The answer is easy but not necessarily acceptable :-).
Cooling will be easiest.
Air conditioner with different refrigerant may suffice - HVAC experts can advise.
System reliability and response to failure must be planned. Does it need to be redundant-failsafe or should it shut down under cooling failure.
You must determine the maximum "local ambient" temperature that the equipment will work in.
You must provide a "local environment that is never any hotter than the acceptable maximum environmental temperature.
If the necessary temperature is below the local ambient of 45 C you MUST provide cooling to maintain the local temperature.
Having established this necessary but unavoidable requirement you can address it in two ways.
Ambient temperature is never > 45 C.
Ambient = mine temperature = Ta.
Local temperature = equipment housing temp = Tl
1 You can make the equipment able to operate at Ta (45C )
2 You can provide cooling of an enclosure to Tl.
1 is superior if you can achieve it as it involves less long term care and management and reliability. If you can find equipment that will run at 50C (Ta+5) you will be able to provide cooling with blown air and very good design. Ability to operate at Ta+10 (55C would be better and TA + 15 or 20 better still). Heatsinks of 1C/Watt rating are getting fairly impressive so at Ta+5 you can only dissipate 5 Watt with a 1C/W heatsink or 20 Watt at Ta+20. If you need substantially more than about 10 Watt dissipation you can use "heat pipe" transfer away from the equipment which then allows as substantial blown heat sinks as required.
You mention "a laptop" but you will probably find that there are custom devices that meet you computing needs and that are aimed at low power and rugged environmental conditions. Ultimately such a purpose designed device may be cheaper than a standard laptop + much pain.
If eg your photo multiplier tubes MUST run at under Ta then you MUST cool them.
There are numerous means of cooling in that environment. The likely easiest and simplest and highest capacity in that environment, provided enough power is available, is a standard "Rankine cycle" airconditioner or refrigerator BUT with the refrigerant type and pressures/fill designed by an airconditioning expert. While standard off the shelf units are unlikely to have refrigerants that are optimum for Thot = Ta = 45C it is quite likely that little more than a refill with a more appropriate refrigerant would be required. This is such an attractive option that talking to an HVAC expert should be one of your first actions. See  at end for some likely starting points.
High efficiency Stirling cycle equipment. Several suppliers. Sunpower et al.
The Coleman Stirling camping cooler may meet your need directly. NOT to be confused with cheaper Peltier type coolers.
I can provide references to the manufacturers of this and similar Stirling cycle equipment.
As per electric chilly bins BUT done properly. Peltier devices can work entirely adequately in the environment you describe. 10C drop is easyish and 20C plus is doable. Needs proper design. A custom solution may be best.
Needs little more than suitable heatsinks, substantial Ta air flow, lower Tl airflow, Peltier devices + power. Peltier coolers of 50W - 100W energy input capacity are available. Each of these would cool perhaps 5W - 10W of equipment load in this environment. TBD by design. Multiple Peltier modules are used to meet need.
Evaporative. An evaporatove cooler using water ("Swamp cooler") MAY suffice and one using other fluids will be able to. This approaches an air conditioner when properly done in this context.
More if needed ...
 Likely useful refrigerants sites:
20 refrigerants for sale ! : http://r22.org/