pvc dosimetry - 30 years of experience

30 years

of

PVC dosimetry in

INCT

Zofia

Stuglik

Institute of Nuclear Chemistry and Technology, Laboratory for Measurement of Technological Doses, Warsaw, Poland

IAEA Training Course, 3-7 December 2007, Warsaw, Zofia Stuglik

History1972

Instalation

of linear electron accelerator LAE 13/9

9 kW, 7 ÷

13 MeV, (Jefremov

Institute,

Leningrad, USSR)

two

regimes:straight electron

beam –

for pulse radiolysis

scanned electron

beam –

for technological studies



LAE 13/9 deflection magnet (270 deg)

Pulse radiolysis apparatus, horizontal beam

Vertical, scanned beam for technological experiments

(1 meter below)

HistoryResponsible persons for electron beam dosimetry

systems

were:

Dr. Przemysław

Panta

(water and graphite quasi-adiabatic calorimeters)

Dr. Zofia

Bułhak

(film dosimeters)


At 60-ties and 70-ties PVC films were intensively investigated by a lot of scientists

(C.Artandi, A.A.Miller, A.Chapiro,

S.Onishi, Y.Nakayama…).

Mrs. Bułhak

joined to this group at the beginning of 70-ties. Dosimetric

applications of PVC films was a subject

of Mrs.

Bułhak

dissertation and because of that they were investigated very carefully.

Some results of these experiments will be presented today

and were compared with the results of the last studies done in Laboratory for Measurement of Technological Doses.



PVC production, method

I

PVC = polyvinyl

chloride

ethylene + chlorine vinyl chloride

vinyl chloride polyvinyl chloridePCV

catalyst

polymerization

n

dichlorethane


PVC production, method

IIPVC = polyvinyl

chloride

acetylene + hydrochloride vinyl chloride

vinyl chloride polyvinyl chloridePCV

catalyst

polymerization

n


How

much of

PVC is

in

the

PVC ?

It

depends……

PVC

hard

(%)

soft (%)

PVC

> 90

40-80

Plasticizer

≤

5

20-60

Stabilizer

0,5 –

0,5 0,5-4

Lubricant ≤5

≤

1

(Fillers)

?

?


The PVC film from Kunstoffwerke, Staufen

which has been used in our Institute for electron beam dosimetry

applications was produced for pharmacy.

It may contain some amounts:

• plasticizers (probably phthalates)

• thermal stabilizers (probably Sn

(stannum) compounds)

• lubricants

The main component is

PVC


Radiolysis

For the simple chemical system as Fricke dosimeter it is possible to establish exact scheme of radiation reactions, to measure their orders, rate constants and G-values.

For PVC it is impossible.

The system is too complicated. We have some information about radiolytical

processes going in

PVC but we are still far for the full knowledge.


What we know?EPR experiments of Ohnishi and Nakayama showed that irradiation of PVC generate 3 types of radicals:

-CH2

C*H-CH2

-

-CHCl-C*H-CHCl-

-CH2

C*Cl-CH2

-

These reactive species initiate chemical reactions leading to more stable structures, for instance -

double bonds systems

C=C.


What else we know?

It is known from many years that the main gaseous product of PVC radiolysis is hydrochloride (HCl).

High G-value of HCl

formation (~26) points to the chain reactions involved in its production.

It is also known that HCl

initiate autocatalytic decay of PVC polymer.


So, among products of PVC radiolysis there are at least two types of species: radicals

and hydrochloride (HCl)

which will destabilize the PVC polymeric system.

But we want to use PVC as dosimeter. So, we should find factors which will stabilize it.

There are two such factors: •time•temperature

In dosimetry

we have not plenty of time,

so we use temperature.

.


At higher temperatures the chemical reactions go faster, the concentration of active species go down and irradiated PVC is going to the more stable state.

Of course, the temperature should be below the softening point

of PVC because very high temperature also lead to

destruction of polymer.


The temperature is

a factor which

can

put in order an

intrinsic

structure of irradiated PVC

films.

Probably

the double bond

of carbon

systems

change

to

carbon couple bond

which

are

known as chromophores

wavelength nm

Dosimetric signal

A396

Electronic

optical

absorption

in

irradiated

PVC films


Optical spectra observed in irradiated PVCThe optical absorption spectra observed in irradiated and

heated

PVC films are ascribed in literature to the coupled carbon bonds

C=C-C=C-C=C-C=C-C=C

The short structures (a number of the coupling bonds ≤

3) absorb the UV light.

The long structures (number of the coupling bonds from 4 to 9) –

absorb the visible light (less energy is necessary to

excite them)


Now, we are

going

to the

results

obtained

in

our

Institute 30-ty years

ago.


Optical

spectra observed

in

different

PVC films

after

irradiation

and

heating

D=30 kGy

Staufen, type

N


After

irradiation

and

heating

After

irradiation

Before

irradiation

PCV from

Kunstoffwerke, Staufen

ΔA396

= dosimetric

signal


Calibration

curves

for different

PVC films

(5 –

50 kGy, gamma)


Calibration

curves

for different

PVC films

(5 –

50 kGy, 10 MeV

electrons)


Vessels

with water

at

different temperature

PVC films

Experimental

scheme

for measurement

of

temperature coefficient of irradiation

40 kGyelectron

beam


Temperature coefficient of irradiation

k=0,25 % per 1°C [Z.Bułhak]


Irradiated

PVC film, probably

Staufen, type

N Influence of

temperature

and

heating

duration

on dosimetric

signal

(A396

)

80 °C

30 min

70 °C

60 °C


Post-effects

(=changes

in

dosimetric

signal

amplitudes) observed

in irradiated

PVC film Staufen, type

N for D=40kGy without

heating

and

after

30min heating

at

different

temperatures

80 °C

70 °C without

heating

50

°C

10 days 20 days


The best stability of dosimetric

signal was obtained for 30min heating at 80°C.

However

So high temperature was near the softening point of PVC film used in this experiment and because of that 70°C was selected for dosimetric

applications.


The protocol of dosimetric

signal measurement

with non-plasticized PVC film from Staufen, Type N

1975

1.

Irradiation

2.

Heating: 30 min at 70°C (oven with water jacket)

3.

Cooling 30 min at room temperature

4.

Absorbance measurement at 396nm

5.

Read-out the dose from a calibration curve ΔA396

=f(D) (performed at the same conditions as dose measurement ones)


On the base of Dr. Z.Bułhak

work INCT bought some tons of non-plasticized PVC film from Kunststoffwerke, Staufen

and used this film for

dose distribution measurements during more than 30 years.

The same film is used also today.

Two years ago LMTD started the experiments to re- validate dosimetric

characteristics of PVC films

which still are using at INCT Sterilization Plant.


Results

•There were observed some differences between sensitivity of different scrolls

of

PVC film.

• No correction for film thickness was necessary.

• CV-values and calibration curves were acceptable.

• Resistance for environmental conditions was excellent.


Calibration curve for PVC (scroll IV) (mean values)


Calibration curve for PVC (scroll IV) (6 dosimeters)


The protocol from 1975 is still valid

However, we observed

that

actually

used PVC films are much more sensitive to

any deviation from the

protocol

because

of

strong

post-effects


Post-effects

in

PVC films

irradiated

with

gamma and high energy

electrons

to 22 kGy

time, h

gamma

electrons


Post-effects

in

PVC films

irradiated

with

gamma and high energy

electrons

to 7 kGy

gamma

electrons


[Z.Peimel-Stuglik, S.Fabisiak, Appl.Radiat.Isot., 2007, in

press]

and heating


What is a reason of such situation?

There were at least three possibilities:

(a) PVC films changed their properties during 30 years

(b) The films which endured to our time are not type N from Kunststoffwerke, Staufen, chosen for dosimetric

application by Dr. Bułhak


(c) The results dealing post-effects and presented in work done at 1975 were obtained for PVC films irradiated to doses ~ 40 kGy.

For such high doses the post-effects in irradiated and heated PVC films are negligible and were omitted.


Advantages

of PVC films

• Very low price

• High sensitivity to ionizing radiation

•

High tolerance (nonsensitivity) to environmental conditions (air humidity, liquid water, visible, UV and Cherenkov light)

• Negligible film thickness variations

•

Slow ageing, significant rigidity, availability in large sheets and the possibility of rough visual dose evaluation

•

Cheap and simple analytical instrument for dosimetric

signal measurements

(spectrofotometer

for visible light)


Disadvantagesof

PVC films

• Significant and complicated post-effects

• Dependence of the post-effects on dose.

•

Repeatability (CV, % ) on the level 2 –

6% (from batch to batch).

•

Chemical composition of PVC films different that mean chemical composition of organic matter (high content of Cl and some amounts (1-3%) of Ca or Sn). Exact chemical composition of PVC film is known only to its producer.


Disadvantagesof

PVC films

• Be careful to use them for low doses (up to 20 kGy)

•

Take attention for technical parameters of oven -

dosimetric signal obtained after heating in oven with ventilation will be

different than after heating in oven with water jacket (for the same dose).

• Each batch should be carefully validated.

•

The protocol of measurement (timing, heating) should be establish and the staff shall follow it.


You

can

use

PVC film for high dose electron

beam

dosimetry

if

you

are

very

accurate

and

carefull


What will be if you are not very accurate

pvc dosimetry - 30 years of experience

Documents