2019/09/16 Poster presentations P1-030
19th International Conference on Solid State Dosimetry (SSD-19) @ Hiroshima

Spectrum estimation of low-energy X-rays radiated fromCrookes tube using Peltie type cloud chamber○Masafumi Akiyoshi*, Kenji Yamamoto
Radiation Research Center, Osaka Prefecture University
and Great Thanks to All Member of Crookes Tube Project.
Masafumi Akiyoshi: akiyoshi@riast.osakafu-u.ac.jp
http://bigbird.riast.osakafu-u.ac.jp/~akiyoshi/Works/index.htm

How to Establish Safty Managementfor Crookes tube?
Crookes tube has been used in junior-high science classes in Japan, and the primary purpose is toteach the characteristics of electrons and current, not for radiological education. Therefore, someteachers are not recognizing the radiation of X-ray from Crookes tube, and most of them have no
information of the dose. However, it is possible to expose high dose of X-ray to students using aCrookes tube, where Hp(0.07) reaches 200mSv/h at a distance of 15cm.
Some discharge tube that use hot cathode isoperated with only several 100V, and even withcold cathode, some equipment can be operated
at about 5kV. With this low voltage, radiated
X-ray is shielded completely by the glass wall.

Basic Plan
By using low voltage type equipment, teachers
never required to consider the radiation andstudents can observe electron beam very safely.
The problem is resolved completely!

Advanced Plan
A Crookes tube operated by 5kV(Horizon Co. VT-7010)

5kV CW high voltage unit
driven by 9V battery

Junior-high school in Japan
have quite limited budget!

1) Cannot replace legacy devices with economicalreason2) Advenced education program that utilize X-rays
radiated from Crookes tube
Anyway, radiation safty guide line to limit X-rays
dose is required.

Problems on Estimation of X-rays from Crookes Tube

Low Energy X-Rays ( arround 20keV )Not only conventional survey meters for public use, but also reliable NaI survey meters forgeneral radiological management are useless for low energy X-rays from Crookes tube.
(below 50keV pulse is ignored by conventioal NaI surver meters )Radiated in Sharp Pulse
Even using detector with Be window for low energy X-rays measurement, radiation in sharp pulse
give pile up and dose is estimated as very small value. Only ionization chamber that measure
averaged current or solid detector that integrate the absorbed dose is useful.

Instability of Induction Coil and Applied Voltage
Induction coil generate high voltage pulse mechanically, that is affected by conditions such astemperature, and firing voltage between discharge electrodes is also affected by temperature orhumidity. For systematic measurement, some physical parameter is required to compare results.

High Voltage Applied by a Induction CoilDistance ofDischarge ElectrodeDDEPW

Output power dial changing the voltage
impressed to the primary side of atransformer, and that control output
voltage continuously.
Since the dielectric breakdown voltage
in the air is about 1 mm for 1 kV, thedistance of discharge electrodes canlimit the maximum voltage. If thedistance is settled to 20mm, themaximum voltage impressed to a
Crookes tube was limitted to 20keV,
therefore it work as a safety device.

Impressed Voltage / kV

30Output power

20
100-10101112Time / ms

13Distance of discharge electrodes 20mm,
Output power 4, Averaged current 80μA

14To measure high voltage pulse, voltagedivider that use a large registor( < 100MΩ) and a small resister (about100kΩ) is used to protect a oscilloscope.
In the most case, an induction coil is notconnected to the ground. In such case,the electric potential is floating and must
be used 2 divider and 2 probe to takedifference between anode and cathode.

Spectrum Measurement by a CZT Detector
Amptek XR-100T-CZTCZT(Cd0.9Zn0.1Te)
detector with Be window,
cooled by Peltier device

200Intensityby CZT Detector
M easurement Distance 80cm
DDE PW Peak Energy
15mm 2 14.0keV
20mm 4 16.4keV
25mm 6 17.3keV
30mm 8 18.3keV

100at DDE 15mm: φ3 lead collimater only
the other: + φ0.5 lead thin plate collimater
Measured time: 10min0010203040
X-ray Energy / keV

50

φ0.5mm lead collimater can reduce count rate to several cps
that avoid pile up in the spectrumCollimaters
φ0.5 lead thin plate
φ1.0 lead thin plate
φ2 co-axial brass
φ3 co-axial lead

Web Site (Japanese)
http://bigbird.riast.osakafu-u.ac.jp/~akiyoshi/Works/CloudChamber.htm

Peltier-cooling-type HighPerformance Cloud Chamber
Features of this products
・Dry-ice Free!
・Clear track observation and Stable long time(hole day operation with no support is abailable)
・Oservation of not only Alpha-rays tracks, but also Beta-rays tracks, andfurthermore, photoelectron tracks by Gamma-rays or X-rays.
→ Intuitively radiological education of the difference in interaction
with materials from a kind of radiation-rays
・Quite Cheap Price! Most parts are common consumer items.

40mm

Observation of radiations using conventional cloud chambers were limited toonly alpha-ray tracks and had many restrictions. While thisPeltier-cooling-type high performance cloud chamber enable us to observe veryclear radiation tracks in a few tens of seconds after a power supply evenunder a bad weather condition, of cause without dry ices. The technicalfeatures such as clearing assorted ions in air using HV-unit, high-intensity
LED illuminations and the fabric of the chamber enable us to observe beta-raytracks. Not only simple observation of alpha-ray tracks, but also comparewith beta-rays track or a delta-ray tracks arisen from gamma rays enables
us to perform far deep radiological education related to interactions of radiations
and materials.

40mm

40mm
Alpha-rays

Beta-rays / Gamma-rays(Photo electrons)With the new EX-type, observation
area is enlarged to 60mm x 60mm.
Large cylindrical chamber enablemulti-people observation, and very
high efficiency enable observationof natural-radiation .

X-rays from Crookes Tube
(Photo electrons)

Main unit of standard type (SD)
Peltier-cooling cloud chamber.

A Cockcroft high voltage unit supplyup to 2kV tension. It sweep motley
ions in air that achieve high efficiencyand all weather operation.

You can watch radiation
track movies using these
QR code!
natural radiations

Am-241

Metal plateshielded source
59.5keV
γ-raysVarious energy ofSpecific X-rays(Changed with metal)

Number of tracks

Measurement of photoelectron track length

Length of photoelectron tracks / mm
Cu Kα8.0keV、Lenge of photoelectron 1.8mm
Nb Kα線 16.5keV Lenge of photoelectron 5.2mm

Intensity

Spectrum of X-rays from a Crookes tube

Number of tracks

X-rays Energy / keV

Very simple system enable energy estimation.
Further study using image analysis with AI isrequired for statistical enough counting.
Length of photoelectron tracks / mmMeasurement at Real Education Field
Measurements of leaked X-rays from37 Crookes tubes at junior-highschool in Japan were performed
using radiophotoluminescence
dosimeters by science teachers.

Setting of induction coil were same as their usually.

In the 37 Crookes tubes, Hp(0.07) with 10min exposure was;
25 tubes < 50μSv @ 1m (extrapolated)
18 tubes < 50μSv @ 15cm (detection limit)

The dosimeter were sticked on 2L PET
bottle and put from distances of 15, 30,
50cm and irradiated during 10min for each.
The radiophotoluminescence dosimeter wasGlass Badge type FX (Chiyoda Technol)that can estimate effective energy and canestimate back ground radiation with Sn
shielded element.

One tube radiate 600μSv in 10min at 1m with minimum output power.
The tube showed intermittent beam and lookes current was small.

Minimum power、at a distance of 30cm

DDE 30mm: 2mSv/h
DDE 50mm: 30mSv/h

DDE 20mm: 40μSv/h
at 1m、10min radiation gives only 0.6μSv exposure

To Reduce the Dose from Crookes tube
First of all

Replace to low voltage equipment
10,000 Junior-High School × 400USD / ea. = 4Million USD is required in Japan
But Crookes tube is used at just a 1 lecture in 3 year.

Inherent Safety
No concern is required

With Economic Factor, Safety Guideline for Conventional Crookes tube is Required
1) Applied Voltage
2) Beam Current
3) Diatance4) Shielding5) TimeReduce Radiated
X-raysRadiation Protection
Three Principles

Reduce applied voltage gives low transmission of
X-rays to the glass wall drastically.
When we shielding, acrylic is not effective butthin glass is enough effective.
To take distane is most easy way to reduce dose.

・ Set output power as low as possible

Provisional Guideline
with our study
Is it enough?
Substantiate study isrequired.

・ Never remove discharge electrodes and set thedistance smaller than 20mm
・ Take distance as far as possible. For student, morethan 1m is recommended.
・ Keep the display time shorter than 10min.

Problems on management of X-rays from Crookes tube
Dose Constraint for Education Field is not Discussed well
Dose limit for general public have been proposed by ICRP Pub1990/2007 as 1mSv/y, but it is for all
additional exposure. ICRP Pub 36 is the only instance for the teaching of science (in effective dose
equivalent, 0.5mSv/y and 0.05mSv for each class).

Definition of X-ray Device is not Clear
In Japanese domestic low, `X-ray device' is not defined strictly. There is no exemption level that gives aconfusion to safety management of X-rays in many fields.
IAEA GSR Part gives exemption level as; in normal operating conditions cause an ambient dose equivalent rate or a directional dose equivalentrate, as appropriate, exceeding 1 ìSv/h at a distance of 0.1 m from any accessible surface of the equipment or the maximum energy of theradiation generated is no greater than 5 keV.

Difficult Estimation of Effective DoseFor 20keV X-rays, the half value layer is about 1cm in human body. The absorbance dose to tissues inbody depends on its depth from the surface. The transmittance is changed with energy of X-ray
drastically around this energy range and the X-rays have broad spectrum.
Further more, dose distribution in horizontal plane is not uniform. Therefore we cannot assume aligned andexpanded radiation field, and we cannot use 1cm dose equivalent as approximation of effective dose.

Dose Control
Distance of discharge electrodes: 30, 20, 15mm
Output power was set to the just firing voltage.
4.50

Reduced

DDE 30mm PW 4DDE 20mm PW 2DDE 15mm PW 0

3.50
3.00
2.50
2.00

Reduced70μm dose equivalent /mSv/h

4.00

1.50
1.00
0.50
0.00
01020

30405060Distance from surface of Crookes tube/ cm・Dose is reduced drastically with thevoltage limitation
Distance of discharge electrodes must be shorter than 20mm

・Dose is changed with distance as theinverse square law
At a distance of 1m, dose is reduced to 1/100 from that of 10cm.

70μm dose equivalent＠30cm / mSv/h

Averaged current was changed with output power.
The averaged current was measured by simple analog current meter.

43DDE 20mm
DDE 25mm
DDE 30mm

2100

Maximum doseat DDE 20mm

20406080

100

Averaged current / μA

・Increasing current rising dose
exponentially
The output power increase current and also voltage. Thevoltage changes the energy of X-rays that changes
transmittance drastically. Therefore, output power must be kept
as small as possible. Furthermore, discharge electrode act as asafety valve to limit the voltage.