal, using a small package.  (Full quantitative measurement of the radiation 'threat' is also possible if a millivolt meter is used to measure the signal amplitude at the PMT output.)

To allow a 'First Responder' to move quickly and freely, a radiation detector 'alert' should not inhibit the 'First Responder' use of hands, eyes, or time.  The sub-millisecond SF/PMT detector, body/head-mounted, can meet these requirements.

The most important function of the SF/PMT detector is to provide an alert to the SF/PMT 'wearer' when health-harming radiation has been detected.
 

GETTING 'PORTABILITY' IN A SF/PMT RADIATION DETECTOR

To achieve portability in a radiation detector, it is necessary that the SF/PMT detector be small and battery-operated.  SFs can be purchased in transverse sizes of 0.1 mm to 5 mm.  The transverse size does not pose problems.  However, for a PMT and its power source to be 'small', it is important to choose a battery-operated, compact, PMT.  Fortunately, these are now available commercially.

The choice of battery type for the PMT is important.  To be comfortably 'portable', and 'reliable', these batteries should be small, light, have a long shelf life and have large 'stored energies'.  The latter properties are important for reliable operation without frequent testing.  Lithium primary batteries are a good choice.

A method to insure that the SF/PMT detector power is 'ON' may be desired.  This may, for example, be provided by a LED which will be powered when the SF/PMT detector is 'ON'.  However, the 'continuous-testing' feature of our device (see next) obviates this
need.
 

QUICKLY AND CONTINUOUSLY TESTING A SF/PMT RADIATION DETECTOR

A complete, rapid, simple, and continuous test for radiation detection can be achieved by putting an 'EXEMPT' radiation source near the SF.  The 'EXEMPT' radiation source will cause an audio signal when placed near the SF, confirming that the SF/PMT radiation detector is powered and functional.  ('EXEMPT' radiation sources are small, cheap, and safe because their radio- activity is very low).  No other radiation detector offers this 'continuous-in-use' testing.

To allow this test, it is important that the voltage-to-audio-frequency converter produce a 'square' wave output.  With a (very low radio-activity) 'EXEMPT' source, the frequency output of the converter will be ~ a few Hz, too low to be heard as a 'sine wave' by the human ear.  However, with 'square' wave output, the human ear can easily detect frequencies of < 1 Hz, because the 'harmonic' content of a 'square' wave leads to easily heard 'click' sounds.

The device test will be 'continuous-in-use'.  A 'continuous-in-use' test is achieved by leaving the 'EXEMPT' radiation source next to the SF at all times.  Since the 'EXEMPT' radiation source emits very low-level (safe) radiation, there will be no health risk to the SF/PMT 'wearer'.   Since the 'EXEMPT' source emits very low-level (safe) radiation, its low frequency ('click') signal can be continuously discerned with a voltage-to-(square wave) frequency converter.  Since the 'EXEMPT' source can be long-lived (e.g., Cs-137 has a half-life of 30.7 years), the 'continuous- in-use' test will be long-lived.

The 'voltage-to-audio-frequency' converter, with EXEMPT radiation, powered by small, simple integrated circuits, would use the same battery source as the SF/PMT detector.  The 'pouch' can easily hold all electronic circuit components for 'continuous-in-use' test operation.

The advantage of this radiation detection device may be represented by the admonition:

'IF YOU HEAR 'CLICKS', THE RADIATION DETECTOR IS WORKING!' (showing the safety of 'continuous-in-use' testing...).

'IF YOU DON'T HEAR 'CLICKS', GET ANOTHER RADIATION DETECTOR!'  (showing the convenience of 'pouch'-carried radiation detectors...).

Thus, testing of the SF/PMT radiation detector will be fast, complete, simple, and reliable.