2021 / 04 / 17
Radiation Safety Federation, the 68th seminar of radiation protection- IAEA Role in Establishment of Radiation Safety Standards and Guidance -A study of Crookes tube
used in the education field
Osaka Prefecture University,
Radiation Research Center,
Masafumi Akiyoshi
Special thanks: Crookes Tube Project Members in Japanakiyoshi@riast.osakafu-u.ac.jp
http://bigbird.riast.osakafu-u.ac.jp/~akiyoshi/Works/index.htm

What is Crookes tube?
Wilhelm Konrad Rontgen
1895, Found the X-ray during the experiment of discharge tube
1901, Got the first Nobel prize in physicsWilliam Crookes①－ －②－－＋

Cathode③－－Accelerationof electron
Vacuum of0.005 - 0.1 Pa① At the cold cathode, cation in
air is accelerated and knockout secondary electrons.

＋electrons are accele② These-rated as the applied HV.

Anodeelectrons hit③ Accelerated
glass wall and radiate

10-20kV HV from an induction coilbremsstrahlung X-rays.

How to Establish Safty Management
for 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 noinformation of the dose. However, it is possible to expose high dose of X-ray to students using a
Crookes 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 economicalreason
2) Advenced education program that utilize X-rays
radiated from Crookes tube
Anyway, radiation safty guideline to limit X-rays
dose is required.

ICRP Basic Radiation Safety Criteria
Justification:
No practice shall be adopted unless its introduction produces a net positive benefit.
・Is the experiment required? Is it accepted to show with photo or video?
→The experiment using Crookes tube gives quite strong impact to students and have excellent
educational effects.

Optimization:
All exposure shall be kept as low as reasonably achievable (ALARA concept), economic andsocial factors being taken into account.
・Low voltage operated equipments can make the exposure to zero and it is firstly recommended.
However, economic factor make it difficult to replace all equipments in all schools. Therefore,optimization on operation conditions (applied voltage, current, distance, time and shielding) is required.

Dose limitation:
The dose equivalent to individuals shall not exceed the limits recommended for the appropriate
circumstances by the Commition.
・In Japan, dose limit for general public proposed by ICRP Pub1990/2007 is not taken into the domestic
low. Now we survey international regulations and the actual control conditions to settle the reasonable
management target dose.

Problems on management of X-rays from Crookes tubeDose 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.

Definition of X-ray Device is not Clear
In Japanese domestic low, `X-ray device' is not defined strictly. There is no exemption levelthat gives a confusion to safety management of X-rays in many fields.

Difficult Estimation of Effective DoseFor 20keV X-rays, the half value layer is about 1cm in human body. Absorbed dose totissues in body 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 broadspectrum.
Further more, dose distribution in horizontal plane is not uniform, so aligned andexpanded radiation field is not applied. Therefore, 1cm dose equivalent does not give
an approximation of effective dose.

Problems on Estimation of X-rays from Crookes Tube

Low Energy X-Rays ( around 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.

Not Fixed Applied Voltage from an Induction CoilInduction coil generate high voltage pulse mechanically, that is affected by electrical conductivityof connected Crookes tube or some conditions such as temperature. Therefore, the output power
dial cannot settle the output voltage to a particular fixed value.

High Voltage Applied by a Induction CoilDistance ofDischarge Electrodes

DDEPW
Impressed Voltage / kV

30

Output power

20
100-10101112Time / ms

13

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

14Output power dial changing the voltage
impressed to the primary side of atransformer, and that control output
voltage, but it cannot settle aparticular fixed voltage. The output
voltage is affected with theconductance of the connected crookes
tube or any other factors.
On the other hand, the dielectric
breakdown voltage in the air is about
1 mm for 1 kV, therefore the distanceof discharge electrodes can limit themaximum voltage to a fixed value. If
the distance is settled to 20mm, themaximum voltage impressed to aCrookes tube was limitted to 20keV,
therefore it work as a safety device.

Exemption and Clearance Level
For a Radiation source: IAEA GSR Part3
Radiation generators of a type approved by the regulatory body, or in the form of an electronic tube,
such as a cathode ray tube for the display of visual images, provided that: 70μm ? 1cm?
(i) They do not in normal operating conditions cause an ambient dose equivalent rate or a directionaldose equivalent rate, as appropriate, exceeding 1 μSv/h at a distance of 0.1m from any accessible
surface of the equipment; orA Crookes tube operated by 5kV(ii) The maximum energy of the radiation generated is no greater than 5 keV.
(Horizon Co. VT-7010)

Most Crookes tubes radiate higher dosethan the clearance level (i).
For a Public People: ICRP-Pub64, IAEA BSS and NCRP Report No.180
ICRP/IAEA: trivial individual dose, NCRP: negligible individual dose
10μSv/y with a source or practice. (under optimized protection)

Achievable level with optimized
operation conditions.

Dose Limit: ICRP-Pub36
ICRP Pub36 Protection against Ionizing
Radiation in the Teaching of Scienc gives thedose limit as: 0.5 mSv/y (effective doseequivalent) or 5mSv/y for each organ (doseequivalent), and 1/10 dose for each class.

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
010

20

30

4050

60

Distance 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/hAveraged 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 bekept as small as possible. Furthermore, discharge electrode act
as a safety valve to limit the voltage.

Drastic Dose Change with Slight Voltage Change
The radiated X-rays are shielded by the grass tube that forms
Crookes tube it self. Slight energy change around 20keV
gives drastic change in transmittance.
1.0
1.E-04

TransmittanceTransmittance

0.9
0.8
0.7
0.6

1.E-101.E-16
1.E-22
1.E-280.5

1.E-340.40200

400600800

X-ray Energy / keV

Above 100keV, transmittance changes slightly

010203040

X-ray Energy / keVTransmittance at 30keV is higher 100 times
than that of 15keV

Transmittance of 2mm glass for X/γ-rays

50

Exploration at Real Education Field
Measurements of leaked X-rays from38 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 38 Crookes tubes, Hp(0.07) with 10min exposure was;
31 tubes < 100μSv @ 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
was Glass Badge type FX (ChiyodaTechnol) that can estimate effectiveenergy and can estimate back groundradiation 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.

Additional study with ionizing chamber: 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

Charactor of High Dose Crookes Tube
Inside of the Crookes tube, vacuum is not so high andsome gas molecules are enclosed. These molecules are
ionized by natural radiation and anions are accelerated tocathode to knock out secondary electrons (cold cathode).
There is some Crookes tube that lack of gas. In oldequipment, gas molecules are absorbed to glass wall gently
or some equipment lack gas on the manufacturing.
With such Crookes tube, electrons are difficult to flow andmore high voltage is required to emit beam. With induction
coil, electromagnetic energy is charged to the coil until
enough voltage is obtained that applies higher voltage than
aimed with the output power . That increasetransmittance of X-ray to glass wall.

At the age of Roentgen,necessity of gas-pressure
control is well known andsome "softener" devices
were incorporated that
released a small amount ofgas.

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 but
thin glass is enough effective.
To take distane is most easy way to reduce dose.

・ Set output power as low as possible

Provisional
Guideline
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.

Validation of the Provisional Guideline

136 tubes in 191 tubes:
Hp(0.07) < 50μSv (detection limit)
exposured at 20cm in 10min.
The other 55 tubes showed dose distribution inlower side than the first exploration. On theother hand, the highest dose was 10.4 mSv.

120
100
80
60
40
20

<2.5

2.4

2.2

2

1.8

1.6

1.4

1.2

10.8

0.6

0.4

0.2

0ND

August: 95tubes from 27schools, September:18tubes from 8schools, October: 67tubes from18schools, Nobember: 11tubes from 4schools wereevaluated based on the provisional guideline.

Number of Crookes tubes

The second exploration was performed
at Aug-Nov/2019 to validate theefficiency of the provisional guideline.

< 50μSv(Detection Limit)

140

Dose ( Hp(0.07), 10min, @ 20cm) / mSv

These doses are the measurements
results obtained Glass Badge type FX
they were put at the distance of20cm and exposured during 10min,
and evaluated in Hp(0.07). We have toestimate the effective dose for true
students and teachers in the trueeducation field.

To Estimate Effective DoseFor Eye Lens, Equivalent
Dose is also must beestimated.

EVENAt the center of front,
Hp(0.07)=200mSv/h at 15cmDosimeter ① At a distance of 1m;

Reduced to 1/44

③ In Holizontally, Peripheral Dose issmaller than that measured at the cente（50cm shift gives 2/3 reduction）

It is assumed that in 10min
demo, Effective Dose isresulted within 50μSv.

② 20keV X-rays are reduced toone half every 1cm in body.
Hp(10) does not represent
Effective Dose.

Calculation with PHITS code using:・Hp(0.07) or Air Kerma
・Energy spectrum
・Horizontal Distribution

ICRP Pub116 Fig.5.2

E(AP) ≒ 1/5 Hp(10)≒ 1/10 Hp(0.07)
at 20keVDifference Between
Protection and Operational Quantity

ICRP Pub116
Fig. 5.2
Ratios of photon effective dose (present report) to ambient dose equivalent.
Students observe a Crookes tube, then orientation is fix to AP.
At 20keV, H*(10) gives 5 times overestimation than Effective Dose.

Exploration at Real Education Field○measurement distance 20cm → students take a distance than 1m : 1/25
○Hp(0.07) @ 20keV → Effective dose : Provisionally 1/10
○Measurement time 10min → Integrated experience time for teachers 10min : SameRaw date byGlass Badges
Number of Crookes tubes

140

Based on the provisional Guideline

120

100

8060

40

200ND～0.5～1～5～10～50～100

Provisional Effective dose @1m, 10min / μSv

Number of Crookes tubes

40

Original setting in the class

3530
25
20
15
1050ND～0.5～1～5～10～50～100

Provisional Effective dose @1m, 10min / μSv

The effective dose of the 187 tubes in 191tubes measured with
provisional guide line were evaluated smaller than 10μSv, that is
exemption level of IAEA BSS, at 1m in 10min.
Only 4 tubes showed larger dose than 10μSv, 3 were smaller
than 20μSv and the highest was evaluated as 42μSv, that issmaller than the dose limit in ICRP-Pub36, 50μSv.

Why several tubes showed a little higher dose is not resolved.
Some screening technique is now under considering using leaf
electrode that is act as open air ionizing chamber orconventional survey meter (with plastic scintillater) calibratedwith low energy X-rays.
If it is turned out that the equipment radiate
rather strong X-rays, it is easy to resolve it.
Take a little more distance is the most easy way.
Shielding with glass aquarium is also goodchoice. Only 1.9mm glass reduced dose to
1/20-1/50, but acrylic resin with low Z reducedose to only 1/3 with 1cm thick plate.