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Medical physics 2: Radiation Safety > Monitoring > Flashcards

Flashcards in Monitoring Deck (24)
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1

What is a TLD?

Thermo-luminescent dosimetry

LiF crystal, x-rays create ions which are then trapped (by doping element) and released by applying heat

No. electron and light is directly proportional to dose (supra lin at high dose)

Clear with high temp

Range: 10uGy to 1Gy

2

What is OSL?

Optically stimulated luminescence

Crystalline material, electron traps -> readout via laser like TLD

Can be reread mult times before annealing

10uSv to 10Gy

3

What is DIS?

Direct ion storage

Similar to DR but also contains an ion chamber to improve dosimetry

Ion-gate, charge liberated in the chamber is collected at the gate. Can store a long term record.

30uGy to 5Gy

4

Other dosimetry methods

Film badge
Real time dosim (DIS for interventional)
Quartz fibre: charge fibre moves over a scale dep on charge. Prev used by emergency services
EPD: Electronic personal dosim GM tech, gives realtime int dose

Blood tests: after acute expo
Looks for breaks and dicentrics in DNA

5

Why is monitoring required?

Required by IRR17

Classified workers must be monitored

Most exposed workers (in healthcare) are not classified

Personal monitoring provides ongoing confirmation that each worker’s status remains appropriate

6

What are examples fluoro staff doses?

Surgeon: 0.4mSv
Rad practitioner: 0.6mSv
Risk from of accidental whole body expo: low

7

What are example cardiology staff doses?

0.25-0.31,Sv per year
Consequence of expo: med

8

IRR2017 monitoring requirements

- Need an Approved dosimetry service; systematic dose assessment and recording

- Individual monitoring where possible or otherwise

- Retain records (to 75 years or 50 years after assessment)

- Provide radiation passbooks

9

Monitoring procedure

- Approved by HSE
- Approve dosimetry service
- Follows written procedures
- Reported doses traceable to primary standards
- external testing and inspection
- Dose audits
- Investigate unexp results
- retains records securely
- Report to ADS

10

What is an ADS ?

- requests estimated doses from the employer
- contains special entries
- makes special entries in the record
- retains details of investigations

Reg22 IRR - est doses and special entries

11

Employer monitoring duties

Takes advice from RPA
Determines what/who/how to monitor
Estab inv. levels
Monitors badge/PPE use
Liases with ADS, regulators, other employers

12

How is Hp10 derived?

Whole body measure

Dose at depth of 10mm to ICRU sphere

Measure air-kerma in standard phantom

13

Issues with Hp10

Significant underestimate due to orientation:

Front badge, radiation from rear
Back badge, radiation from front
Consider field conditions

Shielded dosimeter underestimates dose (~20%)
Unshielded collar badge overestimates dose (x20?)
If critical use two badges
under apron (trunk) badge
over apron (collar) badge

14

Issues with Hp(0.07)

Tip of finger difficult to monitor

Underestimates extremity doses factor 4-5 for Tc99m

15

Deriving Hp3

Eye dose - water phantom under standard conditions, used tld

16

How to measure surface contamination

Surface contamination measurements; floors, work surfaces and skin; slow sweep of entire areas to find maximum levels; routine check against derived working limits consider area and background

17

What is a wipe test?

Test surfaces and storage containers for leaks

Swap areas and measure with a device such as a well counter.

18

What are the issues with wipe testing?

Time consuming
No immediate result
Portion removed (assume 10%?)

19

How to estimate surface contamination

Review local limits
Consider transfer routes

Averaging area 1000cm2 Floors, 300cm2 1cm2 Skin

Typical values for public areas (MDG Notes)
30Bqcm-2 Tc-99m, C-14, S-35, Ga-67,I-123, I-125
300Bqcm-2 H-3, Cr-57
3Bqcm-2 others

20

What is environmental monitoring?

how is it carried out short/long term?

A requirement of the regulations; outside x-rays rooms, radio-pharmacies, LINAC rooms, etc.

Short term: Instantaneous dose rate gives quick indication

For long term review integrated dose (TLD badge or similar, 2 months) which incorporates workload.

21

What are the measures of enviromental doserate

IDR instaneous dose rate

TADR Time average dose rate (avr over 8hr)

TADR2000 av over work year x occupancy factor

22

How is radon monitored?

Passive and active devices, sample basements over 2 months against a derived limit of 400 Bq m-3

23

What is RIMNET?

RIMNET – Monitoring environmental GAMMA background using standard GM equipment (mSv/yr)

24

Dosimetry changes in IRR17

monitoring unchanges
eye limit reduced 150 -> 20mSv
More targeted monitoring
Standard factors still under review